environment.py

"""Classes for managing templates and their runtime and compile time
options.
"""
import os
import sys
import typing
import typing as t
import weakref
from collections import ChainMap
from functools import lru_cache
from functools import partial
from functools import reduce
from types import CodeType

from markupsafe import Markup

from . import nodes
from .compiler import CodeGenerator
from .compiler import generate
from .defaults import BLOCK_END_STRING
from .defaults import BLOCK_START_STRING
from .defaults import COMMENT_END_STRING
from .defaults import COMMENT_START_STRING
from .defaults import DEFAULT_FILTERS
from .defaults import DEFAULT_NAMESPACE
from .defaults import DEFAULT_POLICIES
from .defaults import DEFAULT_TESTS
from .defaults import KEEP_TRAILING_NEWLINE
from .defaults import LINE_COMMENT_PREFIX
from .defaults import LINE_STATEMENT_PREFIX
from .defaults import LSTRIP_BLOCKS
from .defaults import NEWLINE_SEQUENCE
from .defaults import TRIM_BLOCKS
from .defaults import VARIABLE_END_STRING
from .defaults import VARIABLE_START_STRING
from .exceptions import TemplateNotFound
from .exceptions import TemplateRuntimeError
from .exceptions import TemplatesNotFound
from .exceptions import TemplateSyntaxError
from .exceptions import UndefinedError
from .lexer import get_lexer
from .lexer import Lexer
from .lexer import TokenStream
from .nodes import EvalContext
from .parser import Parser
from .runtime import Context
from .runtime import new_context
from .runtime import Undefined
from .utils import _PassArg
from .utils import concat
from .utils import consume
from .utils import import_string
from .utils import internalcode
from .utils import LRUCache
from .utils import missing

if t.TYPE_CHECKING:
    import typing_extensions as te
    from .bccache import BytecodeCache
    from .ext import Extension
    from .loaders import BaseLoader

_env_bound = t.TypeVar("_env_bound", bound="Environment")


# for direct template usage we have up to ten living environments
@lru_cache(maxsize=10)
def get_spontaneous_environment(cls: t.Type[_env_bound], *args: t.Any) -> _env_bound:
    """Return a new spontaneous environment. A spontaneous environment
    is used for templates created directly rather than through an
    existing environment.

    :param cls: Environment class to create.
    :param args: Positional arguments passed to environment.
    """
    env = cls(*args)
    env.shared = True
    return env


def create_cache(
    size: int,
) -> t.Optional[t.MutableMapping[t.Tuple[weakref.ref, str], "Template"]]:
    """Return the cache class for the given size."""
    if size == 0:
        return None

    if size < 0:
        return {}

    return LRUCache(size)  # type: ignore


def copy_cache(
    cache: t.Optional[t.MutableMapping],
) -> t.Optional[t.MutableMapping[t.Tuple[weakref.ref, str], "Template"]]:
    """Create an empty copy of the given cache."""
    if cache is None:
        return None

    if type(cache) is dict:
        return {}

    return LRUCache(cache.capacity)  # type: ignore


def load_extensions(
    environment: "Environment",
    extensions: t.Sequence[t.Union[str, t.Type["Extension"]]],
) -> t.Dict[str, "Extension"]:
    """Load the extensions from the list and bind it to the environment.
    Returns a dict of instantiated extensions.
    """
    result = {}

    for extension in extensions:
        if isinstance(extension, str):
            extension = t.cast(t.Type["Extension"], import_string(extension))

        result[extension.identifier] = extension(environment)

    return result


def _environment_config_check(environment: "Environment") -> "Environment":
    """Perform a sanity check on the environment."""
    assert issubclass(
        environment.undefined, Undefined
    ), "'undefined' must be a subclass of 'jinja2.Undefined'."
    assert (
        environment.block_start_string
        != environment.variable_start_string
        != environment.comment_start_string
    ), "block, variable and comment start strings must be different."
    assert environment.newline_sequence in {
        "\r",
        "\r\n",
        "\n",
    }, "'newline_sequence' must be one of '\\n', '\\r\\n', or '\\r'."
    return environment


class Environment:
    r"""The core component of Jinja is the `Environment`.  It contains
    important shared variables like configuration, filters, tests,
    globals and others.  Instances of this class may be modified if
    they are not shared and if no template was loaded so far.
    Modifications on environments after the first template was loaded
    will lead to surprising effects and undefined behavior.

    Here are the possible initialization parameters:

        `block_start_string`
            The string marking the beginning of a block.  Defaults to ``'{%'``.

        `block_end_string`
            The string marking the end of a block.  Defaults to ``'%}'``.

        `variable_start_string`
            The string marking the beginning of a print statement.
            Defaults to ``'{{'``.

        `variable_end_string`
            The string marking the end of a print statement.  Defaults to
            ``'}}'``.

        `comment_start_string`
            The string marking the beginning of a comment.  Defaults to ``'{#'``.

        `comment_end_string`
            The string marking the end of a comment.  Defaults to ``'#}'``.

        `line_statement_prefix`
            If given and a string, this will be used as prefix for line based
            statements.  See also :ref:`line-statements`.

        `line_comment_prefix`
            If given and a string, this will be used as prefix for line based
            comments.  See also :ref:`line-statements`.

            .. versionadded:: 2.2

        `trim_blocks`
            If this is set to ``True`` the first newline after a block is
            removed (block, not variable tag!).  Defaults to `False`.

        `lstrip_blocks`
            If this is set to ``True`` leading spaces and tabs are stripped
            from the start of a line to a block.  Defaults to `False`.

        `newline_sequence`
            The sequence that starts a newline.  Must be one of ``'\r'``,
            ``'\n'`` or ``'\r\n'``.  The default is ``'\n'`` which is a
            useful default for Linux and OS X systems as well as web
            applications.

        `keep_trailing_newline`
            Preserve the trailing newline when rendering templates.
            The default is ``False``, which causes a single newline,
            if present, to be stripped from the end of the template.

            .. versionadded:: 2.7

        `extensions`
            List of Jinja extensions to use.  This can either be import paths
            as strings or extension classes.  For more information have a
            look at :ref:`the extensions documentation <jinja-extensions>`.

        `optimized`
            should the optimizer be enabled?  Default is ``True``.

        `undefined`
            :class:`Undefined` or a subclass of it that is used to represent
            undefined values in the template.

