path: root/pylint/checkers/exceptions.py

# -*- coding: utf-8 -*-
# Copyright (c) 2006-2011, 2013-2014 LOGILAB S.A. (Paris, FRANCE) <contact@logilab.fr>
# Copyright (c) 2011-2014 Google, Inc.
# Copyright (c) 2012 Tim Hatch <tim@timhatch.com>
# Copyright (c) 2013-2018 Claudiu Popa <pcmanticore@gmail.com>
# Copyright (c) 2014 Brett Cannon <brett@python.org>
# Copyright (c) 2014 Arun Persaud <arun@nubati.net>
# Copyright (c) 2015 Rene Zhang <rz99@cornell.edu>
# Copyright (c) 2015 Florian Bruhin <me@the-compiler.org>
# Copyright (c) 2015 Steven Myint <hg@stevenmyint.com>
# Copyright (c) 2015 Ionel Cristian Maries <contact@ionelmc.ro>
# Copyright (c) 2016 Erik <erik.eriksson@yahoo.com>
# Copyright (c) 2016 Jakub Wilk <jwilk@jwilk.net>
# Copyright (c) 2017 Łukasz Rogalski <rogalski.91@gmail.com>
# Copyright (c) 2017 Martin von Gagern <gagern@google.com>
# Copyright (c) 2018 Mike Frysinger <vapier@gmail.com>
# Copyright (c) 2018 ssolanki <sushobhitsolanki@gmail.com>
# Copyright (c) 2018 Alexander Todorov <atodorov@otb.bg>
# Copyright (c) 2018 Ville Skyttä <ville.skytta@upcloud.com>

# 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

"""Checks for various exception related errors."""
import builtins
import inspect
import sys
import typing

import astroid

from pylint import checkers
from pylint.checkers import utils
from pylint import interfaces


def _builtin_exceptions():
    def predicate(obj):
        return isinstance(obj, type) and issubclass(obj, BaseException)

    members = inspect.getmembers(builtins, predicate)
    return {exc.__name__ for (_, exc) in members}


def _annotated_unpack_infer(stmt, context=None):
    """
    Recursively generate nodes inferred by the given statement.
    If the inferred value is a list or a tuple, recurse on the elements.
    Returns an iterator which yields tuples in the format
    ('original node', 'infered node').
    """
    if isinstance(stmt, (astroid.List, astroid.Tuple)):
        for elt in stmt.elts:
            inferred = utils.safe_infer(elt)
            if inferred and inferred is not astroid.Uninferable:
                yield elt, inferred
        return
    for infered in stmt.infer(context):
        if infered is astroid.Uninferable:
            continue
        yield stmt, infered


def _is_raising(body: typing.List) -> bool:
    """Return true if the given statement node raise an exception"""
    for node in body:
        if isinstance(node, astroid.Raise):
            return True
    return False


PY3K = sys.version_info >= (3, 0)
OVERGENERAL_EXCEPTIONS = ('Exception',)
BUILTINS_NAME = builtins.__name__

