Update pylint to recent version, solve cyclic dependencies (Qiskit#1600)

* Bump pylint version

* Disable pylint unnecessary-pass, no-else-return

Disable pylint checks:
* unnecessary-pass: allow for methods with just "pass", for clarity
* no-else-return: relax "elif" after a clause with a return

* Fix linter: useless-object-inheritance

* Fix new linting errors

* Move version to its own file

Move the reading of Terra version to its own file, avoiding cyclic
import during at `qiskit/providers/basebackend.py`.

* Prefer importing QiskitError from submodule

Revise the imports of `QiskitError` in order to break cyclic
dependencies.

* Break more cyclic imports

* Break more cyclic imports, redux

* Update cyclic import in QuantumCircuit

Move the pylint disable to an inner-most level for QuantumCircuit. There
is still an import cycle (qiskit.circuit.quantumcircuit ->
qiskit.converters.ast_to_dag) due to circuit.Measure and circuit.Reset
being part of the module and the dynamic extension mechanism.

* Unify extension handling in own modules

Revise the Exceptions throughout the codebase, containing them into
specific `.exceptions` modules.

* Update passes imports after rebasing

Update the `qiskit.transpiler` module imports to avoid cyclic import,
after rebasing against master.

* Update .pylintrc

.pylintrc

@@ -20,7 +20,8 @@ persistent=yes
 
 # List of plugins (as comma separated values of python modules names) to load,
 # usually to register additional checkers.
-load-plugins=pylint.extensions.docparams  # enable checking of docstring args
+load-plugins=pylint.extensions.docparams, # enable checking of docstring args
+    pylint.extensions.docstyle  # basic docstring stle checks
 
 # Use multiple processes to speed up Pylint.
 jobs=1
@@ -34,15 +35,6 @@ unsafe-load-any-extension=no
 # run arbitrary code
 extension-pkg-whitelist=numpy
 
-# Allow optimization of some AST trees. This will activate a peephole AST
-# optimizer, which will apply various small optimizations. For instance, it can
-# be used to obtain the result of joining multiple strings with the addition
-# operator. Joining a lot of strings can lead to a maximum recursion error in
-# Pylint and this flag can prevent that. It has one side effect, the resulting
-# AST will be different than the one from reality. This option is deprecated
-# and it will be removed in Pylint 2.0.
-optimize-ast=no
-
 
 [MESSAGES CONTROL]
 
@@ -65,8 +57,7 @@ confidence=
 # --enable=similarities". If you want to run only the classes checker, but have
 # no Warning level messages displayed, use"--disable=all --enable=classes
 # --disable=W"
-disable=print-statement,parameter-unpacking,unpacking-in-except,old-raise-syntax,backtick,import-star-module-level,apply-builtin,basestring-builtin,buffer-builtin,cmp-builtin,coerce-builtin,execfile-builtin,file-builtin,long-builtin,raw_input-builtin,reduce-builtin,standarderror-builtin,unicode-builtin,xrange-builtin,coerce-method,delslice-method,getslice-method,setslice-method,no-absolute-import,old-division,dict-iter-method,dict-view-method,next-method-called,metaclass-assignment,indexing-exception,raising-string,reload-builtin,oct-method,hex-method,nonzero-method,cmp-method,input-builtin,round-builtin,intern-builtin,unichr-builtin,map-builtin-not-iterating,zip-builtin-not-iterating,range-builtin-not-iterating,filter-builtin-not-iterating,using-cmp-argument,long-suffix,old-ne-operator,old-octal-literal,suppressed-message,useless-suppression,
-    no-self-use,        # disabled as it is too verbose
+disable=no-self-use,        # disabled as it is too verbose
     fixme,              # disabled as TODOs would show up as warnings
     protected-access,   # disabled as we don't follow the public vs private
                         # convention strictly
@@ -75,7 +66,11 @@ disable=print-statement,parameter-unpacking,unpacking-in-except,old-raise-syntax
     # disable the "too-many/few-..." refactoring hints
     too-many-lines, too-many-branches, too-many-locals, too-many-nested-blocks,
     too-many-statements, too-many-instance-attributes, too-many-arguments,
-    too-many-public-methods, too-few-public-methods, too-many-ancestors
+    too-many-public-methods, too-few-public-methods, too-many-ancestors,
+    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
+
 
 
 
@@ -338,7 +333,7 @@ callbacks=cb_,_cb
 
 # List of qualified module names which can have objects that can redefine
 # builtins.
-redefining-builtins-modules=six.moves,future.builtins
+redefining-builtins-modules=six.moves,future.builtins,tools.compiler
 
 
 [CLASSES]

qiskit/__init__.py

@@ -6,18 +6,16 @@
 # the LICENSE.txt file in the root directory of this source tree.
 
 # pylint: disable=wrong-import-order
-# pylint: disable=redefined-builtin
 
 """Main Qiskit public functionality."""
 