        `finalize`
            A callable that can be used to process the result of a variable
            expression before it is output.  For example one can convert
            ``None`` implicitly into an empty string here.

        `autoescape`
            If set to ``True`` the XML/HTML autoescaping feature is enabled by
            default.  For more details about autoescaping see
            :class:`~markupsafe.Markup`.  As of Jinja 2.4 this can also
            be a callable that is passed the template name and has to
            return ``True`` or ``False`` depending on autoescape should be
            enabled by default.

            .. versionchanged:: 2.4
               `autoescape` can now be a function

        `loader`
            The template loader for this environment.

        `cache_size`
            The size of the cache.  Per default this is ``400`` which means
            that if more than 400 templates are loaded the loader will clean
            out the least recently used template.  If the cache size is set to
            ``0`` templates are recompiled all the time, if the cache size is
            ``-1`` the cache will not be cleaned.

            .. versionchanged:: 2.8
               The cache size was increased to 400 from a low 50.

        `auto_reload`
            Some loaders load templates from locations where the template
            sources may change (ie: file system or database).  If
            ``auto_reload`` is set to ``True`` (default) every time a template is
            requested the loader checks if the source changed and if yes, it
            will reload the template.  For higher performance it's possible to
            disable that.

        `bytecode_cache`
            If set to a bytecode cache object, this object will provide a
            cache for the internal Jinja bytecode so that templates don't
            have to be parsed if they were not changed.

            See :ref:`bytecode-cache` for more information.

        `enable_async`
            If set to true this enables async template execution which
            allows using async functions and generators.
    """

    #: if this environment is sandboxed.  Modifying this variable won't make
    #: the environment sandboxed though.  For a real sandboxed environment
    #: have a look at jinja2.sandbox.  This flag alone controls the code
    #: generation by the compiler.
    sandboxed = False

    #: True if the environment is just an overlay
    overlayed = False

    #: the environment this environment is linked to if it is an overlay
    linked_to: t.Optional["Environment"] = None

    #: shared environments have this set to `True`.  A shared environment
    #: must not be modified
    shared = False

    #: the class that is used for code generation.  See
    #: :class:`~jinja2.compiler.CodeGenerator` for more information.
    code_generator_class: t.Type["CodeGenerator"] = CodeGenerator

    #: the context class that is used for templates.  See
    #: :class:`~jinja2.runtime.Context` for more information.
    context_class: t.Type[Context] = Context

    template_class: t.Type["Template"]

    def __init__(
        self,
        block_start_string: str = BLOCK_START_STRING,
        block_end_string: str = BLOCK_END_STRING,
        variable_start_string: str = VARIABLE_START_STRING,
        variable_end_string: str = VARIABLE_END_STRING,
        comment_start_string: str = COMMENT_START_STRING,
        comment_end_string: str = COMMENT_END_STRING,
        line_statement_prefix: t.Optional[str] = LINE_STATEMENT_PREFIX,
        line_comment_prefix: t.Optional[str] = LINE_COMMENT_PREFIX,
        trim_blocks: bool = TRIM_BLOCKS,
        lstrip_blocks: bool = LSTRIP_BLOCKS,
        newline_sequence: "te.Literal['\\n', '\\r\\n', '\\r']" = NEWLINE_SEQUENCE,
        keep_trailing_newline: bool = KEEP_TRAILING_NEWLINE,
        extensions: t.Sequence[t.Union[str, t.Type["Extension"]]] = (),
        optimized: bool = True,
        undefined: t.Type[Undefined] = Undefined,
        finalize: t.Optional[t.Callable[..., t.Any]] = None,
        autoescape: t.Union[bool, t.Callable[[t.Optional[str]], bool]] = False,
        loader: t.Optional["BaseLoader"] = None,
        cache_size: int = 400,
        auto_reload: bool = True,
        bytecode_cache: t.Optional["BytecodeCache"] = None,
        enable_async: bool = False,
    ):
        # !!Important notice!!
        #   The constructor accepts quite a few arguments that should be
        #   passed by keyword rather than position.  However it's important to
        #   not change the order of arguments because it's used at least
        #   internally in those cases:
        #       -   spontaneous environments (i18n extension and Template)
        #       -   unittests
        #   If parameter changes are required only add parameters at the end
        #   and don't change the arguments (or the defaults!) of the arguments
        #   existing already.

        # lexer / parser information
        self.block_start_string = block_start_string
        self.block_end_string = block_end_string
        self.variable_start_string = variable_start_string
        self.variable_end_string = variable_end_string
        self.comment_start_string = comment_start_string
        self.comment_end_string = comment_end_string
        self.line_statement_prefix = line_statement_prefix
        self.line_comment_prefix = line_comment_prefix
        self.trim_blocks = trim_blocks
        self.lstrip_blocks = lstrip_blocks
        self.newline_sequence = newline_sequence
        self.keep_trailing_newline = keep_trailing_newline

        # runtime information
        self.undefined: t.Type[Undefined] = undefined
        self.optimized = optimized
        self.finalize = finalize
        self.autoescape = autoescape

        # defaults
        self.filters = DEFAULT_FILTERS.copy()
        self.tests = DEFAULT_TESTS.copy()
        self.globals = DEFAULT_NAMESPACE.copy()

        # set the loader provided
        self.loader = loader
        self.cache = create_cache(cache_size)
        self.bytecode_cache = bytecode_cache
        self.auto_reload = auto_reload

        # configurable policies
        self.policies = DEFAULT_POLICIES.copy()

        # load extensions
        self.extensions = load_extensions(self, extensions)

        self.is_async = enable_async
        _environment_config_check(self)

    def add_extension(self, extension: t.Union[str, t.Type["Extension"]]) -> None:
        """Adds an extension after the environment was created.

