@@ -57,6 +57,7 @@ def _is_node_return_ended(node):
             return False
         exc_name = exc.pytype().split('.')[-1]
         handlers = utils.get_exception_handlers(node, exc_name)
+        handlers = list(handlers) if handlers is not None else list()
         if handlers:
             # among all the handlers handling the exception at least one
             # must end with a return statement
# -*- coding: utf-8 -*-
# Copyright (c) 2016-2017 Claudiu Popa <pcmanticore@gmail.com>
# Copyright (c) 2016-2017 Łukasz Rogalski <rogalski.91@gmail.com>
# Copyright (c) 2016 Moises Lopez <moylop260@vauxoo.com>
# Copyright (c) 2016 Alexander Todorov <atodorov@otb.bg>
# Copyright (c) 2017 Hugo <hugovk@users.noreply.github.com>
# Copyright (c) 2017 Bryce Guinta <bryce.paul.guinta@gmail.com>
# Copyright (c) 2017 hippo91 <guillaume.peillex@gmail.com>
# Copyright (c) 2017 Łukasz Sznuk <ls@rdprojekt.pl>
# Copyright (c) 2017 Alex Hearn <alex.d.hearn@gmail.com>
# Copyright (c) 2017 Antonio Ossa <aaossa@uc.cl>
# Copyright (c) 2017 Ville Skyttä <ville.skytta@iki.fi>

# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
# For details: https://github.com/PyCQA/pylint/blob/master/COPYING

"""Looks for code which can be refactored."""

import collections
import itertools
import tokenize

import astroid
from astroid import decorators
import six

from pylint import interfaces
from pylint import checkers
from pylint import utils as lint_utils
from pylint.checkers import utils


def _all_elements_are_true(gen):
    values = list(gen)
    return values and all(values)


def _is_node_return_ended(node):
    """Check if the node ends with an explicit return statement.

    Args:
        node (astroid.NodeNG): node to be checked.

    Returns:
        bool: True if the node ends with an explicit statement, False otherwise.

    """
    # Recursion base case
    if isinstance(node, astroid.Return):
        return True
    if isinstance(node, astroid.Raise):
        # a Raise statement doesn't need to end with a return statement
        # but if the exception raised is handled, then the handler has to
        # ends with a return statement
        exc = utils.safe_infer(node.exc)
        if exc is None or exc is astroid.Uninferable:
            return False
        exc_name = exc.pytype().split('.')[-1]
        handlers = utils.get_exception_handlers(node, exc_name)
        if handlers:
            # among all the handlers handling the exception at least one
            # must end with a return statement
            return any(_is_node_return_ended(_handler) for _handler in handlers)
        # if no handlers handle the exception then it's ok
        return True
    if isinstance(node, astroid.If):
        # if statement is returning if there are exactly two return statements in its
        # children : one for the body part, the other for the orelse part
        return_stmts = [_is_node_return_ended(_child) for _child in node.get_children()]
        return sum(return_stmts) == 2
    # recurses on the children of the node except for those which are except handler
    # because one cannot be sure that the handler will really be used
    return any(_is_node_return_ended(_child) for _child in node.get_children()
               if not isinstance(_child, astroid.ExceptHandler))


def _if_statement_is_always_returning(if_node):
    def _has_return_node(elems, scope):
        for node in elems:
            if isinstance(node, astroid.If):
                yield _if_statement_is_always_returning(node)
            elif isinstance(node, astroid.Return):
                yield node.scope() is scope

    scope = if_node.scope()
    body_returns = _all_elements_are_true(
        _has_return_node(if_node.body, scope=scope)
    )
    if if_node.orelse:
        orelse_returns = _all_elements_are_true(
            _has_return_node(if_node.orelse, scope=scope)
        )
    else:
        orelse_returns = False

    return body_returns and orelse_returns


class RefactoringChecker(checkers.BaseTokenChecker):
    """Looks for code which can be refactored

    This checker also mixes the astroid and the token approaches
    in order to create knowledge about whether a "else if" node
    is a true "else if" node, or a "elif" node.
    """