MSGS = {
    'E0701': ('Bad except clauses order (%s)',
              'bad-except-order',
              'Used when except clauses are not in the correct order (from the '
              'more specific to the more generic). If you don\'t fix the order, '
              'some exceptions may not be caught by the most specific handler.'),
    'E0702': ('Raising %s while only classes or instances are allowed',
              'raising-bad-type',
              'Used when something which is neither a class, an instance or a '
              'string is raised (i.e. a `TypeError` will be raised).'),
    'E0703': ('Exception context set to something which is not an '
              'exception, nor None',
              'bad-exception-context',
              'Used when using the syntax "raise ... from ...", '
              'where the exception context is not an exception, '
              'nor None.'),
    'E0704': ('The raise statement is not inside an except clause',
              'misplaced-bare-raise',
              'Used when a bare raise is not used inside an except clause. '
              'This generates an error, since there are no active exceptions '
              'to be reraised. An exception to this rule is represented by '
              'a bare raise inside a finally clause, which might work, as long '
              'as an exception is raised inside the try block, but it is '
              'nevertheless a code smell that must not be relied upon.'),
    'E0710': ('Raising a new style class which doesn\'t inherit from BaseException',
              'raising-non-exception',
              'Used when a new style class which doesn\'t inherit from '
              'BaseException is raised.'),
    'E0711': ('NotImplemented raised - should raise NotImplementedError',
              'notimplemented-raised',
              'Used when NotImplemented is raised instead of '
              'NotImplementedError'),
    'E0712': ('Catching an exception which doesn\'t inherit from Exception: %s',
              'catching-non-exception',
              'Used when a class which doesn\'t inherit from '
              'Exception is used as an exception in an except clause.'),
    'W0702': ('No exception type(s) specified',
              'bare-except',
              'Used when an except clause doesn\'t specify exceptions type to '
              'catch.'),
    'W0703': ('Catching too general exception %s',
              'broad-except',
              'Used when an except catches a too general exception, '
              'possibly burying unrelated errors.'),
    'W0705': ('Catching previously caught exception type %s',
              'duplicate-except',
              'Used when an except catches a type that was already caught by '
              'a previous handler.'),
    'W0706': ('The except handler raises immediately',
              'try-except-raise',
              'Used when an except handler uses raise as its first or only '
              'operator. This is useless because it raises back the exception '
              'immediately. Remove the raise operator or the entire '
              'try-except-raise block!'),
    'W0711': ('Exception to catch is the result of a binary "%s" operation',
              'binary-op-exception',
              'Used when the exception to catch is of the form '
              '"except A or B:".  If intending to catch multiple, '
              'rewrite as "except (A, B):"'),
    'W0715': ('Exception arguments suggest string formatting might be intended',
              'raising-format-tuple',
              'Used when passing multiple arguments to an exception '
              'constructor, the first of them a string literal containing what '
              'appears to be placeholders intended for formatting'),
    }


class BaseVisitor:
    """Base class for visitors defined in this module."""

    def __init__(self, checker, node):
        self._checker = checker
        self._node = node

    def visit(self, node):
        name = node.__class__.__name__.lower()
        dispatch_meth = getattr(self, 'visit_' + name, None)
        if dispatch_meth:
            dispatch_meth(node)
        else:
            self.visit_default(node)

    def visit_default(self, node): # pylint: disable=unused-argument
        """Default implementation for all the nodes."""


class ExceptionRaiseRefVisitor(BaseVisitor):
    """Visit references (anything that is not an AST leaf)."""

    def visit_name(self, name):
        if name.name == 'NotImplemented':
            self._checker.add_message(
                'notimplemented-raised',
                node=self._node)

    def visit_call(self, call):
        if isinstance(call.func, astroid.Name):
            self.visit_name(call.func)
        if (len(call.args) > 1 and
                isinstance(call.args[0], astroid.Const) and
                isinstance(call.args[0].value, str)):
            msg = call.args[0].value
            if ('%' in msg or
                    ('{' in msg and '}' in msg)):
                self._checker.add_message(
                    'raising-format-tuple',
                    node=self._node)


class ExceptionRaiseLeafVisitor(BaseVisitor):
    """Visitor for handling leaf kinds of a raise value."""

    def visit_const(self, const):
        if not isinstance(const.value, str):
            # raising-string will be emitted from python3 porting checker.
            self._checker.add_message('raising-bad-type', node=self._node,
                                      args=const.value.__class__.__name__)

    def visit_instance(self, instance):
        # pylint: disable=protected-access
        cls = instance._proxied
        self.visit_classdef(cls)

    # Exception instances have a particular class type
    visit_exceptioninstance = visit_instance

    def visit_classdef(self, cls):
        if (not utils.inherit_from_std_ex(cls) and
                utils.has_known_bases(cls)):
            if cls.newstyle:
                self._checker.add_message('raising-non-exception', node=self._node)
            else:
                self._checker.add_message('nonstandard-exception', node=self._node)

    def visit_tuple(self, tuple_node):
        if PY3K or not tuple_node.elts:
            self._checker.add_message('raising-bad-type',
                                      node=self._node,
                                      args='tuple')
            return

        # On Python 2, using the following is not an error:
        #    raise (ZeroDivisionError, None)
        #    raise (ZeroDivisionError, )
        # What's left to do is to check that the first
        # argument is indeed an exception. Verifying the other arguments
        # is not the scope of this check.
        first = tuple_node.elts[0]
        inferred = utils.safe_infer(first)
        if not inferred or inferred is astroid.Uninferable:
            return

        if (isinstance(inferred, astroid.Instance)
                and inferred.__class__.__name__ != 'Instance'):
            # TODO: explain why
            self.visit_default(tuple_node)
        else:
            self.visit(inferred)

    def visit_default(self, node):
        name = getattr(node, 'name', node.__class__.__name__)
        self._checker.add_message('raising-bad-type',
                                  node=self._node,
                                  args=name)


class ExceptionsChecker(checkers.BaseChecker):
    """Exception related checks."""

