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']

qiskit/__init__.py

@@ -64,7 +64,9 @@
     warnings.warn(
         "Using Qiskit with Python 3.6 is deprecated as of the 0.17.0 release. "
         "Support for running Qiskit with Python 3.6 will be removed in a "
-        "future release.", DeprecationWarning)
+        "future release.",
+        DeprecationWarning,
+    )
 
 
 class AerWrapper:
@@ -77,6 +79,7 @@ def __bool__(self):
         if self.aer is None:
             try:
                 from qiskit.providers import aer
+
                 self.aer = aer.Aer
             except ImportError:
                 return False
@@ -86,11 +89,14 @@ def __getattr__(self, attr):
         if not self.aer:
             try:
                 from qiskit.providers import aer
+
                 self.aer = aer.Aer
             except ImportError as exc:
-                raise ImportError('Could not import the Aer provider from the '
-                                  'qiskit-aer package. Install qiskit-aer or '
-                                  'check your installation.') from exc
+                raise ImportError(
+                    "Could not import the Aer provider from the "
+                    "qiskit-aer package. Install qiskit-aer or "
+                    "check your installation."
+                ) from exc
         return getattr(self.aer, attr)
 
 
@@ -104,6 +110,7 @@ def __bool__(self):
         if self.ibmq is None:
             try:
                 from qiskit.providers import ibmq
+
                 self.ibmq = ibmq.IBMQ
             except ImportError:
                 return False
@@ -113,12 +120,15 @@ def __getattr__(self, attr):
         if not self.ibmq:
             try:
                 from qiskit.providers import ibmq
+
                 self.ibmq = ibmq.IBMQ
             except ImportError as exc:
-                raise ImportError('Could not import the IBMQ provider from the '
-                                  'qiskit-ibmq-provider package. Install '
-                                  'qiskit-ibmq-provider or check your  '
-                                  'installation.') from exc
+                raise ImportError(
+                    "Could not import the IBMQ provider from the "
+                    "qiskit-ibmq-provider package. Install "
+                    "qiskit-ibmq-provider or check your  "
+                    "installation."
+                ) from exc
         return getattr(self.ibmq, attr)
 
 

qiskit/algorithms/__init__.py

@@ -165,60 +165,75 @@
 from .variational_algorithm import VariationalAlgorithm, VariationalResult
 from .amplitude_amplifiers import Grover, GroverResult, AmplificationProblem
 from .amplitude_estimators import (
-    AmplitudeEstimator, AmplitudeEstimatorResult,
-    AmplitudeEstimation, AmplitudeEstimationResult,
-    FasterAmplitudeEstimation, FasterAmplitudeEstimationResult,
-    IterativeAmplitudeEstimation, IterativeAmplitudeEstimationResult,
-    MaximumLikelihoodAmplitudeEstimation, MaximumLikelihoodAmplitudeEstimationResult,
-    EstimationProblem
+    AmplitudeEstimator,
+    AmplitudeEstimatorResult,
+    AmplitudeEstimation,
+    AmplitudeEstimationResult,
+    FasterAmplitudeEstimation,
+    FasterAmplitudeEstimationResult,
+    IterativeAmplitudeEstimation,
+    IterativeAmplitudeEstimationResult,
+    MaximumLikelihoodAmplitudeEstimation,
+    MaximumLikelihoodAmplitudeEstimationResult,
+    EstimationProblem,
 )
 from .eigen_solvers import NumPyEigensolver, Eigensolver, EigensolverResult
 from .factorizers import Shor, ShorResult
 from .linear_solvers import HHL, LinearSolver, NumPyLinearSolver, LinearSolverResult
-from .minimum_eigen_solvers import (VQE, VQEResult, QAOA,
-                                    NumPyMinimumEigensolver,
-                                    MinimumEigensolver, MinimumEigensolverResult)
-from .phase_estimators import (HamiltonianPhaseEstimation, HamiltonianPhaseEstimationResult,
-                               PhaseEstimationScale, PhaseEstimation, PhaseEstimationResult)
+from .minimum_eigen_solvers import (
+    VQE,
+    VQEResult,
+    QAOA,
+    NumPyMinimumEigensolver,
+    MinimumEigensolver,
+    MinimumEigensolverResult,
+)
+from .phase_estimators import (
+    HamiltonianPhaseEstimation,
+    HamiltonianPhaseEstimationResult,
+    PhaseEstimationScale,
+    PhaseEstimation,
+    PhaseEstimationResult,
+)
 from .exceptions import AlgorithmError
 