    __implements__ = (interfaces.ITokenChecker, interfaces.IAstroidChecker)

    name = 'refactoring'

    msgs = {
        'R1701': ("Consider merging these isinstance calls to isinstance(%s, (%s))",
                  "consider-merging-isinstance",
                  "Used when multiple consecutive isinstance calls can be merged into one."),
        'R1706': ("Consider using ternary (%s)",
                  "consider-using-ternary",
                  "Used when one of known pre-python 2.5 ternary syntax is used.",),
        'R1709': ("Boolean expression may be simplified to %s",
                  "simplify-boolean-expression",
                  "Emitted when redundant pre-python 2.5 ternary syntax is used.",),
        'R1702': ('Too many nested blocks (%s/%s)',
                  'too-many-nested-blocks',
                  'Used when a function or a method has too many nested '
                  'blocks. This makes the code less understandable and '
                  'maintainable.',
                  {'old_names': [('R0101', 'too-many-nested-blocks')]}),
        'R1703': ('The if statement can be replaced with %s',
                  'simplifiable-if-statement',
                  'Used when an if statement can be replaced with '
                  '\'bool(test)\'. ',
                  {'old_names': [('R0102', 'simplifiable-if-statement')]}),
        'R1704': ('Redefining argument with the local name %r',
                  'redefined-argument-from-local',
                  'Used when a local name is redefining an argument, which might '
                  'suggest a potential error. This is taken in account only for '
                  'a handful of name binding operations, such as for iteration, '
                  'with statement assignment and exception handler assignment.'
                 ),
        'R1705': ('Unnecessary "else" after "return"',
                  'no-else-return',
                  'Used in order to highlight an unnecessary block of '
                  'code following an if containing a return statement. '
                  'As such, it will warn when it encounters an else '
                  'following a chain of ifs, all of them containing a '
                  'return statement.'
                 ),
        'R1707': ('Disallow trailing comma tuple',
                  'trailing-comma-tuple',
                  'In Python, a tuple is actually created by the comma symbol, '
                  'not by the parentheses. Unfortunately, one can actually create a '
                  'tuple by misplacing a trailing comma, which can lead to potential '
                  'weird bugs in your code. You should always use parentheses '
                  'explicitly for creating a tuple.',
                  {'minversion': (3, 0)}),
        'R1708': ('Do not raise StopIteration in generator, use return statement instead',
                  'stop-iteration-return',
                  'According to PEP479, the raise of StopIteration to end the loop of '
                  'a generator may lead to hard to find bugs. This PEP specify that '
                  'raise StopIteration has to be replaced by a simple return statement',
                  {'minversion': (3, 0)}),
        'R1710': ('Either all return statements in a function should return an expression, '
                  'or none of them should.',
                  'inconsistent-return-statements',
                  'According to PEP8, if any return statement returns an expression, '
                  'any return statements where no value is returned should explicitly '
                  'state this as return None, and an explicit return statement '
                  'should be present at the end of the function (if reachable)'
                 ),
    }
    options = (('max-nested-blocks',
                {'default': 5, 'type': 'int', 'metavar': '<int>',
                 'help': 'Maximum number of nested blocks for function / '
                         'method body'}
               ),)

    priority = 0

    def __init__(self, linter=None):
        checkers.BaseTokenChecker.__init__(self, linter)
        self._return_nodes = {}
        self._init()

    def _init(self):
        self._nested_blocks = []
        self._elifs = []
        self._nested_blocks_msg = None

    @decorators.cachedproperty
    def _dummy_rgx(self):
        return lint_utils.get_global_option(
            self, 'dummy-variables-rgx', default=None)

    @staticmethod
    def _is_bool_const(node):
        return (isinstance(node.value, astroid.Const)
                and isinstance(node.value.value, bool))

    def _is_actual_elif(self, node):
        """Check if the given node is an actual elif

        This is a problem we're having with the builtin ast module,
        which splits `elif` branches into a separate if statement.
        Unfortunately we need to know the exact type in certain
        cases.
        """

        if isinstance(node.parent, astroid.If):
            orelse = node.parent.orelse
            # current if node must directly follow a "else"
            if orelse and orelse == [node]:
                if (node.lineno, node.col_offset) in self._elifs:
                    return True
        return False

    def _check_simplifiable_if(self, node):
        """Check if the given if node can be simplified.