-import os
 import pkgutil
 
 # First, check for required Python and API version
 from . import _util
 
 # qiskit errors operator
-from .qiskiterror import QiskitError, QISKitError
+from .exceptions import QiskitError, QISKitError
 
 # The main qiskit operators
 from qiskit.circuit import ClassicalRegister
@@ -51,10 +49,4 @@
 # TODO: Remove
 from .wrapper._wrapper import (load_qasm_string, load_qasm_file)
 
-# Import the wrapper, to make it available when doing "import qiskit".
-from . import wrapper
-from . import tools
-
-ROOT_DIR = os.path.dirname(os.path.abspath(__file__))
-with open(os.path.join(ROOT_DIR, "VERSION.txt"), "r") as version_file:
-    __version__ = version_file.read().strip()
+from .version import __version__

qiskit/circuit/compositegate.py

@@ -8,7 +8,7 @@
 """
 Composite gate, a container for a sequence of unitary gates.
 """
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 from .gate import Gate
 
 

qiskit/circuit/gate.py

@@ -8,7 +8,7 @@
 """
 Unitary gate.
 """
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 from .instruction import Instruction
 
 

qiskit/circuit/instruction.py

@@ -28,12 +28,12 @@
 import sympy
 
 from qiskit.qasm._node import _node
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 from .quantumregister import QuantumRegister
 from .classicalregister import ClassicalRegister
 
 
-class Instruction(object):
+class Instruction:
     """Generic quantum instruction."""
 
     def __init__(self, name, param, qargs, cargs, circuit=None):

qiskit/circuit/instructionset.py

@@ -8,11 +8,11 @@
 """
 Instruction collection.
 """
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 from .instruction import Instruction
 
 
-class InstructionSet(object):
+class InstructionSet:
     """Instruction collection."""
 
     def __init__(self):

qiskit/circuit/measure.py

@@ -8,7 +8,7 @@
 """
 Quantum measurement in the computational basis.
 """
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 
 from .instruction import Instruction
 from .instructionset import InstructionSet

qiskit/circuit/quantumcircuit.py

@@ -5,11 +5,10 @@
 # This source code is licensed under the Apache License, Version 2.0 found in
 # the LICENSE.txt file in the root directory of this source tree.
 
-# pylint: disable=cyclic-import,invalid-name
-
 """
 Quantum circuit object.
 """
+
 from collections import OrderedDict
 from copy import deepcopy
 import itertools
@@ -18,12 +17,12 @@
 import multiprocessing as mp
 
 from qiskit.qasm import _qasm
-from qiskit.qiskiterror import QiskitError
+from qiskit.exceptions import QiskitError
 from .quantumregister import QuantumRegister
 from .classicalregister import ClassicalRegister
 
 
-class QuantumCircuit(object):
+class QuantumCircuit:
     """Quantum circuit."""
     instances = 0
     prefix = 'circuit'
@@ -413,6 +412,7 @@ def from_qasm_str(qasm_str):
 
 
 def _circuit_from_qasm(qasm):
+    # pylint: disable=cyclic-import
     from qiskit.converters import ast_to_dag
     from qiskit.converters import dag_to_circuit
     ast = qasm.parse()

qiskit/circuit/register.py

@@ -12,12 +12,12 @@
 import logging
 import itertools
 
-from qiskit.qiskiterror import QiskitError, QiskitIndexError
+from qiskit.exceptions import QiskitError, QiskitIndexError
 
 logger = logging.getLogger(__name__)
 
 
-class Register(object):
+class Register:
     """Implement a generic register."""
 
     # Counter for the number of instances in this class.

qiskit/converters/__init__.py

@@ -5,8 +5,6 @@
 # This source code is licensed under the Apache License, Version 2.0 found in
 # the LICENSE.txt file in the root directory of this source tree.
 
-# pylint: disable=redefined-builtin
-
 """Helper module for simplified Qiskit usage.
 
 The functions in this module provide convenience converters

qiskit/converters/ast_to_dag.py

@@ -14,7 +14,7 @@
 from qiskit.circuit import QuantumRegister
 from qiskit.circuit import ClassicalRegister
 from qiskit.dagcircuit import DAGCircuit
-from qiskit import QiskitError
+from qiskit.exceptions import QiskitError
 
 from qiskit.circuit.measure import Measure
 from qiskit.circuit.reset import Reset
@@ -70,7 +70,7 @@ def ast_to_dag(ast):
     return dag
 
 
-class AstInterpreter(object):
+class AstInterpreter:
     """Interprets an OpenQASM by expanding subroutines and unrolling loops."""
 