 __all__ = [
-    'AlgorithmResult',
-    'VariationalAlgorithm',
-    'VariationalResult',
-    'AmplificationProblem',
-    'Grover',
-    'GroverResult',
-    'AmplitudeEstimator',
-    'AmplitudeEstimatorResult',
-    'AmplitudeEstimation',
-    'AmplitudeEstimationResult',
-    'FasterAmplitudeEstimation',
-    'FasterAmplitudeEstimationResult',
-    'IterativeAmplitudeEstimation',
-    'IterativeAmplitudeEstimationResult',
-    'MaximumLikelihoodAmplitudeEstimation',
-    'MaximumLikelihoodAmplitudeEstimationResult',
-    'EstimationProblem',
-    'NumPyEigensolver',
-    'LinearSolverResult',
-    'Eigensolver',
-    'EigensolverResult',
-    'Shor',
-    'ShorResult',
-    'VQE',
-    'VQEResult',
-    'QAOA',
-    'LinearSolver',
-    'HHL',
-    'NumPyLinearSolver',
-    'NumPyMinimumEigensolver',
-    'MinimumEigensolver',
-    'MinimumEigensolverResult',
-    'HamiltonianPhaseEstimation',
-    'HamiltonianPhaseEstimationResult',
-    'PhaseEstimationScale',
-    'PhaseEstimation',
-    'PhaseEstimationResult',
-    'AlgorithmError',
+    "AlgorithmResult",
+    "VariationalAlgorithm",
+    "VariationalResult",
+    "AmplificationProblem",
+    "Grover",
+    "GroverResult",
+    "AmplitudeEstimator",
+    "AmplitudeEstimatorResult",
+    "AmplitudeEstimation",
+    "AmplitudeEstimationResult",
+    "FasterAmplitudeEstimation",
+    "FasterAmplitudeEstimationResult",
+    "IterativeAmplitudeEstimation",
+    "IterativeAmplitudeEstimationResult",
+    "MaximumLikelihoodAmplitudeEstimation",
+    "MaximumLikelihoodAmplitudeEstimationResult",
+    "EstimationProblem",
+    "NumPyEigensolver",
+    "LinearSolverResult",
+    "Eigensolver",
+    "EigensolverResult",
+    "Shor",
+    "ShorResult",
+    "VQE",
+    "VQEResult",
+    "QAOA",
+    "LinearSolver",
+    "HHL",
+    "NumPyLinearSolver",
+    "NumPyMinimumEigensolver",
+    "MinimumEigensolver",
+    "MinimumEigensolverResult",
+    "HamiltonianPhaseEstimation",
+    "HamiltonianPhaseEstimationResult",
+    "PhaseEstimationScale",
+    "PhaseEstimation",
+    "PhaseEstimationResult",
+    "AlgorithmError",
 ]

qiskit/algorithms/algorithm_result.py

@@ -20,20 +20,23 @@
 
 
 class AlgorithmResult(ABC):
-    """ Abstract Base Class for algorithm results."""
+    """Abstract Base Class for algorithm results."""
 
     def __str__(self) -> str:
         result = {}
         for name, value in inspect.getmembers(self):
-            if not name.startswith('_') and \
-                    not inspect.ismethod(value) and not inspect.isfunction(value) and \
-                    hasattr(self, name):
+            if (
+                not name.startswith("_")
+                and not inspect.ismethod(value)
+                and not inspect.isfunction(value)
+                and hasattr(self, name)
+            ):
 
                 result[name] = value
 
         return pprint.pformat(result, indent=4)
 