        The if statement can be reduced to a boolean expression
        in some cases. For instance, if there are two branches
        and both of them return a boolean value that depends on
        the result of the statement's test, then this can be reduced
        to `bool(test)` without losing any functionality.
        """

        if self._is_actual_elif(node):
            # Not interested in if statements with multiple branches.
            return
        if len(node.orelse) != 1 or len(node.body) != 1:
            return

        # Check if both branches can be reduced.
        first_branch = node.body[0]
        else_branch = node.orelse[0]
        if isinstance(first_branch, astroid.Return):
            if not isinstance(else_branch, astroid.Return):
                return
            first_branch_is_bool = self._is_bool_const(first_branch)
            else_branch_is_bool = self._is_bool_const(else_branch)
            reduced_to = "'return bool(test)'"
        elif isinstance(first_branch, astroid.Assign):
            if not isinstance(else_branch, astroid.Assign):
                return
            first_branch_is_bool = self._is_bool_const(first_branch)
            else_branch_is_bool = self._is_bool_const(else_branch)
            reduced_to = "'var = bool(test)'"
        else:
            return

        if not first_branch_is_bool or not else_branch_is_bool:
            return
        if not first_branch.value.value:
            # This is a case that can't be easily simplified and
            # if it can be simplified, it will usually result in a
            # code that's harder to understand and comprehend.
            # Let's take for instance `arg and arg <= 3`. This could theoretically be
            # reduced to `not arg or arg > 3`, but the net result is that now the
            # condition is harder to understand, because it requires understanding of
            # an extra clause:
            #   * first, there is the negation of truthness with `not arg`
            #   * the second clause is `arg > 3`, which occurs when arg has a
            #     a truth value, but it implies that `arg > 3` is equivalent
            #     with `arg and arg > 3`, which means that the user must
            #     think about this assumption when evaluating `arg > 3`.
            #     The original form is easier to grasp.
            return

        self.add_message('simplifiable-if-statement', node=node,
                         args=(reduced_to,))

    def process_tokens(self, tokens):
        # Process tokens and look for 'if' or 'elif'
        for index, token in enumerate(tokens):
            token_string = token[1]
            if token_string == 'elif':
                # AST exists by the time process_tokens is called, so
                # it's safe to assume tokens[index+1]
                # exists. tokens[index+1][2] is the elif's position as
                # reported by CPython and PyPy,
                # tokens[index][2] is the actual position and also is
                # reported by IronPython.
                self._elifs.extend([tokens[index][2], tokens[index+1][2]])
            elif six.PY3 and is_trailing_comma(tokens, index):
                if self.linter.is_message_enabled('trailing-comma-tuple'):
                    self.add_message('trailing-comma-tuple',
                                     line=token.start[0])

    def leave_module(self, _):
        self._init()

    @utils.check_messages('too-many-nested-blocks')
    def visit_tryexcept(self, node):
        self._check_nested_blocks(node)

    visit_tryfinally = visit_tryexcept
    visit_while = visit_tryexcept

    def _check_redefined_argument_from_local(self, name_node):
        if self._dummy_rgx and self._dummy_rgx.match(name_node.name):
            return
        if not name_node.lineno:
            # Unknown position, maybe it is a manually built AST?
            return

        scope = name_node.scope()
        if not isinstance(scope, astroid.FunctionDef):
            return

        for defined_argument in scope.args.nodes_of_class(astroid.AssignName):
            if defined_argument.name == name_node.name:
                self.add_message('redefined-argument-from-local',
                                 node=name_node,
                                 args=(name_node.name, ))

    @utils.check_messages('redefined-argument-from-local',
                          'too-many-nested-blocks')
    def visit_for(self, node):
        self._check_nested_blocks(node)

        for name in node.target.nodes_of_class(astroid.AssignName):
            self._check_redefined_argument_from_local(name)

