DOC: update contributing guidelines for black (pandas-dev#27233)

* DOC: update contributing guidelines for black

* add pre-commit-config file

* update pre-commit hook for pandas

* correct name for config file

Author: jorisvandenbossche

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,5 @@
 - [ ] closes #xxxx
 - [ ] tests added / passed
+- [ ] passes `black pandas`
 - [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
 - [ ] whatsnew entry

.pre-commit-config.yaml

@@ -0,0 +1,16 @@
+repos:
+    - repo: https://github.com/python/black
+      rev: stable
+      hooks:
+          - id: black
+            language_version: python3.7
+    - repo: https://gitlab.com/pycqa/flake8
+      rev: 3.7.7
+      hooks:
+          - id: flake8
+            language: python_venv
+    - repo: https://github.com/pre-commit/mirrors-isort
+      rev: v4.3.20
+      hooks:
+          - id: isort
+            language: python_venv

doc/source/development/contributing.rst

@@ -562,23 +562,38 @@ many errors as possible, but it may not correct *all* of them. Thus, it is
 recommended that you run ``cpplint`` to double check and make any other style
 fixes manually.
 
-Python (PEP8)
-~~~~~~~~~~~~~
-
-*pandas* uses the `PEP8 <http://www.python.org/dev/peps/pep-0008/>`_ standard.
-There are several tools to ensure you abide by this standard. Here are *some* of
-the more common ``PEP8`` issues:
+Python (PEP8 / black)
+~~~~~~~~~~~~~~~~~~~~~
 
-* we restrict line-length to 79 characters to promote readability
-* passing arguments should have spaces after commas, e.g. ``foo(arg1, arg2, kw1='bar')``
+*pandas* follows the `PEP8 <http://www.python.org/dev/peps/pep-0008/>`_ standard
+and uses `Black <https://black.readthedocs.io/en/stable/>`_ and
+`Flake8 <http://flake8.pycqa.org/en/latest/>`_ to ensure a consistent code
+format throughout the project.
 
-:ref:`Continuous Integration <contributing.ci>` will run
-the `flake8 <https://pypi.org/project/flake8>`_ tool
-and report any stylistic errors in your code. Therefore, it is helpful before
-submitting code to run the check yourself on the diff::
+:ref:`Continuous Integration <contributing.ci>` will run those tools and
+report any stylistic errors in your code. Therefore, it is helpful before
+submitting code to run the check yourself::
 
+   black pandas
    git diff upstream/master -u -- "*.py" | flake8 --diff
 
+to auto-format your code. Additionally, many editors have plugins that will
+apply ``black`` as you edit files.
+
+Optionally, you may wish to setup `pre-commit hooks <https://pre-commit.com/>`_
+to automatically run ``black`` and ``flake8`` when you make a git commit. This
+can be done by installing ``pre-commit``::
+
+   pip install pre-commit
+
+and then running::
+
+   pre-commit install
+
+from the root of the pandas repository. Now ``black`` and ``flake8`` will be run
+each time you commit changes. You can skip these checks with
+``git commit --no-verify``.
+
 This command will catch any stylistic errors in your changes specifically, but
 be beware it may not catch all of them. For example, if you delete the only
 usage of an imported function, it is stylistically incorrect to import an