        .. versionadded:: 2.5
        """
        self.extensions.update(load_extensions(self, [extension]))

    def extend(self, **attributes: t.Any) -> None:
        """Add the items to the instance of the environment if they do not exist
        yet.  This is used by :ref:`extensions <writing-extensions>` to register
        callbacks and configuration values without breaking inheritance.
        """
        for key, value in attributes.items():
            if not hasattr(self, key):
                setattr(self, key, value)

    def overlay(
        self,
        block_start_string: str = missing,
        block_end_string: str = missing,
        variable_start_string: str = missing,
        variable_end_string: str = missing,
        comment_start_string: str = missing,
        comment_end_string: str = missing,
        line_statement_prefix: t.Optional[str] = missing,
        line_comment_prefix: t.Optional[str] = missing,
        trim_blocks: bool = missing,
        lstrip_blocks: bool = missing,
        extensions: t.Sequence[t.Union[str, t.Type["Extension"]]] = missing,
        optimized: bool = missing,
        undefined: t.Type[Undefined] = missing,
        finalize: t.Optional[t.Callable[..., t.Any]] = missing,
        autoescape: t.Union[bool, t.Callable[[t.Optional[str]], bool]] = missing,
        loader: t.Optional["BaseLoader"] = missing,
        cache_size: int = missing,
        auto_reload: bool = missing,
        bytecode_cache: t.Optional["BytecodeCache"] = missing,
    ) -> "Environment":
        """Create a new overlay environment that shares all the data with the
        current environment except for cache and the overridden attributes.
        Extensions cannot be removed for an overlayed environment.  An overlayed
        environment automatically gets all the extensions of the environment it
        is linked to plus optional extra extensions.

        Creating overlays should happen after the initial environment was set
        up completely.  Not all attributes are truly linked, some are just
        copied over so modifications on the original environment may not shine
        through.
        """
        args = dict(locals())
        del args["self"], args["cache_size"], args["extensions"]

        rv = object.__new__(self.__class__)
        rv.__dict__.update(self.__dict__)
        rv.overlayed = True
        rv.linked_to = self

        for key, value in args.items():
            if value is not missing:
                setattr(rv, key, value)

        if cache_size is not missing:
            rv.cache = create_cache(cache_size)
        else:
            rv.cache = copy_cache(self.cache)

        rv.extensions = {}
        for key, value in self.extensions.items():
            rv.extensions[key] = value.bind(rv)
        if extensions is not missing:
            rv.extensions.update(load_extensions(rv, extensions))

        return _environment_config_check(rv)

    @property
    def lexer(self) -> Lexer:
        """The lexer for this environment."""
        return get_lexer(self)

    def iter_extensions(self) -> t.Iterator["Extension"]:
        """Iterates over the extensions by priority."""
        return iter(sorted(self.extensions.values(), key=lambda x: x.priority))

    def getitem(
        self, obj: t.Any, argument: t.Union[str, t.Any]
    ) -> t.Union[t.Any, Undefined]:
        """Get an item or attribute of an object but prefer the item."""
        try:
            return obj[argument]
        except (AttributeError, TypeError, LookupError):
            if isinstance(argument, str):
                try:
                    attr = str(argument)
                except Exception:
                    pass
                else:
                    try:
                        return getattr(obj, attr)
                    except AttributeError:
                        pass
            return self.undefined(obj=obj, name=argument)

    def getattr(self, obj: t.Any, attribute: str) -> t.Any:
        """Get an item or attribute of an object but prefer the attribute.
        Unlike :meth:`getitem` the attribute *must* be a string.
        """
        try:
            return getattr(obj, attribute)
        except AttributeError:
            pass
        try:
            return obj[attribute]
        except (TypeError, LookupError, AttributeError):
            return self.undefined(obj=obj, name=attribute)

    def _filter_test_common(
        self,
        name: t.Union[str, Undefined],
        value: t.Any,
        args: t.Optional[t.Sequence[t.Any]],
        kwargs: t.Optional[t.Mapping[str, t.Any]],
        context: t.Optional[Context],
        eval_ctx: t.Optional[EvalContext],
        is_filter: bool,
    ) -> t.Any:
        if is_filter:
            env_map = self.filters
            type_name = "filter"
        else:
            env_map = self.tests
            type_name = "test"

        func = env_map.get(name)  # type: ignore

        if func is None:
            msg = f"No {type_name} named {name!r}."

            if isinstance(name, Undefined):
                try:
                    name._fail_with_undefined_error()
                except Exception as e:
                    msg = f"{msg} ({e}; did you forget to quote the callable name?)"

            raise TemplateRuntimeError(msg)

        args = [value, *(args if args is not None else ())]
        kwargs = kwargs if kwargs is not None else {}
        pass_arg = _PassArg.from_obj(func)

        if pass_arg is _PassArg.context:
            if context is None:
                raise TemplateRuntimeError(
                    f"Attempted to invoke a context {type_name} without context."
                )

            args.insert(0, context)
        elif pass_arg is _PassArg.eval_context:
            if eval_ctx is None:
                if context is not None:
                    eval_ctx = context.eval_ctx
                else:
                    eval_ctx = EvalContext(self)

            args.insert(0, eval_ctx)
        elif pass_arg is _PassArg.environment:
            args.insert(0, self)

        return func(*args, **kwargs)

    def call_filter(
        self,
        name: str,
        value: t.Any,
        args: t.Optional[t.Sequence[t.Any]] = None,
        kwargs: t.Optional[t.Mapping[str, t.Any]] = None,
        context: t.Optional[Context] = None,
        eval_ctx: t.Optional[EvalContext] = None,
    ) -> t.Any:
        """Invoke a filter on a value the same way the compiler does.

        This might return a coroutine if the filter is running from an
        environment in async mode and the filter supports async
        execution. It's your responsibility to await this if needed.

        .. versionadded:: 2.7
        """
        return self._filter_test_common(
            name, value, args, kwargs, context, eval_ctx, True
        )

    def call_test(
        self,
        name: str,
        value: t.Any,
        args: t.Optional[t.Sequence[t.Any]] = None,
        kwargs: t.Optional[t.Mapping[str, t.Any]] = None,
        context: t.Optional[Context] = None,
        eval_ctx: t.Optional[EvalContext] = None,
    ) -> t.Any:
        """Invoke a test on a value the same way the compiler does.

        This might return a coroutine if the test is running from an
        environment in async mode and the test supports async execution.
        It's your responsibility to await this if needed.

        .. versionchanged:: 3.0
            Tests support ``@pass_context``, etc. decorators. Added
            the ``context`` and ``eval_ctx`` parameters.

        .. versionadded:: 2.7
        """
        return self._filter_test_common(
            name, value, args, kwargs, context, eval_ctx, False
        )

    @internalcode
    def parse(
        self,
        source: str,
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
    ) -> nodes.Template:
        """Parse the sourcecode and return the abstract syntax tree.  This
        tree of nodes is used by the compiler to convert the template into
        executable source- or bytecode.  This is useful for debugging or to
        extract information from templates.

        If you are :ref:`developing Jinja extensions <writing-extensions>`
        this gives you a good overview of the node tree generated.
        """
        try:
            return self._parse(source, name, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source)

    def _parse(
        self, source: str, name: t.Optional[str], filename: t.Optional[str]
    ) -> nodes.Template:
        """Internal parsing function used by `parse` and `compile`."""
        return Parser(self, source, name, filename).parse()

    def lex(
        self,
        source: str,
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
    ) -> t.Iterator[t.Tuple[int, str, str]]:
        """Lex the given sourcecode and return a generator that yields
        tokens as tuples in the form ``(lineno, token_type, value)``.
        This can be useful for :ref:`extension development <writing-extensions>`
        and debugging templates.

        This does not perform preprocessing.  If you want the preprocessing
        of the extensions to be applied you have to filter source through
        the :meth:`preprocess` method.
        """
        source = str(source)
        try:
            return self.lexer.tokeniter(source, name, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source)

    def preprocess(
        self,
        source: str,
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
    ) -> str:
        """Preprocesses the source with all extensions.  This is automatically
        called for all parsing and compiling methods but *not* for :meth:`lex`
        because there you usually only want the actual source tokenized.
        """
        return reduce(
            lambda s, e: e.preprocess(s, name, filename),
            self.iter_extensions(),
            str(source),
        )

    def _tokenize(
        self,
        source: str,
        name: t.Optional[str],
        filename: t.Optional[str] = None,
        state: t.Optional[str] = None,
    ) -> TokenStream:
        """Called by the parser to do the preprocessing and filtering
        for all the extensions.  Returns a :class:`~jinja2.lexer.TokenStream`.
        """
        source = self.preprocess(source, name, filename)
        stream = self.lexer.tokenize(source, name, filename, state)

        for ext in self.iter_extensions():
            stream = ext.filter_stream(stream)  # type: ignore

            if not isinstance(stream, TokenStream):
                stream = TokenStream(stream, name, filename)  # type: ignore

        return stream

    def _generate(
        self,
        source: nodes.Template,
        name: t.Optional[str],
        filename: t.Optional[str],
        defer_init: bool = False,
    ) -> str:
        """Internal hook that can be overridden to hook a different generate
        method in.

        .. versionadded:: 2.5
        """
        return generate(  # type: ignore
            source,
            self,
            name,
            filename,
            defer_init=defer_init,
            optimized=self.optimized,
        )

    def _compile(self, source: str, filename: str) -> CodeType:
        """Internal hook that can be overridden to hook a different compile
        method in.

        .. versionadded:: 2.5
        """
        return compile(source, filename, "exec")  # type: ignore

    @typing.overload
    def compile(  # type: ignore
        self,
        source: t.Union[str, nodes.Template],
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
        raw: "te.Literal[False]" = False,
        defer_init: bool = False,
    ) -> CodeType:
        ...

    @typing.overload
    def compile(
        self,
        source: t.Union[str, nodes.Template],
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
        raw: "te.Literal[True]" = ...,
        defer_init: bool = False,
    ) -> str:
        ...

    @internalcode
    def compile(
        self,
        source: t.Union[str, nodes.Template],
        name: t.Optional[str] = None,
        filename: t.Optional[str] = None,
        raw: bool = False,
        defer_init: bool = False,
    ) -> t.Union[str, CodeType]:
        """Compile a node or template source code.  The `name` parameter is
        the load name of the template after it was joined using
        :meth:`join_path` if necessary, not the filename on the file system.
        the `filename` parameter is the estimated filename of the template on
        the file system.  If the template came from a database or memory this
        can be omitted.

        The return value of this method is a python code object.  If the `raw`
        parameter is `True` the return value will be a string with python
        code equivalent to the bytecode returned otherwise.  This method is
        mainly used internally.

        `defer_init` is use internally to aid the module code generator.  This
        causes the generated code to be able to import without the global
        environment variable to be set.

        .. versionadded:: 2.4
           `defer_init` parameter added.
        """
        source_hint = None
        try:
            if isinstance(source, str):
                source_hint = source
                source = self._parse(source, name, filename)
            source = self._generate(source, name, filename, defer_init=defer_init)
            if raw:
                return source
            if filename is None:
                filename = "<template>"
            return self._compile(source, filename)
        except TemplateSyntaxError:
            self.handle_exception(source=source_hint)

    def compile_expression(
        self, source: str, undefined_to_none: bool = True
    ) -> "TemplateExpression":
        """A handy helper method that returns a callable that accepts keyword
        arguments that appear as variables in the expression.  If called it
        returns the result of the expression.

        This is useful if applications want to use the same rules as Jinja
        in template "configuration files" or similar situations.

        Example usage:

        >>> env = Environment()
        >>> expr = env.compile_expression('foo == 42')
        >>> expr(foo=23)
        False
        >>> expr(foo=42)
        True

        Per default the return value is converted to `None` if the
        expression returns an undefined value.  This can be changed
        by setting `undefined_to_none` to `False`.

        >>> env.compile_expression('var')() is None
        True
        >>> env.compile_expression('var', undefined_to_none=False)()
        Undefined

        .. versionadded:: 2.1
        """
        parser = Parser(self, source, state="variable")
        try:
            expr = parser.parse_expression()
            if not parser.stream.eos:
                raise TemplateSyntaxError(
                    "chunk after expression", parser.stream.current.lineno, None, None
                )
            expr.set_environment(self)
        except TemplateSyntaxError:
            self.handle_exception(source=source)

        body = [nodes.Assign(nodes.Name("result", "store"), expr, lineno=1)]
        template = self.from_string(nodes.Template(body, lineno=1))
        return TemplateExpression(template, undefined_to_none)

    def compile_templates(
        self,
        target: t.Union[str, os.PathLike],
        extensions: t.Optional[t.Collection[str]] = None,
        filter_func: t.Optional[t.Callable[[str], bool]] = None,
        zip: t.Optional[str] = "deflated",
        log_function: t.Optional[t.Callable[[str], None]] = None,
        ignore_errors: bool = True,
    ) -> None:
        """Finds all the templates the loader can find, compiles them
        and stores them in `target`.  If `zip` is `None`, instead of in a
        zipfile, the templates will be stored in a directory.
        By default a deflate zip algorithm is used. To switch to
        the stored algorithm, `zip` can be set to ``'stored'``.