    __implements__ = interfaces.IAstroidChecker

    name = 'exceptions'
    msgs = MSGS
    priority = -4
    options = (('overgeneral-exceptions',
                {'default' : OVERGENERAL_EXCEPTIONS,
                 'type' : 'csv', 'metavar' : '<comma-separated class names>',
                 'help' : 'Exceptions that will emit a warning '
                          'when being caught. Defaults to "%s".' % (
                              ', '.join(OVERGENERAL_EXCEPTIONS),)}
               ),
              )

    def open(self):
        self._builtin_exceptions = _builtin_exceptions()
        super(ExceptionsChecker, self).open()

    @utils.check_messages('nonstandard-exception', 'misplaced-bare-raise',
                          'raising-bad-type', 'raising-non-exception',
                          'notimplemented-raised', 'bad-exception-context',
                          'raising-format-tuple')
    def visit_raise(self, node):
        if node.exc is None:
            self._check_misplaced_bare_raise(node)
            return

        if PY3K and node.cause:
            self._check_bad_exception_context(node)

        expr = node.exc
        ExceptionRaiseRefVisitor(self, node).visit(expr)

        try:
            inferred_value = expr.inferred()[-1]
        except astroid.InferenceError:
            pass
        else:
            if inferred_value:
                ExceptionRaiseLeafVisitor(self, node).visit(inferred_value)

    def _check_misplaced_bare_raise(self, node):
        # Filter out if it's present in __exit__.
        scope = node.scope()
        if (isinstance(scope, astroid.FunctionDef)
                and scope.is_method()
                and scope.name == '__exit__'):
            return

        current = node
        # Stop when a new scope is generated or when the raise
        # statement is found inside a TryFinally.
        ignores = (astroid.ExceptHandler, astroid.FunctionDef,)
        while current and not isinstance(current.parent, ignores):
            current = current.parent

        expected = (astroid.ExceptHandler,)
        if not current or not isinstance(current.parent, expected):
            self.add_message('misplaced-bare-raise', node=node)

    def _check_bad_exception_context(self, node):
        """Verify that the exception context is properly set.

        An exception context can be only `None` or an exception.
        """
        cause = utils.safe_infer(node.cause)
        if cause in (astroid.Uninferable, None):
            return

        if isinstance(cause, astroid.Const):
            if cause.value is not None:
                self.add_message('bad-exception-context',
                                 node=node)
        elif (not isinstance(cause, astroid.ClassDef) and
              not utils.inherit_from_std_ex(cause)):
            self.add_message('bad-exception-context',
                             node=node)

    def _check_catching_non_exception(self, handler, exc, part):
        if isinstance(exc, astroid.Tuple):
            # Check if it is a tuple of exceptions.
            inferred = [utils.safe_infer(elt) for elt in exc.elts]
            if any(node is astroid.Uninferable for node in inferred):
                # Don't emit if we don't know every component.
                return
            if all(node and (utils.inherit_from_std_ex(node) or
                             not utils.has_known_bases(node))
                   for node in inferred):
                return

        if not isinstance(exc, astroid.ClassDef):
            # Don't emit the warning if the infered stmt
            # is None, but the exception handler is something else,
            # maybe it was redefined.
            if (isinstance(exc, astroid.Const) and
                    exc.value is None):
                if ((isinstance(handler.type, astroid.Const) and
                     handler.type.value is None) or
                        handler.type.parent_of(exc)):
                    # If the exception handler catches None or
                    # the exception component, which is None, is
                    # defined by the entire exception handler, then
                    # emit a warning.
                    self.add_message('catching-non-exception',
                                     node=handler.type,
                                     args=(part.as_string(), ))
            else:
                self.add_message('catching-non-exception',
                                 node=handler.type,
                                 args=(part.as_string(), ))
            return