-    def combine(self, result: 'AlgorithmResult') -> None:
+    def combine(self, result: "AlgorithmResult") -> None:
         """
         Any property from the argument that exists in the receiver is
         updated.
@@ -43,15 +46,18 @@ def combine(self, result: 'AlgorithmResult') -> None:
             TypeError: Argument is None
         """
         if result is None:
-            raise TypeError('Argument result expected.')
+            raise TypeError("Argument result expected.")
         if result == self:
             return
 
         # find any result public property that exists in the receiver
         for name, value in inspect.getmembers(result):
-            if not name.startswith('_') and \
-                    not inspect.ismethod(value) and not inspect.isfunction(value) and \
-                    hasattr(self, name):
+            if (
+                not name.startswith("_")
+                and not inspect.ismethod(value)
+                and not inspect.isfunction(value)
+                and hasattr(self, name)
+            ):
                 try:
                     setattr(self, name, value)
                 except AttributeError:

qiskit/algorithms/amplitude_amplifiers/__init__.py

@@ -17,9 +17,9 @@
 from .grover import Grover, GroverResult
 
 __all__ = [
-    'AmplitudeAmplifier',
-    'AmplitudeAmplifierResult',
-    'AmplificationProblem',
-    'Grover',
-    'GroverResult'
+    "AmplitudeAmplifier",
+    "AmplitudeAmplifierResult",
+    "AmplificationProblem",
+    "Grover",
+    "GroverResult",
 ]

qiskit/algorithms/amplitude_amplifiers/amplification_problem.py

@@ -27,15 +27,17 @@ class AmplificationProblem:
     on the optimal bitstring.
     """
 
-    def __init__(self,
-                 oracle: Union[QuantumCircuit, Statevector],
-                 state_preparation: Optional[QuantumCircuit] = None,
-                 grover_operator: Optional[QuantumCircuit] = None,
-                 post_processing: Optional[Callable[[str], Any]] = None,
-                 objective_qubits: Optional[Union[int, List[int]]] = None,
-                 is_good_state: Optional[Union[
-                     Callable[[str], bool], List[int], List[str], Statevector]] = None,
-                 ) -> None:
+    def __init__(
+        self,
+        oracle: Union[QuantumCircuit, Statevector],
+        state_preparation: Optional[QuantumCircuit] = None,
+        grover_operator: Optional[QuantumCircuit] = None,
+        post_processing: Optional[Callable[[str], Any]] = None,
+        objective_qubits: Optional[Union[int, List[int]]] = None,
+        is_good_state: Optional[
+            Union[Callable[[str], bool], List[int], List[str], Statevector]
+        ] = None,
+    ) -> None:
         r"""
         Args:
             oracle: The oracle reflecting about the bad states.
@@ -159,15 +161,17 @@ def is_good_state(self) -> Callable[[str], bool]:
             if all(isinstance(good_bitstr, str) for good_bitstr in self._is_good_state):
                 return lambda bitstr: bitstr in self._is_good_state
             else:
-                return lambda bitstr: all(bitstr[good_index] == '1'  # type:ignore
-                                          for good_index in self._is_good_state)
+                return lambda bitstr: all(
+                    bitstr[good_index] == "1"  # type:ignore
+                    for good_index in self._is_good_state
+                )
 
         return lambda bitstr: bitstr in self._is_good_state.probabilities_dict()
 
     @is_good_state.setter
-    def is_good_state(self,
-                      is_good_state: Union[Callable[[str], bool], List[int], List[str], Statevector]
-                      ) -> None:
+    def is_good_state(
+        self, is_good_state: Union[Callable[[str], bool], List[int], List[str], Statevector]
+    ) -> None:
         """Set the ``is_good_state`` function.
 
         Args:

qiskit/algorithms/amplitude_amplifiers/amplitude_amplifier.py

@@ -25,7 +25,7 @@ class AmplitudeAmplifier(ABC):
     """The interface for amplification algorithms."""
 
     @abstractmethod
-    def amplify(self, amplification_problem: AmplificationProblem) -> 'AmplificationResult':
+    def amplify(self, amplification_problem: AmplificationProblem) -> "AmplificationResult":
         """Run the amplification algorithm.
 
         Args: