Switch to using black for code formatting

.pylintrc

@@ -71,7 +71,8 @@ disable=spelling,       # way too noisy
     unnecessary-pass,   # allow for methods with just "pass", for clarity
     no-else-return,     # relax "elif" after a clause with a return
     docstring-first-line-empty, # relax docstring style
-    import-outside-toplevel
+    import-outside-toplevel,
+    bad-continuation, bad-whitespace # differences of opinion with black
 
 
 
@@ -212,7 +213,7 @@ max-nested-blocks=5
 [FORMAT]
 
 # Maximum number of characters on a single line.
-max-line-length=100
+max-line-length=105
 
 # Regexp for a line that is allowed to be longer than the limit.
 ignore-long-lines=^\s*(# )?<?https?://\S+>?$

CONTRIBUTING.md

@@ -18,6 +18,10 @@ please ensure that:
 1. The code follows the code style of the project and successfully
    passes the tests. For convenience, you can execute `tox` locally,
    which will run these checks and report any issues.
+
+   If your code fails the local style checks (specifically the black
+   code formatting check) you can use `tox -eblack` to automatically
+   fix update the code formatting.
 2. The documentation has been updated accordingly. In particular, if a
    function or class has been modified during the PR, please update the
    *docstring* accordingly.
@@ -358,7 +362,24 @@ you just need to update the reference images as follows:
     new tests should now pass.
 
 Note: If you have run `test/ipynb/mpl_tester.ipynb` locally it is possible some file metadata has changed, **please do not commit and push changes to this file unless they were intentional**.
-### Development Cycle
+
+## Style and lint
+
+Qiskit Terra uses 2 tools for verify code formatting and lint checking. The
+first tool is [black](https://github.com/psf/black) which is a code formatting
+tool that will automatically update the code formatting to a consistent style.
+The second tool is [pylint]https://www.pylint.org/) which is a code linter
+which does a deeper analysis of the Python code to find both style issues and
+potential bugs and other common issues in Python.
+
+You can check that your local modifications conform to the style rules
+by running `tox -elint` which will run `black` and `pylint` to check the local
+code formatting and lint. If black returns a code formatting error you can
+run `tox -eblack` to automatically update the code formatting to conform to
+the style. However, if `pylint` returns any error you will have to fix these
+issues by manually updating your code.
+
+## Development Cycle
 
 The development cycle for qiskit-terra is all handled in the open using
 the project boards in Github for project management. We use milestones

Makefile

@@ -51,7 +51,10 @@ lint:
 	python tools/find_optional_imports.py
 
 style:
-	pycodestyle qiskit test
+	black --check qiskit test tools
+
+black:
+	black qiskit test tools
 
 # Use the -s (starting directory) flag for "unittest discover" is necessary,
 # otherwise the QuantumCircuit header will be modified during the discovery.

azure-pipelines.yml

@@ -274,7 +274,7 @@ stages:
         - bash: |
             set -e
             source test-job/bin/activate
-            pycodestyle qiskit test
+            black --check qiskit test tools
             pylint -rn qiskit test
             tools/verify_headers.py qiskit test
             python tools/find_optional_imports.py

pyproject.toml

@@ -1,2 +1,6 @@
 [build-system]
 requires = ["Cython>=0.27.1", "setuptools", "wheel"]
+
+[tool.black]
+line-length = 100
+target-version = ['py36', 'py37', 'py38', 'py39']

requirements-dev.txt

@@ -6,7 +6,7 @@ ipywidgets>=7.3.0
 jupyter
 matplotlib>=2.1,<3.4
 pillow>=4.2.1
-pycodestyle
+black==21.4b2
 pydot
 astroid==2.5
 pylint==2.7.1

tox.ini

@@ -19,14 +19,19 @@ commands =
   stestr run {posargs}
 
 [testenv:lint]
+envdir = .tox/lint
 basepython = python3
 commands =
-  pycodestyle qiskit test
+  black --check {posargs} qiskit test tools
   pylint -rn qiskit test
   {toxinidir}/tools/verify_headers.py qiskit test
   {toxinidir}/tools/find_optional_imports.py
   reno lint
 
+[testenv:black]
+envdir = .tox/lint
+commands = black {posargs} qiskit test tools
+
 [testenv:coverage]
 basepython = python3
 setenv =
@@ -49,7 +54,12 @@ commands =
   sphinx-build -W -b html -j auto docs/ docs/_build/html {posargs}
 
 [pycodestyle]
-max-line-length = 100
+max-line-length = 105
 # default ignores + E741 because of opflow global variable I
+# + E203 because of a difference of opinion with black
 # codebase does currently comply with: E133, E242, E704, W505
-ignore = E121, E123, E126, E133, E226, E241, E242, E704, W503, W504, W505, E741
+ignore = E121, E123, E126, E133, E226, E241, E242, E704, W503, W504, W505, E741, E203
+
+[flake8]
+max-line-length = 105
+extend-ignore = E203, E741