    @utils.check_messages('redefined-argument-from-local')
    def visit_excepthandler(self, node):
        if node.name and isinstance(node.name, astroid.AssignName):
            self._check_redefined_argument_from_local(node.name)

    @utils.check_messages('redefined-argument-from-local')
    def visit_with(self, node):
        for _, names in node.items:
            if not names:
                continue
            for name in names.nodes_of_class(astroid.AssignName):
                self._check_redefined_argument_from_local(name)

    def _check_superfluous_else_return(self, node):
        if not node.orelse:
            # Not interested in if statements without else.
            return

        if _if_statement_is_always_returning(node) and not self._is_actual_elif(node):
            self.add_message('no-else-return', node=node)

    @utils.check_messages('too-many-nested-blocks', 'simplifiable-if-statement',
                          'no-else-return',)
    def visit_if(self, node):
        self._check_simplifiable_if(node)
        self._check_nested_blocks(node)
        self._check_superfluous_else_return(node)

    @utils.check_messages('too-many-nested-blocks', 'inconsistent-return-statements')
    def leave_functiondef(self, node):
        # check left-over nested blocks stack
        self._emit_nested_blocks_message_if_needed(self._nested_blocks)
        # new scope = reinitialize the stack of nested blocks
        self._nested_blocks = []
        # check consistent return statements
        self._check_consistent_returns(node)
        self._return_nodes[node.name] = []

    @utils.check_messages('stop-iteration-return')
    def visit_raise(self, node):
        self._check_stop_iteration_inside_generator(node)

    def _check_stop_iteration_inside_generator(self, node):
        """Check if an exception of type StopIteration is raised inside a generator"""
        frame = node.frame()
        if not isinstance(frame, astroid.FunctionDef) or not frame.is_generator():
            return
        if utils.node_ignores_exception(node, StopIteration):
            return
        if not node.exc:
            return
        exc = utils.safe_infer(node.exc)
        if exc is not None and self._check_exception_inherit_from_stopiteration(exc):
            self.add_message('stop-iteration-return', node=node)

    @staticmethod
    def _check_exception_inherit_from_stopiteration(exc):
        """Return True if the exception node in argument inherit from StopIteration"""
        stopiteration_qname = '{}.StopIteration'.format(utils.EXCEPTIONS_MODULE)
        return any(_class.qname() == stopiteration_qname for _class in exc.mro())

    @utils.check_messages('stop-iteration-return')
    def visit_call(self, node):
        self._check_raising_stopiteration_in_generator_next_call(node)

    def _check_raising_stopiteration_in_generator_next_call(self, node):
        """Check if a StopIteration exception is raised by the call to next function"""
        inferred = utils.safe_infer(node.func)
        if getattr(inferred, 'name', '') == 'next':
            frame = node.frame()
            if (isinstance(frame, astroid.FunctionDef) and frame.is_generator()
                    and not utils.node_ignores_exception(node, StopIteration)):
                self.add_message('stop-iteration-return', node=node)

    def _check_nested_blocks(self, node):
        """Update and check the number of nested blocks
        """
        # only check block levels inside functions or methods
        if not isinstance(node.scope(), astroid.FunctionDef):
            return
        # messages are triggered on leaving the nested block. Here we save the
        # stack in case the current node isn't nested in the previous one
        nested_blocks = self._nested_blocks[:]
        if node.parent == node.scope():
            self._nested_blocks = [node]
        else:
            # go through ancestors from the most nested to the less
            for ancestor_node in reversed(self._nested_blocks):
                if ancestor_node == node.parent:
                    break
                self._nested_blocks.pop()
            # if the node is a elif, this should not be another nesting level
            if isinstance(node, astroid.If) and self._is_actual_elif(node):
                if self._nested_blocks:
                    self._nested_blocks.pop()
            self._nested_blocks.append(node)

        # send message only once per group of nested blocks
        if len(nested_blocks) > len(self._nested_blocks):
            self._emit_nested_blocks_message_if_needed(nested_blocks)

    def _emit_nested_blocks_message_if_needed(self, nested_blocks):
        if len(nested_blocks) > self.config.max_nested_blocks:
            self.add_message('too-many-nested-blocks', node=nested_blocks[0],
                             args=(len(nested_blocks), self.config.max_nested_blocks))

    @staticmethod
    def _duplicated_isinstance_types(node):
        """Get the duplicated types from the underlying isinstance calls.

