Initial rough version, incomplete

.gitignore

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+#Ignore the build directory (and its sub-directories):
+build
+
+#Ignore the distribution directory
+dist
+
+#Ignore another Python specific build folder:
+flake8_black.egg-info/
+
+#Ignore backup files from some Unix editors,
+*~
+*.swp
+*.bak
+
+#Ignore patches and any original files created by patch command
+*.diff
+*.orig
+*.rej
+
+#Ignore these hidden files from Mac OS X
+.DS_Store
+
+#Ignore hidden files from Dolphin window manager
+.directory

LICENSE.rst

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+Copyright 2019, Peter Cock, The James Hutton Institute, UK.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

MANIFEST.in

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+include README.rst
+include LICENSE.rst
+include requirements.txt

README.rst

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+flake-black
+===========
+
+.. image:: https://img.shields.io/pypi/v/flake8-black.svg
+   :alt: Released on the Python Package Index (PyPI)
+   :target: https://pypi.python.org/pypi/flake8-black
+.. image:: https://img.shields.io/travis/peterjc/flake8-black/master.svg
+   :alt: Testing with TravisCI
+   :target: https://travis-ci.org/peterjc/flake8-black/branches
+.. image:: https://landscape.io/github/peterjc/flake8-black/master/landscape.svg?style=flat
+   :alt: Landscape Code Metrics
+   :target: https://landscape.io/github/peterjc/flake8-black/
+
+Introduction
+------------
+
+This is an MIT licensed `flake8 <https://gitlab.com/pycqa/flake8>`_ plugin
+for validating Python code style with the command line code formatting tool
+`black <https://github.com/ambv/black>`_. It is available to install from
+the Python Package Index (PyPI):
+
+- https://pypi.python.org/pypi/flake8-black
+
+Black, "The Uncompromising Code Formatter", is normally run to edit your
+Python code in place to match their coding style, a strict subset of the
+`PEP 8 style guide <https://www.python.org/dev/peps/pep-0008/>`_.
+
+The point of this plugin is to be able to run ``black --check ...`` from
+within the ``flake8`` plugin ecosystem. You might use this via a ``git``
+pre-commit hook, or as part of your continuous integration testing.
+
+Flake8 Validation codes
+-----------------------
+
+Early versions of flake8 assumed a single character prefix for the validation
+codes, which became problematic with collisions in the plugin ecosystem. Since
+v3.0, flake8 has supported longer prefixes therefore this plugin uses ``BLK``
+as its prefix.
+
+====== =======================================================================
+Code   Description
+------ -----------------------------------------------------------------------
+BLK100 Black would edit your Python file(s).
+BLK900 Error invoking Black.
+====== =======================================================================
+
+
+Installation and usage
+----------------------
+
+Python 3.6 or later is required to run ``black``, so that is recommanded but
+``black`` can be used on Python code written for older versions of Python.
+
+Install ``flake8-black`` using ``pip``, which should install ``flake8`` and
+``black`` as well if not already present::
+
+    $ pip install flake8-black
+
+The new validator should be automatically included when using ``flake8`` which
+may now report additional validation codes starting with ``BLK`` (as defined
+above). For example::
+
+    $ flake8 example.py
+
+You can request only the ``BLK`` codes be shown using::
+
+    $ flake8 --select BLK example.py
+
+We recommend using the following settings in your ``flake8`` configuration,
+for example in your ``.flake8``  configuration::
+
+    [flake8]
+    # Recommend matching the black default line length of 88,
+    # rather than the flake8 default of 79:
+    max-line-length = 88
+    extend-ignore =
+        # See https://github.com/PyCQA/pycodestyle/issues/373
+        E203,    
+
+In order not to trigger flake8's ``E501 line too long`` errors, the plugin
+passes the ``flake8`` maximum line length when it calls ``black``,
+equivalent to doing ``black -l 88 --check *.py`` at the command line.
+
+Note currently ``pycodestyle`` gives false positives on the spaces ``black``
+uses for slices, which ``flake8`` reports as ``E203: whitespace before ':'``.
+Until this is fixed, and ``flake8`` is updated, we suggest disabling this
+style check.
+
+
+Version History
+---------------
+
+======= ========== ===========================================================
+Version Released   Changes
+------- ---------- -----------------------------------------------------------
+v0.0.1  *pending*   - Initial public release.
+======= ========== ===========================================================
+
+
+Developers
+----------
+
+This plugin is on GitHub at https://github.com/peterjc/flake8-black
+
+To make a new release once tested locally and on TravisCI::
+
+    $ git tag vX.Y.Z
+    $ python setup.py sdist --formats=gztar
+    $ twine upload dist/flake8-black-X.Y.Z.tar.gz
+    $ git push origin master --tags
+
+TODO
+----
+
+- Define different error codes based on what changes black would make?
+- Create a full test suite and use this for continuous integration.

