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Parameters

click

Click supports two types of parameters for scripts: options and arguments. There is generally some confusion among authors of command line scripts of when to use which, so here is a quick overview of the differences. As its name indicates, an option is optional. While arguments can be optional within reason, they are much more restricted in how optional they can be.

To help you decide between options and arguments, the recommendation is to use arguments exclusively for things like going to subcommands or input filenames / URLs, and have everything else be an option instead.

Differences

Arguments can do less than options. The following features are only available for options:

  • automatic prompting for missing input
  • act as flags (boolean or otherwise)
  • option values can be pulled from environment variables, arguments can not
  • options are fully documented in the help page, arguments are not (this is intentional <documenting-arguments> as arguments might be too specific to be automatically documented)

On the other hand arguments, unlike options, can accept an arbitrary number of arguments. Options can strictly ever only accept a fixed number of arguments (defaults to 1), or they may be specified multiple times using multiple-options.

Parameter Types

Parameters can be of different types. Types can be implemented with different behavior and some are supported out of the box:

str / click.STRING:

The default parameter type which indicates unicode strings.

int / click.INT:

A parameter that only accepts integers.

float / click.FLOAT:

A parameter that only accepts floating point values.

bool / click.BOOL:

A parameter that accepts boolean values. This is automatically used for boolean flags. The string values "1", "true", "t", "yes", "y", and "on" convert to True. "0", "false", "f", "no", "n", and "off" convert to False.

click.UUID:

A parameter that accepts UUID values. This is not automatically guessed but represented as uuid.UUID.

File

Path

Choice

IntRange

FloatRange

DateTime

Custom parameter types can be implemented by subclassing click.ParamType. For simple cases, passing a Python function that fails with a ValueError is also supported, though discouraged.

Parameter Names

Parameters (both options and arguments) have a name that will be used as the Python argument name when calling the decorated function with values.

Arguments take only one positional name. To provide a different name for use in help text, see doc-meta-variables.

Options can have many names that may be prefixed with one or two dashes. Names with one dash are parsed as short options, names with two are parsed as long options. If a name is not prefixed, it is used as the Python argument name and not parsed as an option name. Otherwise, the first name with a two dash prefix is used, or the first with a one dash prefix if there are none with two. The prefix is removed and dashes are converted to underscores to get the Python argument name.

Implementing Custom Types

To implement a custom type, you need to subclass the ParamType class. Override the ~ParamType.convert method to convert the value from a string to the correct type.

The following code implements an integer type that accepts hex and octal numbers in addition to normal integers, and converts them into regular integers.

import click

class BasedIntParamType(click.ParamType):
    name = "integer"

    def convert(self, value, param, ctx):
        if isinstance(value, int):
            return value

        try:
            if value[:2].lower() == "0x":
                return int(value[2:], 16)
            elif value[:1] == "0":
                return int(value, 8)
            return int(value, 10)
        except ValueError:
            self.fail(f"{value!r} is not a valid integer", param, ctx)

BASED_INT = BasedIntParamType()

The ~ParamType.name attribute is optional and is used for documentation. Call ~ParamType.fail if conversion fails. The param and ctx arguments may be None in some cases such as prompts.

Values from user input or the command line will be strings, but default values and Python arguments may already be the correct type. The custom type should check at the top if the value is already valid and pass it through to support those cases.