        `extensions` and `filter_func` are passed to :meth:`list_templates`.
        Each template returned will be compiled to the target folder or
        zipfile.

        By default template compilation errors are ignored.  In case a
        log function is provided, errors are logged.  If you want template
        syntax errors to abort the compilation you can set `ignore_errors`
        to `False` and you will get an exception on syntax errors.

        .. versionadded:: 2.4
        """
        from .loaders import ModuleLoader

        if log_function is None:

            def log_function(x: str) -> None:
                pass

        assert log_function is not None
        assert self.loader is not None, "No loader configured."

        def write_file(filename: str, data: str) -> None:
            if zip:
                info = ZipInfo(filename)
                info.external_attr = 0o755 << 16
                zip_file.writestr(info, data)
            else:
                with open(os.path.join(target, filename), "wb") as f:
                    f.write(data.encode("utf8"))

        if zip is not None:
            from zipfile import ZipFile, ZipInfo, ZIP_DEFLATED, ZIP_STORED

            zip_file = ZipFile(
                target, "w", dict(deflated=ZIP_DEFLATED, stored=ZIP_STORED)[zip]
            )
            log_function(f"Compiling into Zip archive {target!r}")
        else:
            if not os.path.isdir(target):
                os.makedirs(target)
            log_function(f"Compiling into folder {target!r}")

        try:
            for name in self.list_templates(extensions, filter_func):
                source, filename, _ = self.loader.get_source(self, name)
                try:
                    code = self.compile(source, name, filename, True, True)
                except TemplateSyntaxError as e:
                    if not ignore_errors:
                        raise
                    log_function(f'Could not compile "{name}": {e}')
                    continue

                filename = ModuleLoader.get_module_filename(name)

                write_file(filename, code)
                log_function(f'Compiled "{name}" as {filename}')
        finally:
            if zip:
                zip_file.close()

        log_function("Finished compiling templates")

    def list_templates(
        self,
        extensions: t.Optional[t.Collection[str]] = None,
        filter_func: t.Optional[t.Callable[[str], bool]] = None,
    ) -> t.List[str]:
        """Returns a list of templates for this environment.  This requires
        that the loader supports the loader's
        :meth:`~BaseLoader.list_templates` method.

        If there are other files in the template folder besides the
        actual templates, the returned list can be filtered.  There are two
        ways: either `extensions` is set to a list of file extensions for
        templates, or a `filter_func` can be provided which is a callable that
        is passed a template name and should return `True` if it should end up
        in the result list.

        If the loader does not support that, a :exc:`TypeError` is raised.

        .. versionadded:: 2.4
        """
        assert self.loader is not None, "No loader configured."
        names = self.loader.list_templates()

        if extensions is not None:
            if filter_func is not None:
                raise TypeError(
                    "either extensions or filter_func can be passed, but not both"
                )

            def filter_func(x: str) -> bool:
                return "." in x and x.rsplit(".", 1)[1] in extensions  # type: ignore

        if filter_func is not None:
            names = [name for name in names if filter_func(name)]

        return names

    def handle_exception(self, source: t.Optional[str] = None) -> "te.NoReturn":
        """Exception handling helper.  This is used internally to either raise
        rewritten exceptions or return a rendered traceback for the template.
        """
        from .debug import rewrite_traceback_stack

        raise rewrite_traceback_stack(source=source)

    def join_path(self, template: str, parent: str) -> str:
        """Join a template with the parent.  By default all the lookups are
        relative to the loader root so this method returns the `template`
        parameter unchanged, but if the paths should be relative to the
        parent template, this function can be used to calculate the real
        template name.

        Subclasses may override this method and implement template path
        joining here.
        """
        return template

    @internalcode
    def _load_template(
        self, name: str, globals: t.Optional[t.Mapping[str, t.Any]]
    ) -> "Template":
        if self.loader is None:
            raise TypeError("no loader for this environment specified")
        cache_key = (weakref.ref(self.loader), name)
        if self.cache is not None:
            template = self.cache.get(cache_key)
            if template is not None and (
                not self.auto_reload or template.is_up_to_date
            ):
                # template.globals is a ChainMap, modifying it will only
                # affect the template, not the environment globals.
                if globals:
                    template.globals.update(globals)

                return template

        template = self.loader.load(self, name, self.make_globals(globals))

        if self.cache is not None:
            self.cache[cache_key] = template
        return template

    @internalcode
    def get_template(
        self,
        name: t.Union[str, "Template"],
        parent: t.Optional[str] = None,
        globals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> "Template":
        """Load a template by name with :attr:`loader` and return a
        :class:`Template`. If the template does not exist a
        :exc:`TemplateNotFound` exception is raised.

        :param name: Name of the template to load.
        :param parent: The name of the parent template importing this
            template. :meth:`join_path` can be used to implement name
            transformations with this.
        :param globals: Extend the environment :attr:`globals` with
            these extra variables available for all renders of this
            template. If the template has already been loaded and
            cached, its globals are updated with any new items.

        .. versionchanged:: 3.0
            If a template is loaded from cache, ``globals`` will update
            the template's globals instead of ignoring the new values.

        .. versionchanged:: 2.4
            If ``name`` is a :class:`Template` object it is returned
            unchanged.
        """
        if isinstance(name, Template):
            return name
        if parent is not None:
            name = self.join_path(name, parent)

        return self._load_template(name, globals)

    @internalcode
    def select_template(
        self,
        names: t.Iterable[t.Union[str, "Template"]],
        parent: t.Optional[str] = None,
        globals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> "Template":
        """Like :meth:`get_template`, but tries loading multiple names.
        If none of the names can be loaded a :exc:`TemplatesNotFound`
        exception is raised.

        :param names: List of template names to try loading in order.
        :param parent: The name of the parent template importing this
            template. :meth:`join_path` can be used to implement name
            transformations with this.
        :param globals: Extend the environment :attr:`globals` with
            these extra variables available for all renders of this
            template. If the template has already been loaded and
            cached, its globals are updated with any new items.

        .. versionchanged:: 3.0
            If a template is loaded from cache, ``globals`` will update
            the template's globals instead of ignoring the new values.

        .. versionchanged:: 2.11
            If ``names`` is :class:`Undefined`, an :exc:`UndefinedError`
            is raised instead. If no templates were found and ``names``
            contains :class:`Undefined`, the message is more helpful.

        .. versionchanged:: 2.4
            If ``names`` contains a :class:`Template` object it is
            returned unchanged.

        .. versionadded:: 2.3
        """
        if isinstance(names, Undefined):
            names._fail_with_undefined_error()

        if not names:
            raise TemplatesNotFound(
                message="Tried to select from an empty list of templates."
            )

        for name in names:
            if isinstance(name, Template):
                return name
            if parent is not None:
                name = self.join_path(name, parent)
            try:
                return self._load_template(name, globals)
            except (TemplateNotFound, UndefinedError):
                pass
        raise TemplatesNotFound(names)  # type: ignore

    @internalcode
    def get_or_select_template(
        self,
        template_name_or_list: t.Union[
            str, "Template", t.List[t.Union[str, "Template"]]
        ],
        parent: t.Optional[str] = None,
        globals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> "Template":
        """Use :meth:`select_template` if an iterable of template names
        is given, or :meth:`get_template` if one name is given.

        .. versionadded:: 2.3
        """
        if isinstance(template_name_or_list, (str, Undefined)):
            return self.get_template(template_name_or_list, parent, globals)
        elif isinstance(template_name_or_list, Template):
            return template_name_or_list
        return self.select_template(template_name_or_list, parent, globals)

    def from_string(
        self,
        source: t.Union[str, nodes.Template],
        globals: t.Optional[t.Mapping[str, t.Any]] = None,
        template_class: t.Optional[t.Type["Template"]] = None,
    ) -> "Template":
        """Load a template from a source string without using
        :attr:`loader`.

        :param source: Jinja source to compile into a template.
        :param globals: Extend the environment :attr:`globals` with
            these extra variables available for all renders of this
            template. If the template has already been loaded and
            cached, its globals are updated with any new items.
        :param template_class: Return an instance of this
            :class:`Template` class.
        """
        gs = self.make_globals(globals)
        cls = template_class or self.template_class
        return cls.from_code(self, self.compile(source), gs, None)

    def make_globals(
        self, d: t.Optional[t.Mapping[str, t.Any]]
    ) -> t.MutableMapping[str, t.Any]:
        """Make the globals map for a template. Any given template
        globals overlay the environment :attr:`globals`.

        Returns a :class:`collections.ChainMap`. This allows any changes
        to a template's globals to only affect that template, while
        changes to the environment's globals are still reflected.
        However, avoid modifying any globals after a template is loaded.

        :param d: Dict of template-specific globals.

        .. versionchanged:: 3.0
            Use :class:`collections.ChainMap` to always prevent mutating
            environment globals.
        """
        if d is None:
            d = {}

        return ChainMap(d, self.globals)


class Template:
    """The central template object.  This class represents a compiled template
    and is used to evaluate it.

    Normally the template object is generated from an :class:`Environment` but
    it also has a constructor that makes it possible to create a template
    instance directly using the constructor.  It takes the same arguments as
    the environment constructor but it's not possible to specify a loader.

    Every template object has a few methods and members that are guaranteed
    to exist.  However it's important that a template object should be
    considered immutable.  Modifications on the object are not supported.

    Template objects created from the constructor rather than an environment
    do have an `environment` attribute that points to a temporary environment
    that is probably shared with other templates created with the constructor
    and compatible settings.

    >>> template = Template('Hello {{ name }}!')
    >>> template.render(name='John Doe') == u'Hello John Doe!'
    True
    >>> stream = template.stream(name='John Doe')
    >>> next(stream) == u'Hello John Doe!'
    True
    >>> next(stream)
    Traceback (most recent call last):
        ...
    StopIteration
    """

    #: Type of environment to create when creating a template directly
    #: rather than through an existing environment.
    environment_class: t.Type[Environment] = Environment

    environment: Environment
    globals: t.MutableMapping[str, t.Any]
    name: t.Optional[str]
    filename: t.Optional[str]
    blocks: t.Dict[str, t.Callable[[Context], t.Iterator[str]]]
    root_render_func: t.Callable[[Context], t.Iterator[str]]
    _module: t.Optional["TemplateModule"]
    _debug_info: str
    _uptodate: t.Optional[t.Callable[[], bool]]

    def __new__(
        cls,
        source: t.Union[str, nodes.Template],
        block_start_string: str = BLOCK_START_STRING,
        block_end_string: str = BLOCK_END_STRING,
        variable_start_string: str = VARIABLE_START_STRING,
        variable_end_string: str = VARIABLE_END_STRING,
        comment_start_string: str = COMMENT_START_STRING,
        comment_end_string: str = COMMENT_END_STRING,
        line_statement_prefix: t.Optional[str] = LINE_STATEMENT_PREFIX,
        line_comment_prefix: t.Optional[str] = LINE_COMMENT_PREFIX,
        trim_blocks: bool = TRIM_BLOCKS,
        lstrip_blocks: bool = LSTRIP_BLOCKS,
        newline_sequence: "te.Literal['\\n', '\\r\\n', '\\r']" = NEWLINE_SEQUENCE,
        keep_trailing_newline: bool = KEEP_TRAILING_NEWLINE,
        extensions: t.Sequence[t.Union[str, t.Type["Extension"]]] = (),
        optimized: bool = True,
        undefined: t.Type[Undefined] = Undefined,
        finalize: t.Optional[t.Callable[..., t.Any]] = None,
        autoescape: t.Union[bool, t.Callable[[t.Optional[str]], bool]] = False,
        enable_async: bool = False,
    ) -> t.Any:  # it returns a `Template`, but this breaks the sphinx build...
        env = get_spontaneous_environment(
            cls.environment_class,  # type: ignore
            block_start_string,
            block_end_string,
            variable_start_string,
            variable_end_string,
            comment_start_string,
            comment_end_string,
            line_statement_prefix,
            line_comment_prefix,
            trim_blocks,
            lstrip_blocks,
            newline_sequence,
            keep_trailing_newline,
            frozenset(extensions),
            optimized,
            undefined,  # type: ignore
            finalize,
            autoescape,
            None,
            0,
            False,
            None,
            enable_async,
        )
        return env.from_string(source, template_class=cls)

    @classmethod
    def from_code(
        cls,
        environment: Environment,
        code: CodeType,
        globals: t.MutableMapping[str, t.Any],
        uptodate: t.Optional[t.Callable[[], bool]] = None,
    ) -> "Template":
        """Creates a template object from compiled code and the globals.  This
        is used by the loaders and environment to create a template object.
        """
        namespace = {"environment": environment, "__file__": code.co_filename}
        exec(code, namespace)
        rv = cls._from_namespace(environment, namespace, globals)
        rv._uptodate = uptodate
        return rv

    @classmethod
    def from_module_dict(
        cls,
        environment: Environment,
        module_dict: t.MutableMapping[str, t.Any],
        globals: t.MutableMapping[str, t.Any],
    ) -> "Template":
        """Creates a template object from a module.  This is used by the
        module loader to create a template object.