        :param astroid.BoolOp node: Node which should contain a bunch of isinstance calls.
        :returns: Dictionary of the comparison objects from the isinstance calls,
                  to duplicate values from consecutive calls.
        :rtype: dict
        """
        duplicated_objects = set()
        all_types = collections.defaultdict(set)

        for call in node.values:
            if not isinstance(call, astroid.Call) or len(call.args) != 2:
                continue

            inferred = utils.safe_infer(call.func)
            if not inferred or not utils.is_builtin_object(inferred):
                continue

            if inferred.name != 'isinstance':
                continue

            isinstance_object = call.args[0].as_string()
            isinstance_types = call.args[1]

            if isinstance_object in all_types:
                duplicated_objects.add(isinstance_object)

            if isinstance(isinstance_types, astroid.Tuple):
                elems = [class_type.as_string() for class_type in isinstance_types.itered()]
            else:
                elems = [isinstance_types.as_string()]
            all_types[isinstance_object].update(elems)

        # Remove all keys which not duplicated
        return {key: value for key, value in all_types.items()
                if key in duplicated_objects}

    @utils.check_messages('consider-merging-isinstance')
    def visit_boolop(self, node):
        '''Check isinstance calls which can be merged together.'''
        if node.op != 'or':
            return

        first_args = self._duplicated_isinstance_types(node)
        for duplicated_name, class_names in first_args.items():
            names = sorted(name for name in class_names)
            self.add_message('consider-merging-isinstance',
                             node=node,
                             args=(duplicated_name, ', '.join(names)))

    @utils.check_messages('simplify-boolean-expression', 'consider-using-ternary')
    def visit_assign(self, node):
        if self._is_and_or_ternary(node.value):
            cond, truth_value, false_value = self._and_or_ternary_arguments(node.value)
        elif self._is_seq_based_ternary(node.value):
            cond, truth_value, false_value = self._seq_based_ternary_params(node.value)
        else:
            return

        if truth_value.bool_value() is False:
            message = 'simplify-boolean-expression'
            suggestion = false_value.as_string()
        else:
            message = 'consider-using-ternary'
            suggestion = '{truth} if {cond} else {false}'.format(
                truth=truth_value.as_string(),
                cond=cond.as_string(),
                false=false_value.as_string()
            )
        self.add_message(message, node=node, args=(suggestion,))

    visit_return = visit_assign

    @staticmethod
    def _is_and_or_ternary(node):
        """
        Returns true if node is 'condition and true_value else false_value' form.

        All of: condition, true_value and false_value should not be a complex boolean expression
        """
        return (isinstance(node, astroid.BoolOp)
                and node.op == 'or' and len(node.values) == 2
                and isinstance(node.values[0], astroid.BoolOp)
                and not isinstance(node.values[1], astroid.BoolOp)
                and node.values[0].op == 'and'
                and not isinstance(node.values[0].values[1], astroid.BoolOp)
                and len(node.values[0].values) == 2)

    @staticmethod
    def _and_or_ternary_arguments(node):
        false_value = node.values[1]
        condition, true_value = node.values[0].values
        return condition, true_value, false_value

    @staticmethod
    def _is_seq_based_ternary(node):
        """Returns true if node is '[false_value,true_value][condition]' form"""
        return (isinstance(node, astroid.Subscript)
                and isinstance(node.value, (astroid.Tuple, astroid.List))
                and len(node.value.elts) == 2 and isinstance(node.slice, astroid.Index))

    @staticmethod
    def _seq_based_ternary_params(node):
        false_value, true_value = node.value.elts
        condition = node.slice.value
        return condition, true_value, false_value

    def visit_functiondef(self, node):
        self._return_nodes[node.name] = []
        return_nodes = node.nodes_of_class(astroid.Return)
        self._return_nodes[node.name] = [_rnode for _rnode in return_nodes
                                         if _rnode.frame() == node.frame()]

    def _check_consistent_returns(self, node):
        """Check that all return statements inside a function are consistent.

