diff --git a/docs/conf.py b/docs/conf.py
index b38f474..d87b1b6 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -16,6 +16,7 @@
 
 import os
 import sys
+
 sys.path.insert(0, os.path.abspath('..'))
 
 # -- Project information -----------------------------------------------------
diff --git a/pyproject.toml b/pyproject.toml
index 5788a21..306024f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -39,6 +39,16 @@ pytest = "^6"
 [tool.poetry.extras]
 docs = ["m2r", "sphinx"]
 
+[tool.isort]
+profile = "google"
+# Upstream version within Google actually uses contextlib2 and mock for Python 2
+# compatibility, and this is transformed away by copybara.
+# Adding contextlib2 to the stdlib makes the sorting correct in the canonical
+# github version even when run on the upstream version pre-copybara. Adding
+# mock... doesn't help as much (sorts as "mock", not "unittest").
+# These can both go away starting in 2021.
+extra_standard_library = ["contextlib2", "mock"]
+
 # https://tox.readthedocs.io/
 [tool.tox]
 legacy_tox_ini = """
diff --git a/refex/cli.py b/refex/cli.py
index abe8c7c..1f3ff3d 100755
--- a/refex/cli.py
+++ b/refex/cli.py
@@ -36,22 +36,23 @@
 import io
 import json
 import os
-import pkg_resources
 import re
 import sys
 import tempfile
 import textwrap
 import traceback
-from typing import Dict, List, Iterable, Optional, Text, Tuple, Union
+from typing import Dict, Iterable, List, Optional, Text, Tuple, Union
 
 from absl import app
 import attr
 import colorama
+import pkg_resources
+import six
+
 from refex import formatting
 from refex import search
 from refex.fix import find_fixer
 from refex.python import syntactic_template
-import six
 
 _IGNORABLE_ERRNO = frozenset([
     errno.ENOENT,  # file was removed after we went looking
diff --git a/refex/fix/fixer.py b/refex/fix/fixer.py
index 392dc0f..565e60a 100644
--- a/refex/fix/fixer.py
+++ b/refex/fix/fixer.py
@@ -20,16 +20,17 @@
 
 import abc
 import operator
+from typing import Callable, List, Mapping, Optional, Text, TypeVar
 
 import attr
+import six
+
 from refex import formatting
 from refex import future_string
 from refex import search
 from refex import substitution
 from refex.python import matcher
 from refex.python.matchers import syntax_matchers
-import six
-from typing import Callable, List, Mapping, Optional, Text, TypeVar
 
 
 class ParsedPythonFixer(
diff --git a/refex/fix/fixers/correctness_fixers.py b/refex/fix/fixers/correctness_fixers.py
index 15d44fd..a1c405d 100644
--- a/refex/fix/fixers/correctness_fixers.py
+++ b/refex/fix/fixers/correctness_fixers.py
@@ -25,14 +25,14 @@
 from __future__ import print_function
 from __future__ import unicode_literals  # for convenience
 
+import six
+
 from refex import formatting
 from refex.fix import fixer
 from refex.python import syntactic_template
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
 from refex.python.matchers import syntax_matchers
-import six
-
 
 # Python 2 compatibility hack to be able to get b'...' and '...'.
 if six.PY2:
diff --git a/refex/fix/fixers/idiom_fixers.py b/refex/fix/fixers/idiom_fixers.py
index 781341a..d36f4a1 100644
--- a/refex/fix/fixers/idiom_fixers.py
+++ b/refex/fix/fixers/idiom_fixers.py
@@ -22,6 +22,7 @@
 from __future__ import unicode_literals  # for convenience
 
 import textwrap
+
 from refex import formatting
 from refex import future_string
 from refex.fix import fixer
diff --git a/refex/fix/fixers/modern_python_fixers.py b/refex/fix/fixers/modern_python_fixers.py
index 7bf3ddc..7de3b0c 100644
--- a/refex/fix/fixers/modern_python_fixers.py
+++ b/refex/fix/fixers/modern_python_fixers.py
@@ -17,14 +17,15 @@
 from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals  # for convenience
+
+import six
+
 from refex import formatting
 from refex.fix import fixer
 from refex.python import syntactic_template
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
 from refex.python.matchers import syntax_matchers
-import six
-
 
 SIMPLE_PYTHON_FIXERS = []  # Disabled except when running in Python 2.
 