        .. versionadded:: 2.4
        """
        return cls._from_namespace(environment, module_dict, globals)

    @classmethod
    def _from_namespace(
        cls,
        environment: Environment,
        namespace: t.MutableMapping[str, t.Any],
        globals: t.MutableMapping[str, t.Any],
    ) -> "Template":
        t: "Template" = object.__new__(cls)
        t.environment = environment
        t.globals = globals
        t.name = namespace["name"]
        t.filename = namespace["__file__"]
        t.blocks = namespace["blocks"]

        # render function and module
        t.root_render_func = namespace["root"]  # type: ignore
        t._module = None

        # debug and loader helpers
        t._debug_info = namespace["debug_info"]
        t._uptodate = None

        # store the reference
        namespace["environment"] = environment
        namespace["__jinja_template__"] = t

        return t

    def render(self, *args: t.Any, **kwargs: t.Any) -> str:
        """This method accepts the same arguments as the `dict` constructor:
        A dict, a dict subclass or some keyword arguments.  If no arguments
        are given the context will be empty.  These two calls do the same::

            template.render(knights='that say nih')
            template.render({'knights': 'that say nih'})

        This will return the rendered template as a string.
        """
        if self.environment.is_async:
            import asyncio

            close = False

            if sys.version_info < (3, 7):
                loop = asyncio.get_event_loop()
            else:
                try:
                    loop = asyncio.get_running_loop()
                except RuntimeError:
                    loop = asyncio.new_event_loop()
                    close = True

            try:
                return loop.run_until_complete(self.render_async(*args, **kwargs))
            finally:
                if close:
                    loop.close()

        ctx = self.new_context(dict(*args, **kwargs))

        try:
            return concat(self.root_render_func(ctx))  # type: ignore
        except Exception:
            self.environment.handle_exception()

    async def render_async(self, *args: t.Any, **kwargs: t.Any) -> str:
        """This works similar to :meth:`render` but returns a coroutine
        that when awaited returns the entire rendered template string.  This
        requires the async feature to be enabled.

        Example usage::

            await template.render_async(knights='that say nih; asynchronously')
        """
        if not self.environment.is_async:
            raise RuntimeError(
                "The environment was not created with async mode enabled."
            )

        ctx = self.new_context(dict(*args, **kwargs))

        try:
            return concat([n async for n in self.root_render_func(ctx)])  # type: ignore
        except Exception:
            return self.environment.handle_exception()

    def stream(self, *args: t.Any, **kwargs: t.Any) -> "TemplateStream":
        """Works exactly like :meth:`generate` but returns a
        :class:`TemplateStream`.
        """
        return TemplateStream(self.generate(*args, **kwargs))

    def generate(self, *args: t.Any, **kwargs: t.Any) -> t.Iterator[str]:
        """For very large templates it can be useful to not render the whole
        template at once but evaluate each statement after another and yield
        piece for piece.  This method basically does exactly that and returns
        a generator that yields one item after another as strings.

        It accepts the same arguments as :meth:`render`.
        """
        if self.environment.is_async:
            import asyncio

            async def to_list() -> t.List[str]:
                return [x async for x in self.generate_async(*args, **kwargs)]

            if sys.version_info < (3, 7):
                loop = asyncio.get_event_loop()
                out = loop.run_until_complete(to_list())
            else:
                out = asyncio.run(to_list())

            yield from out
            return

        ctx = self.new_context(dict(*args, **kwargs))

        try:
            yield from self.root_render_func(ctx)  # type: ignore
        except Exception:
            yield self.environment.handle_exception()

    async def generate_async(
        self, *args: t.Any, **kwargs: t.Any
    ) -> t.AsyncIterator[str]:
        """An async version of :meth:`generate`.  Works very similarly but
        returns an async iterator instead.
        """
        if not self.environment.is_async:
            raise RuntimeError(
                "The environment was not created with async mode enabled."
            )

        ctx = self.new_context(dict(*args, **kwargs))

        try:
            async for event in self.root_render_func(ctx):  # type: ignore
                yield event
        except Exception:
            yield self.environment.handle_exception()

    def new_context(
        self,
        vars: t.Optional[t.Dict[str, t.Any]] = None,
        shared: bool = False,
        locals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> Context:
        """Create a new :class:`Context` for this template.  The vars
        provided will be passed to the template.  Per default the globals
        are added to the context.  If shared is set to `True` the data
        is passed as is to the context without adding the globals.

        `locals` can be a dict of local variables for internal usage.
        """
        return new_context(
            self.environment, self.name, self.blocks, vars, shared, self.globals, locals
        )

    def make_module(
        self,
        vars: t.Optional[t.Dict[str, t.Any]] = None,
        shared: bool = False,
        locals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> "TemplateModule":
        """This method works like the :attr:`module` attribute when called
        without arguments but it will evaluate the template on every call
        rather than caching it.  It's also possible to provide
        a dict which is then used as context.  The arguments are the same
        as for the :meth:`new_context` method.
        """
        ctx = self.new_context(vars, shared, locals)
        return TemplateModule(self, ctx)

    async def make_module_async(
        self,
        vars: t.Optional[t.Dict[str, t.Any]] = None,
        shared: bool = False,
        locals: t.Optional[t.Mapping[str, t.Any]] = None,
    ) -> "TemplateModule":
        """As template module creation can invoke template code for
        asynchronous executions this method must be used instead of the
        normal :meth:`make_module` one.  Likewise the module attribute
        becomes unavailable in async mode.
        """
        ctx = self.new_context(vars, shared, locals)
        return TemplateModule(
            self, ctx, [x async for x in self.root_render_func(ctx)]  # type: ignore
        )

    @internalcode
    def _get_default_module(self, ctx: t.Optional[Context] = None) -> "TemplateModule":
        """If a context is passed in, this means that the template was
        imported. Imported templates have access to the current
        template's globals by default, but they can only be accessed via
        the context during runtime.