        Return statements are consistent if:
            - all returns are explicit and if there is no implicit return;
            - all returns are empty and if there is, possibly, an implicit return.

        Args:
            node (astroid.FunctionDef): the function holding the return statements.

        """
        # explicit return statements are those with a not None value
        explicit_returns = [_node for _node in self._return_nodes[node.name]
                            if _node.value is not None]
        if not explicit_returns:
            return
        if (len(explicit_returns) == len(self._return_nodes[node.name])
                and _is_node_return_ended(node)):
            return
        self.add_message('inconsistent-return-statements', node=node)


class RecommandationChecker(checkers.BaseChecker):
    __implements__ = (interfaces.IAstroidChecker,)
    name = 'refactoring'
    msgs = {'C0200': ('Consider using enumerate instead of iterating with range and len',
                      'consider-using-enumerate',
                      'Emitted when code that iterates with range and len is '
                      'encountered. Such code can be simplified by using the '
                      'enumerate builtin.'),
            'C0201': ('Consider iterating the dictionary directly instead of calling .keys()',
                      'consider-iterating-dictionary',
                      'Emitted when the keys of a dictionary are iterated through the .keys() '
                      'method. It is enough to just iterate through the dictionary itself, as '
                      'in "for key in dictionary".'),
           }

    @staticmethod
    def _is_builtin(node, function):
        inferred = utils.safe_infer(node)
        if not inferred:
            return False
        return utils.is_builtin_object(inferred) and inferred.name == function

    @utils.check_messages('consider-iterating-dictionary')
    def visit_call(self, node):
        inferred = utils.safe_infer(node.func)
        if not inferred:
            return
        if not isinstance(inferred, astroid.BoundMethod):
            return
        if not isinstance(inferred.bound, astroid.Dict) or inferred.name != 'keys':
            return

        if isinstance(node.parent, (astroid.For, astroid.Comprehension)):
            self.add_message('consider-iterating-dictionary', node=node)

    @utils.check_messages('consider-using-enumerate')
    def visit_for(self, node):
        """Emit a convention whenever range and len are used for indexing."""
        # Verify that we have a `range([start], len(...), [stop])` call and
        # that the object which is iterated is used as a subscript in the
        # body of the for.

        # Is it a proper range call?
        if not isinstance(node.iter, astroid.Call):
            return
        if not self._is_builtin(node.iter.func, 'range'):
            return
        if len(node.iter.args) == 2 and not _is_constant_zero(node.iter.args[0]):
            return
        if len(node.iter.args) > 2:
            return

        # Is it a proper len call?
        if not isinstance(node.iter.args[-1], astroid.Call):
            return
        second_func = node.iter.args[-1].func
        if not self._is_builtin(second_func, 'len'):
            return
        len_args = node.iter.args[-1].args
        if not len_args or len(len_args) != 1:
            return
        iterating_object = len_args[0]
        if not isinstance(iterating_object, astroid.Name):
            return

        # Verify that the body of the for loop uses a subscript
        # with the object that was iterated. This uses some heuristics
        # in order to make sure that the same object is used in the
        # for body.
        for child in node.body:
            for subscript in child.nodes_of_class(astroid.Subscript):
                if not isinstance(subscript.value, astroid.Name):
                    continue
                if not isinstance(subscript.slice, astroid.Index):
                    continue
                if not isinstance(subscript.slice.value, astroid.Name):
                    continue
                if subscript.slice.value.name != node.target.name:
                    continue
                if iterating_object.name != subscript.value.name:
                    continue
                if subscript.value.scope() != node.scope():
                    # Ignore this subscript if it's not in the same
                    # scope. This means that in the body of the for
                    # loop, another scope was created, where the same
                    # name for the iterating object was used.
                    continue
                self.add_message('consider-using-enumerate', node=node)
                return


class NotChecker(checkers.BaseChecker):
    """checks for too many not in comparison expressions