flake8_black.py

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+"""Check Python code passes black style validation via flake8.
+
+This is a plugin for the tool flake8 tool for checking Python
+soucre code using the tool black.
+"""
+
+import io
+import logging
+import sys
+import tokenize
+
+import black
+
+
+__version__ = "0.0.1"
+
+log = logging.getLogger(__name__)
+
+black_prefix = "BLK"
+
+
+def find_diff_start(old_src, new_src):
+    old_lines = old_src.split("\n")
+    new_lines = new_src.split("\n")
+
+    for line in range(min(len(old_lines), len(new_lines))):
+        old = old_lines[line]
+        new = new_lines[line]
+        if old == new:
+            continue
+        for col in range(min(len(old), len(new))):
+            if old[col] != new[col]:
+                return line, col
+        # Difference at the end of the line...
+        return line, min(len(old), len(new)) + 1
+    # Difference at the of the file...
+    return min(len(old_lines), min(new_lines)) + 1, 0
+
+
+class BlackStyleChecker(object):
+    """Checker of Python code using black."""
+
+    name = "black"
+    version = __version__
+
+    STDIN_NAMES = set(['stdin', '-', '(none)', None])
+
+    def __init__(self, tree, filename='(none)', builtins=None):
+        """Initialise."""
+        self.tree = tree
+        self.filename = filename
+        try:
+            self.load_source()
+            self.err = None
+        except Exception as err:
+            self.source = None
+            self.err = err
+        # TODO, read flake8 config
+        self.line_length = 79
+
+    def run(self):
+        """Use black to check code style."""
+        # Is there any reason not to call load_source here?
+        msg = None
+        line = 0
+        col = 0
+        if self.err is not None:
+            assert self.source is None
+            msg = black_prefix + "900 Failed to load file: %s" % self.err
+            yield 0, 0, msg, type(self)
+        elif not self.source:
+            # Empty file, nothing to change
+            pass
+        else:
+            # Call black...
+            try:
+                # Set mode?
+                new_code = black.format_file_contents(
+                    self.source,
+                    line_length=self.line_length,
+                    fast=False)
+            except black.NothingChanged:
+                pass
+            except black.InvalidInput:
+                msg = "901 Invalid input"
+            except Exception as e:
+                msg = "999 Unexpected exception: %s" % e
+            else:
+                assert new_code != self.source, \
+                    "Black made changes without raising NothingChanged"
+                line, col = find_diff_start(self.source, new_code)
+                msg = "100 Black would make changes"
+        if msg:
+            # If we don't know the line or column numbers, leaving as zero.
+            yield line, col, black_prefix + msg, type(self)
+
+    def load_source(self):
+        """Load the source for the specified file."""
+        if self.filename in self.STDIN_NAMES:
+            self.filename = 'stdin'
+            if sys.version_info[0] < 3:
+                self.source = sys.stdin.read()
+            else:
+                self.source = io.TextIOWrapper(sys.stdin.buffer,
+                                               errors='ignore').read()
+        else:
+            with tokenize.open(self.filename) as handle:
+                self.source = handle.read()

requirements.txt

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+# This is a plugin for flake8, so we require that.
+# Our prefix is not single letter, so we need v3:
+flake8 >= 3.0.0
+
+# We need black, which in turn needs Python 3.6+
+black

setup.cfg

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+[wheel]
+universal = 1

setup.py

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+from __future__ import with_statement
+
+from setuptools import setup
+
+
+def get_version(fname="flake8_black.py"):
+    with open(fname) as f:
+        for line in f:
+            if line.startswith("__version__"):
+                return eval(line.split("=")[-1])
+
+
+setup(
+    name="flake8-black",
+    version=get_version(),
+    description="flake8 plugin to call black as a code style validator",
+    long_description=open("README.rst").read(),
+    license="MIT",
+    author="Peter J. A. Cock",
+    author_email="p.j.a.cock@googlemail.com",
+    url="https://github.com/peterjc/flake8-black",
+    classifiers=[
+        "Intended Audience :: Developers",
+        "Framework :: Flake8",
+        "Programming Language :: Python",
+        "Programming Language :: Python :: 3",
+        "Operating System :: OS Independent",
+        "License :: OSI Approved :: MIT License",
+        "Topic :: Software Development :: Libraries :: Python Modules",
+        "Topic :: Software Development :: Quality Assurance",
+    ],
+    keywords="PEP8",
+    py_modules=["flake8_black"],
+    install_requires=["flake8 >= 3.0.0", "black"],
+    entry_points={"flake8.extension": ["BLK = flake8_black:BlackStyleChecker"]},
+)