        If there are new globals, we need to create a new module because
        the cached module is already rendered and will not have access
        to globals from the current context. This new module is not
        cached because the template can be imported elsewhere, and it
        should have access to only the current template's globals.
        """
        if self.environment.is_async:
            raise RuntimeError("Module is not available in async mode.")

        if ctx is not None:
            keys = ctx.globals_keys - self.globals.keys()

            if keys:
                return self.make_module({k: ctx.parent[k] for k in keys})

        if self._module is None:
            self._module = self.make_module()

        return self._module

    async def _get_default_module_async(
        self, ctx: t.Optional[Context] = None
    ) -> "TemplateModule":
        if ctx is not None:
            keys = ctx.globals_keys - self.globals.keys()

            if keys:
                return await self.make_module_async({k: ctx.parent[k] for k in keys})

        if self._module is None:
            self._module = await self.make_module_async()

        return self._module

    @property
    def module(self) -> "TemplateModule":
        """The template as module.  This is used for imports in the
        template runtime but is also useful if one wants to access
        exported template variables from the Python layer:

        >>> t = Template('{% macro foo() %}42{% endmacro %}23')
        >>> str(t.module)
        '23'
        >>> t.module.foo() == u'42'
        True

        This attribute is not available if async mode is enabled.
        """
        return self._get_default_module()

    def get_corresponding_lineno(self, lineno: int) -> int:
        """Return the source line number of a line number in the
        generated bytecode as they are not in sync.
        """
        for template_line, code_line in reversed(self.debug_info):
            if code_line <= lineno:
                return template_line
        return 1

    @property
    def is_up_to_date(self) -> bool:
        """If this variable is `False` there is a newer version available."""
        if self._uptodate is None:
            return True
        return self._uptodate()

    @property
    def debug_info(self) -> t.List[t.Tuple[int, int]]:
        """The debug info mapping."""
        if self._debug_info:
            return [
                tuple(map(int, x.split("=")))  # type: ignore
                for x in self._debug_info.split("&")
            ]

        return []

    def __repr__(self) -> str:
        if self.name is None:
            name = f"memory:{id(self):x}"
        else:
            name = repr(self.name)
        return f"<{type(self).__name__} {name}>"


class TemplateModule:
    """Represents an imported template.  All the exported names of the
    template are available as attributes on this object.  Additionally
    converting it into a string renders the contents.
    """

    def __init__(
        self,
        template: Template,
        context: Context,
        body_stream: t.Optional[t.Iterable[str]] = None,
    ) -> None:
        if body_stream is None:
            if context.environment.is_async:
                raise RuntimeError(
                    "Async mode requires a body stream to be passed to"
                    " a template module. Use the async methods of the"
                    " API you are using."
                )

            body_stream = list(template.root_render_func(context))  # type: ignore

        self._body_stream = body_stream
        self.__dict__.update(context.get_exported())
        self.__name__ = template.name

    def __html__(self) -> Markup:
        return Markup(concat(self._body_stream))

    def __str__(self) -> str:
        return concat(self._body_stream)

    def __repr__(self) -> str:
        if self.__name__ is None:
            name = f"memory:{id(self):x}"
        else:
            name = repr(self.__name__)
        return f"<{type(self).__name__} {name}>"


class TemplateExpression:
    """The :meth:`jinja2.Environment.compile_expression` method returns an
    instance of this object.  It encapsulates the expression-like access
    to the template with an expression it wraps.
    """

    def __init__(self, template: Template, undefined_to_none: bool) -> None:
        self._template = template
        self._undefined_to_none = undefined_to_none

    def __call__(self, *args: t.Any, **kwargs: t.Any) -> t.Optional[t.Any]:
        context = self._template.new_context(dict(*args, **kwargs))
        consume(self._template.root_render_func(context))  # type: ignore
        rv = context.vars["result"]
        if self._undefined_to_none and isinstance(rv, Undefined):
            rv = None
        return rv


class TemplateStream:
    """A template stream works pretty much like an ordinary python generator
    but it can buffer multiple items to reduce the number of total iterations.
    Per default the output is unbuffered which means that for every unbuffered
    instruction in the template one string is yielded.

    If buffering is enabled with a buffer size of 5, five items are combined
    into a new string.  This is mainly useful if you are streaming
    big templates to a client via WSGI which flushes after each iteration.
    """

    def __init__(self, gen: t.Iterator[str]) -> None:
        self._gen = gen
        self.disable_buffering()

    def dump(
        self,
        fp: t.Union[str, t.IO],
        encoding: t.Optional[str] = None,
        errors: t.Optional[str] = "strict",
    ) -> None:
        """Dump the complete stream into a file or file-like object.
        Per default strings are written, if you want to encode
        before writing specify an `encoding`.

        Example usage::

            Template('Hello {{ name }}!').stream(name='foo').dump('hello.html')
        """
        close = False

        if isinstance(fp, str):
            if encoding is None:
                encoding = "utf-8"

            fp = open(fp, "wb")
            close = True
        try:
            if encoding is not None:
                iterable = (x.encode(encoding, errors) for x in self)  # type: ignore
            else:
                iterable = self  # type: ignore

            if hasattr(fp, "writelines"):
                fp.writelines(iterable)
            else:
                for item in iterable:
                    fp.write(item)
        finally:
            if close:
                fp.close()

    def disable_buffering(self) -> None:
        """Disable the output buffering."""
        self._next = partial(next, self._gen)
        self.buffered = False

    def _buffered_generator(self, size: int) -> t.Iterator[str]:
        buf: t.List[str] = []
        c_size = 0
        push = buf.append

        while True:
            try:
                while c_size < size:
                    c = next(self._gen)
                    push(c)
                    if c:
                        c_size += 1
            except StopIteration:
                if not c_size:
                    return
            yield concat(buf)
            del buf[:]
            c_size = 0

    def enable_buffering(self, size: int = 5) -> None:
        """Enable buffering.  Buffer `size` items before yielding them."""
        if size <= 1:
            raise ValueError("buffer size too small")

        self.buffered = True
        self._next = partial(next, self._buffered_generator(size))

    def __iter__(self) -> "TemplateStream":
        return self

    def __next__(self) -> str:
        return self._next()  # type: ignore


# hook in default template class.  if anyone reads this comment: ignore that
# it's possible to use custom templates ;-)
Environment.template_class = Template