TYPING: add some type hints to core.generic

doc/source/whatsnew/v1.0.0.rst

@@ -42,6 +42,45 @@ Backwards incompatible API changes
 - :class:`pandas.core.groupby.GroupBy.transform` now raises on invalid operation names (:issue:`27489`).
 -
 
+.. _whatsnew_1000.api_breaking.deps:
+
+Increased minimum versions for dependencies
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Supporting Python 3.6, we are not able to take advantage of new types added to the typing module. The typing_extensions module
+contains both backports of these changes as well as experimental types that will eventually be added to the typing module.
+
+If installed, we now require:
+
++-------------------------+-----------------+----------+
+| Package                 | Minimum Version | Required |
++=========================+=================+==========+
+|                         |                 |          |
++-------------------------+-----------------+----------+
+|                         |                 |          |
++-------------------------+-----------------+----------+
+| typing_extensions (dev) |                 |          |
++-------------------------+-----------------+----------+
+
+For `optional libraries <https://dev.pandas.io/install.html#dependencies>`_ the general recommendation is to use the latest version.
+The following table lists the lowest version per library that is currently being tested throughout the development of pandas.
+Optional libraries below the lowest tested version may still work, but are not considered supported.
+
++-----------------+-----------------+
+| Package         | Minimum Version |
++=================+=================+
+|                 |                 |
++-----------------+-----------------+
+|                 |                 |
++-----------------+-----------------+
+|                 |                 |
++-----------------+-----------------+
+|                 |                 |
++-----------------+-----------------+
+
+See :ref:`install.dependencies` and :ref:`install.optional_dependencies` for more.
+
+
 Other API changes
 ^^^^^^^^^^^^^^^^^
 

environment.yml

@@ -24,6 +24,7 @@ dependencies:
   - isort  # check that imports are in the right order
   - mypy
   - pycodestyle  # used by flake8
+  - typing_extensions
 
   # documentation
   - gitpython  # obtain contributors from git for whatsnew

pandas/_typing.py

@@ -14,6 +14,13 @@
     from pandas.core.frame import DataFrame  # noqa: F401
     from pandas.core.series import Series  # noqa: F401
     from pandas.core.sparse.series import SparseSeries  # noqa: F401
+    from typing_extensions import Literal
+
+    IgnoreRaise = Literal["ignore", "raise"]
+    FirstLast = Literal["first", "last"]
+else:
+    IgnoreRaise = str
+    FirstLast = str
 
 
 AnyArrayLike = TypeVar(
@@ -28,3 +35,4 @@
 Scalar = Union[str, int, float]
 Axis = Union[str, int]
 Ordered = Optional[bool]
+Level = Union[str, int]