    - "not not" should trigger a warning
    - "not" followed by a comparison should trigger a warning
    """
    __implements__ = (interfaces.IAstroidChecker,)
    msgs = {'C0113': ('Consider changing "%s" to "%s"',
                      'unneeded-not',
                      'Used when a boolean expression contains an unneeded '
                      'negation.'),
           }
    name = 'basic'
    reverse_op = {'<': '>=', '<=': '>', '>': '<=', '>=': '<', '==': '!=',
                  '!=': '==', 'in': 'not in', 'is': 'is not'}
    # sets are not ordered, so for example "not set(LEFT_VALS) <= set(RIGHT_VALS)" is
    # not equivalent to "set(LEFT_VALS) > set(RIGHT_VALS)"
    skipped_nodes = (astroid.Set,)
    # 'builtins' py3, '__builtin__' py2
    skipped_classnames = ['%s.%s' % (six.moves.builtins.__name__, qname)
                          for qname in ('set', 'frozenset')]

    @utils.check_messages('unneeded-not')
    def visit_unaryop(self, node):
        if node.op != 'not':
            return
        operand = node.operand

        if isinstance(operand, astroid.UnaryOp) and operand.op == 'not':
            self.add_message('unneeded-not', node=node,
                             args=(node.as_string(),
                                   operand.operand.as_string()))
        elif isinstance(operand, astroid.Compare):
            left = operand.left
            # ignore multiple comparisons
            if len(operand.ops) > 1:
                return
            operator, right = operand.ops[0]
            if operator not in self.reverse_op:
                return
            # Ignore __ne__ as function of __eq__
            frame = node.frame()
            if frame.name == '__ne__' and operator == '==':
                return
            for _type in (utils.node_type(left), utils.node_type(right)):
                if not _type:
                    return
                if isinstance(_type, self.skipped_nodes):
                    return
                if (isinstance(_type, astroid.Instance) and
                        _type.qname() in self.skipped_classnames):
                    return
            suggestion = '%s %s %s' % (left.as_string(),
                                       self.reverse_op[operator],
                                       right.as_string())
            self.add_message('unneeded-not', node=node,
                             args=(node.as_string(), suggestion))


def _is_len_call(node):
    """Checks if node is len(SOMETHING)."""
    return (isinstance(node, astroid.Call) and isinstance(node.func, astroid.Name) and
            node.func.name == 'len')

def _is_constant_zero(node):
    return isinstance(node, astroid.Const) and node.value == 0

def _node_is_test_condition(node):
    """ Checks if node is an if, while, assert or if expression statement."""
    return isinstance(node, (astroid.If, astroid.While, astroid.Assert, astroid.IfExp))


class LenChecker(checkers.BaseChecker):
    """Checks for incorrect usage of len() inside conditions.
    Pep8 states:
    For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

        Yes: if not seq:
             if seq:

        No: if len(seq):
            if not len(seq):

    Problems detected:
    * if len(sequence):
    * if not len(sequence):
    * if len(sequence) == 0:
    * if len(sequence) != 0:
    * if len(sequence) > 0:
    """

    __implements__ = (interfaces.IAstroidChecker,)

    # configuration section name
    name = 'len'
    msgs = {'C1801': ('Do not use `len(SEQUENCE)` to determine if a sequence is empty',
                      'len-as-condition',
                      'Used when Pylint detects that len(sequence) is being used inside '
                      'a condition to determine if a sequence is empty. Instead of '
                      'comparing the length to 0, rely on the fact that empty sequences '
                      'are false.'),
           }

    priority = -2
    options = ()

    @utils.check_messages('len-as-condition')
    def visit_call(self, node):
        # a len(S) call is used inside a test condition
        # could be if, while, assert or if expression statement
        # e.g. `if len(S):`
        if _is_len_call(node):
            # the len() call could also be nested together with other
            # boolean operations, e.g. `if z or len(x):`
            parent = node.parent
            while isinstance(parent, astroid.BoolOp):
                parent = parent.parent