diff --git a/refex/fix/fixers/test_correctness_fixers.py b/refex/fix/fixers/test_correctness_fixers.py
index aa0bfdf..3fbafcb 100644
--- a/refex/fix/fixers/test_correctness_fixers.py
+++ b/refex/fix/fixers/test_correctness_fixers.py
@@ -22,6 +22,7 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+
 from refex import search
 from refex.fix import fixer
 from refex.fix.fixers import correctness_fixers
diff --git a/refex/fix/fixers/test_idiom_fixers.py b/refex/fix/fixers/test_idiom_fixers.py
index a02c623..1abf822 100644
--- a/refex/fix/fixers/test_idiom_fixers.py
+++ b/refex/fix/fixers/test_idiom_fixers.py
@@ -23,10 +23,11 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+import six
+
 from refex import search
 from refex.fix import fixer
 from refex.fix.fixers import idiom_fixers
-import six
 
 
 def _rewrite(fixer_, code):
diff --git a/refex/fix/fixers/test_modern_python_fixers.py b/refex/fix/fixers/test_modern_python_fixers.py
index 7db146c..06c764c 100644
--- a/refex/fix/fixers/test_modern_python_fixers.py
+++ b/refex/fix/fixers/test_modern_python_fixers.py
@@ -19,6 +19,7 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+
 from refex import search
 from refex.fix.fixers import modern_python_fixers
 
diff --git a/refex/fix/fixers/unittest_fixers.py b/refex/fix/fixers/unittest_fixers.py
index b4307a0..e0f8aaa 100644
--- a/refex/fix/fixers/unittest_fixers.py
+++ b/refex/fix/fixers/unittest_fixers.py
@@ -24,6 +24,7 @@
 from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals  # for convenience
+
 from refex import future_string
 from refex.fix import fixer
 from refex.python import syntactic_template
diff --git a/refex/fix/test_fixer.py b/refex/fix/test_fixer.py
index 52df39c..c8dd636 100644
--- a/refex/fix/test_fixer.py
+++ b/refex/fix/test_fixer.py
@@ -18,23 +18,23 @@
 from __future__ import print_function
 from __future__ import unicode_literals  # for convenience.
 
-import re
 from unittest import mock
+import re
 
 from absl.testing import absltest
 from absl.testing import parameterized
 import attr
+
 from refex import formatting
 from refex import future_string
 from refex import search
 from refex import substitution
+from refex.fix import find_fixer
+from refex.fix import fixer
 from refex.python import syntactic_template
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import syntax_matchers
 
-from refex.fix import find_fixer
-from refex.fix import fixer
-
 
 def _search_replace_fixer(search_expr, replace, message=None, url='', **kwargs):
   return fixer.SimplePythonFixer(
diff --git a/refex/fix/test_generate_example.py b/refex/fix/test_generate_example.py
index df3e475..bff06ed 100644
--- a/refex/fix/test_generate_example.py
+++ b/refex/fix/test_generate_example.py
@@ -20,6 +20,7 @@
 import ast
 
 from absl.testing import absltest
+
 from refex.fix import generate_example
 
 
diff --git a/refex/formatting.py b/refex/formatting.py
index c45af83..aa58024 100644
--- a/refex/formatting.py
+++ b/refex/formatting.py
@@ -75,7 +75,6 @@
 import subprocess
 import sys
 import tempfile
-
 import typing
 from typing import Any, Iterable, Mapping, Optional, Text, Tuple
 
@@ -89,7 +88,6 @@
 from refex import parsed_file
 from refex import substitution
 
-
 _DEFAULT_STYLES = (
     colorama.Style.BRIGHT + colorama.Fore.YELLOW,
     colorama.Style.BRIGHT + colorama.Fore.BLUE,
diff --git a/refex/future_string.py b/refex/future_string.py
index 9456faf..920dd72 100644
--- a/refex/future_string.py
+++ b/refex/future_string.py
@@ -49,10 +49,9 @@
 
 import collections
 import string
+from typing import Any, Mapping, Text
 
 import six
-from typing import Any, Text, Mapping
-
 
 # For refex_doctest.py
 # The examples are specific to Python 2.
diff --git a/refex/match.py b/refex/match.py
index df1729a..5179349 100644
--- a/refex/match.py
+++ b/refex/match.py
@@ -59,10 +59,10 @@
 # from __future__ import google_type_annotations
 from __future__ import print_function
 