     def __init__(self, dag):

qiskit/dagcircuit/__init__.py

@@ -7,4 +7,4 @@
 
 """Module for DAG Circuits."""
 from ._dagcircuit import DAGCircuit
-from ._dagcircuiterror import DAGCircuitError
+from .exceptions import DAGCircuitError

qiskit/dagcircuit/_dagcircuit.py

@@ -23,9 +23,10 @@
 import itertools
 import networkx as nx
 
-from qiskit import QuantumRegister, ClassicalRegister
-from qiskit.circuit import Gate
-from ._dagcircuiterror import DAGCircuitError
+from qiskit.circuit.quantumregister import QuantumRegister
+from qiskit.circuit.classicalregister import ClassicalRegister
+from qiskit.circuit.gate import Gate
+from .exceptions import DAGCircuitError
 
 
 class DAGCircuit:

qiskit/dagcircuit/_dagcircuiterror.py → qiskit/dagcircuit/exceptions.py

@@ -8,7 +8,7 @@
 """
 Exception for errors raised by the DAGCircuit object.
 """
-from qiskit import QiskitError
+from qiskit.exceptions import QiskitError
 
 
 class DAGCircuitError(QiskitError):

qiskit/extensions/_extensionerror.py → qiskit/extensions/exceptions.py

@@ -8,7 +8,7 @@
 """
 Exception for errors raised by extensions module.
 """
-from qiskit import QiskitError
+from qiskit.exceptions import QiskitError
 
 
 class ExtensionError(QiskitError):

qiskit/extensions/quantum_initializer/_initializer.py

@@ -13,8 +13,8 @@
 import numpy as np
 import scipy
 
-from qiskit import QiskitError
-from qiskit import QuantumCircuit
+from qiskit.exceptions import QiskitError
+from qiskit.circuit import QuantumCircuit
 from qiskit.circuit import CompositeGate
 from qiskit.circuit import Gate
 from qiskit.extensions.standard.cx import CnotGate

qiskit/extensions/simulator/load.py

@@ -11,7 +11,7 @@
 from qiskit import QuantumCircuit
 from qiskit import QuantumRegister
 from qiskit.circuit import Instruction
-from qiskit.extensions._extensionerror import ExtensionError
+from qiskit.extensions.exceptions import ExtensionError
 from qiskit.extensions.standard import header  # pylint: disable=unused-import
 
 

qiskit/extensions/simulator/noise.py

@@ -11,7 +11,7 @@
 from qiskit import QuantumCircuit
 from qiskit import QuantumRegister
 from qiskit.circuit import Instruction
-from qiskit.extensions._extensionerror import ExtensionError
+from qiskit.extensions.exceptions import ExtensionError
 from qiskit.extensions.standard import header  # pylint: disable=unused-import
 
 

qiskit/extensions/simulator/save.py

@@ -11,7 +11,7 @@
 from qiskit import QuantumCircuit
 from qiskit import QuantumRegister
 from qiskit.circuit import Instruction
-from qiskit.extensions._extensionerror import ExtensionError
+from qiskit.extensions.exceptions import ExtensionError
 from qiskit.extensions.standard import header  # pylint: disable=unused-import
 
 

qiskit/extensions/simulator/snapshot.py

@@ -11,7 +11,7 @@
 from qiskit import QuantumCircuit
 from qiskit import QuantumRegister
 from qiskit.circuit import Instruction
-from qiskit.extensions._extensionerror import ExtensionError
+from qiskit.extensions.exceptions import ExtensionError
 from qiskit.extensions.standard import header  # pylint: disable=unused-import
 
 

qiskit/extensions/standard/barrier.py

@@ -8,8 +8,8 @@
 """
 Barrier instruction.
 """
-from qiskit import QuantumCircuit
-from qiskit import QuantumRegister
+from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import QuantumRegister
 from qiskit.circuit import Instruction
 from qiskit.extensions.standard import header  # pylint: disable=unused-import
 

qiskit/mapper/__init__.py

@@ -9,7 +9,6 @@
 
 from ._compiling import two_qubit_kak, euler_angles_1q
 from ._coupling import CouplingMap
-from ._couplingerror import CouplingError
 from ._layout import Layout
-from ._mappererror import MapperError
+from .exceptions import CouplingError, MapperError
 from ._mapping import swap_mapper

qiskit/mapper/_compiling.py

@@ -15,7 +15,7 @@
 import numpy as np
 import scipy.linalg as la
 
-from ._mappererror import MapperError
+from .exceptions import MapperError
 
 
 def euler_angles_1q(unitary_matrix):

qiskit/mapper/_coupling.py

@@ -18,7 +18,7 @@
 
 import warnings
 import networkx as nx
-from ._couplingerror import CouplingError
+from .exceptions import CouplingError
 
 
 class CouplingMap:

qiskit/mapper/_layout.py

@@ -12,8 +12,7 @@
 Virtual (qu)bits are tuples (eg, `(QuantumRegister(3, 'qr'),2)`.
 Physical (qu)bits are numbers.
 """
-
-from qiskit import QiskitError
+from qiskit.mapper.exceptions import LayoutError
 
 
 class Layout(dict):
@@ -182,16 +181,3 @@ def combine_into_edge_map(self, another_layout):
             edge_map[virtual] = another_layout[physical]
 
         return edge_map
-
-
-class LayoutError(QiskitError):
-    """Errors raised by the layout object."""
-
-    def __init__(self, *msg):
-        """Set the error message."""
-        super().__init__(*msg)
-        self.msg = ' '.join(msg)
-
-    def __str__(self):
-        """Return the message."""
-        return repr(self.msg)