            # we're finally out of any nested boolean operations so check if
            # this len() call is part of a test condition
            if not _node_is_test_condition(parent):
                return
            if not (node is parent.test or parent.test.parent_of(node)):
                return
            self.add_message('len-as-condition', node=node)

    @utils.check_messages('len-as-condition')
    def visit_unaryop(self, node):
        """`not len(S)` must become `not S` regardless if the parent block
        is a test condition or something else (boolean expression)
        e.g. `if not len(S):`"""
        if isinstance(node, astroid.UnaryOp) and node.op == 'not' and _is_len_call(node.operand):
            self.add_message('len-as-condition', node=node)

    @utils.check_messages('len-as-condition')
    def visit_compare(self, node):
        # compare nodes are trickier because the len(S) expression
        # may be somewhere in the middle of the node

        # note: astroid.Compare has the left most operand in node.left
        # while the rest are a list of tuples in node.ops
        # the format of the tuple is ('compare operator sign', node)
        # here we squash everything into `ops` to make it easier for processing later
        ops = [('', node.left)]
        ops.extend(node.ops)
        ops = list(itertools.chain(*ops))

        for ops_idx in range(len(ops) - 2):
            op_1 = ops[ops_idx]
            op_2 = ops[ops_idx + 1]
            op_3 = ops[ops_idx + 2]
            error_detected = False

            # 0 ?? len()
            if _is_constant_zero(op_1) and op_2 in ['==', '!=', '<'] and _is_len_call(op_3):
                error_detected = True
            # len() ?? 0
            elif _is_len_call(op_1) and op_2 in ['==', '!=', '>'] and _is_constant_zero(op_3):
                error_detected = True

            if error_detected:
                parent = node.parent
                # traverse the AST to figure out if this comparison was part of
                # a test condition
                while parent and not _node_is_test_condition(parent):
                    parent = parent.parent

                # report only if this len() comparison is part of a test condition
                # for example: return len() > 0 should not report anything
                if _node_is_test_condition(parent):
                    self.add_message('len-as-condition', node=node)


def is_trailing_comma(tokens, index):
    """Check if the given token is a trailing comma

    :param tokens: Sequence of modules tokens
    :type tokens: list[tokenize.TokenInfo]
    :param int index: Index of token under check in tokens
    :returns: True if the token is a comma which trails an expression
    :rtype: bool
    """
    token = tokens[index]
    if token.exact_type != tokenize.COMMA:
        return False
    # Must have remaining tokens on the same line such as NEWLINE
    left_tokens = itertools.islice(tokens, index + 1, None)
    same_line_remaining_tokens = list(itertools.takewhile(
        lambda other_token, _token=token: other_token.start[0] == _token.start[0],
        left_tokens
    ))
    # Note: If the newline is tokenize.NEWLINE and not tokenize.NL
    # then the newline denotes the end of expression
    is_last_element = all(
        other_token.type in (tokenize.NEWLINE, tokenize.COMMENT)
        for other_token in same_line_remaining_tokens
    )
    if not same_line_remaining_tokens or not is_last_element:
        return False
    def get_curline_index_start():
        """Get the index denoting the start of the current line"""
        for subindex, token in enumerate(reversed(tokens[:index])):
            # See Lib/tokenize.py and Lib/token.py in cpython for more info
            if token.type in (tokenize.NEWLINE, tokenize.NL):
                return index - subindex
        return 0
    curline_start = get_curline_index_start()
    for prevtoken in tokens[curline_start:index]:
        if '=' in prevtoken.string:
            return True
    return False


def register(linter):
    """Required method to auto register this checker."""
    linter.register_checker(RefactoringChecker(linter))
    linter.register_checker(NotChecker(linter))
    linter.register_checker(RecommandationChecker(linter))
    linter.register_checker(LenChecker(linter))

Not sure what pylint's coding style is here, but usually `[]` is more idiomatic than `list()`.