Make Python 2 support an extra

docs/source/cheat_sheet.rst

@@ -13,6 +13,12 @@ language represents various common types in Python 2.
    many of the examples have a dual purpose: show how to write the
    annotation, and show the inferred types.
 
+.. note::
+
+   To check Python 2 code with mypy, you'll need to install mypy with
+   ``pip install 'mypy[python2]'``.
+
+
 
 Built-in types
 **************

docs/source/command_line.rst

@@ -232,6 +232,11 @@ For more information on how to use these flags, see :ref:`version_and_platform_c
 
     Equivalent to running :option:`--python-version 2.7 <--python-version>`.
 
+    .. note::
+
+        To check Python 2 code with mypy, you'll need to install mypy with
+        ``pip install 'mypy[python2]'``.
+
 .. option:: --platform PLATFORM
 
     This flag will make mypy type check your code as if it were

docs/source/python2.rst

@@ -8,6 +8,9 @@ annotations are given in comments, since the function annotation
 syntax was introduced in Python 3. The comment-based syntax is
 specified in :pep:`484`.
 
+Mypy requires typed-ast in order to check Python 2 code. You can install it
+using ``pip install 'mypy[python2]'``.
+
 Run mypy in Python 2 mode by using the :option:`--py2 <mypy --py2>` option::
 
     $ mypy --py2 program.py

mypy/fastparse2.py

@@ -68,7 +68,8 @@
         from typed_ast import ast35  # type: ignore[attr-defined]  # noqa: F401
     except ImportError:
         print('The typed_ast package is not installed.\n'
-              'You can install it with `python3 -m pip install typed-ast`.',
+              'For Python 2 support, install mypy using `python3 -m pip install "mypy[python2]"`'
+              'Alternatively, you can install typed_ast with `python3 -m pip install typed-ast`.',
               file=sys.stderr)
     else:
         print('You need a more recent version of the typed_ast package.\n'

setup.py

@@ -191,15 +191,15 @@ def run(self):
       classifiers=classifiers,
       cmdclass=cmdclass,
       # When changing this, also update mypy-requirements.txt.
-      install_requires=['typed_ast >= 1.4.0, < 1.5.0',
+      install_requires=["typed_ast >= 1.4.0, < 1.5.0; python_version<'3.8'",
                         'typing_extensions>=3.7.4',
                         'mypy_extensions >= 0.4.3, < 0.5.0',
                         'types-typing-extensions>=3.7.0',
                         'types-mypy-extensions>=0.4.0',
                         'toml',
                         ],
       # Same here.
-      extras_require={'dmypy': 'psutil >= 4.0'},
+      extras_require={'dmypy': 'psutil >= 4.0', 'python2': 'typed_ast >= 1.4.0, < 1.5.0'},
       python_requires=">=3.5",
       include_package_data=True,
       project_urls={