        if (not utils.inherit_from_std_ex(exc) and
                exc.name not in self._builtin_exceptions):
            if utils.has_known_bases(exc):
                self.add_message('catching-non-exception',
                                 node=handler.type,
                                 args=(exc.name, ))

    def _check_try_except_raise(self, node):

        def gather_exceptions_from_handler(handler):
            exceptions = []
            if handler.type:
                exceptions_in_handler = utils.safe_infer(handler.type)
                if isinstance(exceptions_in_handler, astroid.Tuple):
                    exceptions = {exception
                                  for exception in exceptions_in_handler.elts
                                  if isinstance(exception, astroid.Name)}
                elif exceptions_in_handler:
                    exceptions = [exceptions_in_handler]
            return exceptions

        bare_raise = False
        handler_having_bare_raise = None
        excs_in_bare_handler = []
        for handler in node.handlers:
            if bare_raise:
                # check that subsequent handler is not parent of handler which had bare raise.
                # since utils.safe_infer can fail for bare except, check it before.
                # also break early if bare except is followed by bare except.

                excs_in_current_handler = gather_exceptions_from_handler(handler)
                if not excs_in_current_handler:
                    bare_raise = False
                    break

                for exc_in_current_handler in excs_in_current_handler:
                    inferred_current = utils.safe_infer(exc_in_current_handler)
                    if any(utils.is_subclass_of(utils.safe_infer(exc_in_bare_handler),
                                                inferred_current)
                           for exc_in_bare_handler in excs_in_bare_handler):
                        bare_raise = False
                        break

            # `raise` as the first operator inside the except handler
            if _is_raising([handler.body[0]]):
                # flags when there is a bare raise
                if handler.body[0].exc is None:
                    bare_raise = True
                    handler_having_bare_raise = handler
                    excs_in_bare_handler = gather_exceptions_from_handler(handler)
        if bare_raise:
            self.add_message('try-except-raise', node=handler_having_bare_raise)

    @utils.check_messages('bare-except', 'broad-except', 'try-except-raise',
                          'binary-op-exception', 'bad-except-order',
                          'catching-non-exception', 'duplicate-except')
    def visit_tryexcept(self, node):
        """check for empty except"""
        self._check_try_except_raise(node)
        exceptions_classes = []
        nb_handlers = len(node.handlers)
        for index, handler in enumerate(node.handlers):
            if handler.type is None:
                if not _is_raising(handler.body):
                    self.add_message('bare-except', node=handler)

                # check if an "except:" is followed by some other
                # except
                if index < (nb_handlers - 1):
                    msg = 'empty except clause should always appear last'
                    self.add_message('bad-except-order', node=node, args=msg)

            elif isinstance(handler.type, astroid.BoolOp):
                self.add_message('binary-op-exception',
                                 node=handler, args=handler.type.op)
            else:
                try:
                    excs = list(_annotated_unpack_infer(handler.type))
                except astroid.InferenceError:
                    continue

                for part, exc in excs:
                    if exc is astroid.Uninferable:
                        continue
                    if (isinstance(exc, astroid.Instance)
                            and utils.inherit_from_std_ex(exc)):
                        # pylint: disable=protected-access
                        exc = exc._proxied

                    self._check_catching_non_exception(handler, exc, part)

                    if not isinstance(exc, astroid.ClassDef):
                        continue

                    exc_ancestors = [anc for anc in exc.ancestors()
                                     if isinstance(anc, astroid.ClassDef)]

                    for previous_exc in exceptions_classes:
                        if previous_exc in exc_ancestors:
                            msg = '%s is an ancestor class of %s' % (
                                previous_exc.name, exc.name)
                            self.add_message('bad-except-order',
                                             node=handler.type, args=msg)
                    if (exc.name in self.config.overgeneral_exceptions
                            and exc.root().name == utils.EXCEPTIONS_MODULE
                            and not _is_raising(handler.body)):
                        self.add_message('broad-except',
                                         args=exc.name, node=handler.type)

                    if exc in exceptions_classes:
                        self.add_message('duplicate-except',
                                         args=exc.name, node=handler.type)

                exceptions_classes += [exc for _, exc in excs]


def register(linter):
    """required method to auto register this checker"""
    linter.register_checker(ExceptionsChecker(linter))