-import attr
-
 from typing import Any, Tuple
 
+import attr
+
 
 @attr.s(frozen=True)
 class Match(object):
diff --git a/refex/parsed_file.py b/refex/parsed_file.py
index 0c1f35e..f93777f 100644
--- a/refex/parsed_file.py
+++ b/refex/parsed_file.py
@@ -16,21 +16,20 @@
 ------------------------
 """
 
+# No portable raw unicode literal exists without unicode_literals.
+# see https://stackoverflow.com/questions/33027281
 from __future__ import absolute_import
 from __future__ import division
 # from __future__ import google_type_annotations
 from __future__ import print_function
-
-# No portable raw unicode literal exists without unicode_literals.
-# see https://stackoverflow.com/questions/33027281
 from __future__ import unicode_literals
 
 import re
+from typing import Iterable, Mapping, Optional, Text
 
 import asttokens
 import attr
 import cached_property
-from typing import Mapping, Optional, Iterable, Text
 
 
 @attr.s(frozen=True, eq=True, order=False)
diff --git a/refex/python/evaluate.py b/refex/python/evaluate.py
index 7fa8c55..e449101 100644
--- a/refex/python/evaluate.py
+++ b/refex/python/evaluate.py
@@ -30,19 +30,17 @@
 # from __future__ import google_type_annotations
 from __future__ import print_function
 
-
 import textwrap
 
 from refex.python import error_strings
 from refex.python import matcher
 from refex.python import matchers
 from refex.python import semiliteral_eval
-
 # Actually collect all the matchers into the matchers module, so they can be
 # enumerated.
+import refex.python.matchers.ast_matchers  # pylint: disable=unused-import
 import refex.python.matchers.base_matchers  # pylint: disable=unused-import
 import refex.python.matchers.lexical_matchers  # pylint: disable=unused-import
-import refex.python.matchers.ast_matchers  # pylint: disable=unused-import
 import refex.python.matchers.syntax_matchers  # pylint: disable=unused-import
 
 
diff --git a/refex/python/matcher.py b/refex/python/matcher.py
index 86bf6d7..b9626af 100644
--- a/refex/python/matcher.py
+++ b/refex/python/matcher.py
@@ -80,21 +80,22 @@
 import collections
 import contextlib
 import copy
+import enum
 import functools
 import sys
 import tokenize
+from typing import Any, Dict, Iterator, Optional, Text
 import weakref
 
 from absl import logging
 import asttokens
 import attr
 import cached_property
-import enum
-from refex import match
-from refex import parsed_file
 import six
 from six.moves import reprlib
-from typing import Any, Dict, Iterator, Text, Optional
+
+from refex import match
+from refex import parsed_file
 
 _match = match  # when `match` is shadowed, e.g. class attributes.
 
diff --git a/refex/python/matcher_test_util.py b/refex/python/matcher_test_util.py
index ff840bf..7a237a3 100644
--- a/refex/python/matcher_test_util.py
+++ b/refex/python/matcher_test_util.py
@@ -18,6 +18,7 @@
 from __future__ import print_function
 
 from absl.testing import absltest
+
 from refex.python import matcher
 
 
diff --git a/refex/python/matchers/base_matchers.py b/refex/python/matchers/base_matchers.py
index c288654..c6eb51c 100644
--- a/refex/python/matchers/base_matchers.py
+++ b/refex/python/matchers/base_matchers.py
@@ -70,13 +70,14 @@
 from __future__ import print_function
 
 import re
+from typing import Container, List
 import weakref
 
 import attr
 import cached_property
+
 from refex import match
 from refex.python import matcher
-from typing import Container, List
 
 
 @matcher.safe_to_eval
diff --git a/refex/python/matchers/syntax_matchers.py b/refex/python/matchers/syntax_matchers.py
index 901ebd2..4898f67 100644
--- a/refex/python/matchers/syntax_matchers.py
+++ b/refex/python/matchers/syntax_matchers.py
@@ -91,11 +91,12 @@
 
 import attr
 import cached_property
+import six
+
 from refex.python import matcher
 from refex.python import python_pattern
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
-import six
 
 
 def _remap_macro_variables(pattern):
diff --git a/refex/python/matchers/test_ast_matchers.py b/refex/python/matchers/test_ast_matchers.py
index 4947832..23b714c 100644
--- a/refex/python/matchers/test_ast_matchers.py
+++ b/refex/python/matchers/test_ast_matchers.py
@@ -21,11 +21,12 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+import six
+
 from refex import match
 from refex.python import matcher
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
-import six
 
 
 def expression(e):
diff --git a/refex/python/matchers/test_base_matchers.py b/refex/python/matchers/test_base_matchers.py
index 807b8c7..1fdc2f7 100644
--- a/refex/python/matchers/test_base_matchers.py
+++ b/refex/python/matchers/test_base_matchers.py
@@ -26,11 +26,11 @@
 from absl.testing import parameterized
 
 from refex import match
+from refex.python import evaluate
 from refex.python import matcher
 from refex.python import matcher_test_util
-from refex.python.matchers import base_matchers
 from refex.python.matchers import ast_matchers
-from refex.python import evaluate
+from refex.python.matchers import base_matchers
 
 _NOTHING = base_matchers.Unless(base_matchers.Anything())
 _FAKE_CONTEXT = matcher.MatchContext(matcher.parse_ast('', 'foo.py'))
diff --git a/refex/python/matchers/test_lexical_matchers.py b/refex/python/matchers/test_lexical_matchers.py
index ea2f709..c04aa28 100644
--- a/refex/python/matchers/test_lexical_matchers.py
+++ b/refex/python/matchers/test_lexical_matchers.py
@@ -18,6 +18,7 @@
 from __future__ import print_function
 
 from absl.testing import absltest
+
 from refex.python import matcher_test_util
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import lexical_matchers
diff --git a/refex/python/matchers/test_syntax_matchers.py b/refex/python/matchers/test_syntax_matchers.py
index 3c1d69b..859134f 100644
--- a/refex/python/matchers/test_syntax_matchers.py
+++ b/refex/python/matchers/test_syntax_matchers.py
@@ -19,9 +19,9 @@
 from __future__ import division
 from __future__ import print_function
 
+from unittest import mock
 import textwrap
 import unittest
-from unittest import mock
 
 from absl.testing import absltest
 from absl.testing import parameterized
@@ -29,11 +29,10 @@
 
 from refex.python import matcher
 from refex.python import matcher_test_util
-from refex.python.matchers import base_matchers
 from refex.python.matchers import ast_matchers
+from refex.python.matchers import base_matchers
 from refex.python.matchers import syntax_matchers
 
-
 _FAKE_CONTEXT = matcher.MatchContext(matcher.parse_ast('', 'foo.py'))
 
 
diff --git a/refex/python/syntactic_template.py b/refex/python/syntactic_template.py
index 78d93ce..4991486 100644
--- a/refex/python/syntactic_template.py
+++ b/refex/python/syntactic_template.py
@@ -41,18 +41,19 @@
 
 import ast
 import tokenize
+from typing import Text
 
 from absl import logging
 import attr
 import cached_property
+import six
+
 from refex import formatting
 from refex.python import matcher
 from refex.python import python_pattern
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
 from refex.python.matchers import syntax_matchers
-import six
-from typing import Text
 
 
 @attr.s(frozen=True)
diff --git a/refex/python/test_evaluate.py b/refex/python/test_evaluate.py
index 5687a64..3b60f18 100644
--- a/refex/python/test_evaluate.py
+++ b/refex/python/test_evaluate.py
@@ -18,6 +18,7 @@
 from __future__ import print_function
 
 from absl.testing import absltest
+
 from refex.python import evaluate
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
diff --git a/refex/python/test_syntactic_template.py b/refex/python/test_syntactic_template.py
index b76611a..e0376fb 100644
--- a/refex/python/test_syntactic_template.py
+++ b/refex/python/test_syntactic_template.py
@@ -20,12 +20,13 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+import six
+
 from refex import formatting
 from refex.python import matcher
 from refex.python import syntactic_template
 from refex.python.matchers import ast_matchers
 from refex.python.matchers import base_matchers
-import six
 
 
 class LexicalTemplateTest(parameterized.TestCase):
diff --git a/refex/refex_doctest.py b/refex/refex_doctest.py
index ae21a44..51f32f5 100644
--- a/refex/refex_doctest.py
+++ b/refex/refex_doctest.py
@@ -22,11 +22,14 @@
 import sys
 
 from absl.testing import absltest
+
 import refex.python.matcher_test_util  # so that it's found by _submodules: pylint: disable=unused-import
 import refex.search
 
+# isort: split
 # We put doctest after absltest so that it picks up the unittest monkeypatch.
-# Otherwise doctest tests aren't runnable at all.
+# Otherwise doctest tests aren't runnable at all with Bazel.
+
 import doctest
 
 
diff --git a/refex/search.py b/refex/search.py
index 662e7a5..e891163 100644
--- a/refex/search.py
+++ b/refex/search.py
@@ -96,10 +96,15 @@
 import itertools
 import re
 import sys
+from typing import (Dict, Iterable, Mapping, MutableMapping, MutableSequence,
+                    MutableSet, Optional, Pattern, Sequence, Text, Tuple,
+                    Union)
 
 from absl import logging
 import attr
 import cached_property
+import six
+
 from refex import formatting
 from refex import match
 from refex import parsed_file
@@ -108,8 +113,6 @@
 from refex.python import matcher
 from refex.python.matchers import base_matchers
 from refex.python.matchers import syntax_matchers
-import six
-from typing import Dict, Iterable, Mapping, MutableMapping, MutableSequence, MutableSet, Optional, Pattern, Sequence, Text, Tuple, Union
 
 Span = Tuple[int, int]
 # TODO(b/118783544): Only string keys.
diff --git a/refex/substitution.py b/refex/substitution.py
index 52613b0..c9725d1 100644
--- a/refex/substitution.py
+++ b/refex/substitution.py
@@ -51,13 +51,12 @@
 import collections
 import operator
 import re
+from typing import (FrozenSet, Iterable, List, Mapping, Optional, Text, Tuple,
+                    Union)
 
 import attr
 import six
 
-from typing import FrozenSet, Iterable, List, Mapping, Optional, Text, Tuple, Union
-
-
 # Only slightly structured category name: dot-separated, no empty intra-dot
 # sequences, no whitespace, doesn't begin with a -, and doesn't begin/end on a
 # dot.
diff --git a/refex/test_cli.py b/refex/test_cli.py
index 4f55749..0ed1351 100644
--- a/refex/test_cli.py
+++ b/refex/test_cli.py
@@ -20,6 +20,7 @@
 import argparse
 import contextlib
 import json
+from unittest import mock
 import os
 import re
 import sys
@@ -28,10 +29,10 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
-from unittest import mock
-from refex import cli
 import six
 
+from refex import cli
+
 
 class ParseArgsLeftoversTest(absltest.TestCase):
 
diff --git a/refex/test_example_binary.py b/refex/test_example_binary.py
index 72a8259..b46949a 100644
--- a/refex/test_example_binary.py
+++ b/refex/test_example_binary.py
@@ -20,7 +20,6 @@
 
 from absl.testing import absltest
 
-
 _EXECUTABLE = [sys.executable, 'examples/example_binary.py']
 
 
diff --git a/refex/test_formatting.py b/refex/test_formatting.py
index 334fb6c..e315aac 100644
--- a/refex/test_formatting.py
+++ b/refex/test_formatting.py
@@ -22,6 +22,7 @@
 from absl.testing import absltest
 from absl.testing import parameterized
 import colorama
+
 from refex import formatting
 from refex import match
 from refex import parsed_file
diff --git a/refex/test_future_string.py b/refex/test_future_string.py
index b5d72d4..b7c6034 100644
--- a/refex/test_future_string.py
+++ b/refex/test_future_string.py
@@ -24,9 +24,10 @@
 from absl.testing import absltest
 from absl.testing import parameterized
 import attr
-from refex import future_string
 import six
 
+from refex import future_string
+
 
 @attr.s
 class Stringifiable(object):
diff --git a/refex/test_parsed_file.py b/refex/test_parsed_file.py
index a1c2f6b..2405202 100644
--- a/refex/test_parsed_file.py
+++ b/refex/test_parsed_file.py
@@ -19,6 +19,7 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+
 from refex import parsed_file
 
 
diff --git a/refex/test_substitution.py b/refex/test_substitution.py
index 3d02739..711ec5d 100644
--- a/refex/test_substitution.py
+++ b/refex/test_substitution.py
@@ -19,6 +19,7 @@
 
 from absl.testing import absltest
 from absl.testing import parameterized
+
 from refex import substitution