Emitted when an ambiguous looking with statement is used. For example `with open() as first, second` which looks like a tuple assignment but is actually 2 context managers.
These warnings were promoted since they could uncover potential bugs in the code and since most people are using `pylint -E` anyway, it's good to have them as errors. These warnings are: assignment-from-none, unbalanced-tuple-unpacking, unpacking-non-sequence, non-iterator-returned. Closes issue #388.
The option ``--optimize-ast`` will control the peephole optimizer, which is used to optimize a couple of AST subtrees. The current problem solved by the peephole optimizer is when multiple joined strings, with the addition operator, are encountered. If the numbers of such strings is high enough, Pylint will then fail with a maximum recursion depth exceeded error, due to its visitor architecture. The peephole just transforms such calls, if it can, into the final resulting string and this exhibit a problem, because the visit_binop method stops being called (in the optimized AST it will be a Const node).
Also, this patch fixes the leaks of error messages from the Python 3 checker when the errors mode was activated. Closes issue #437.
This warning is emitted when the `cmp` argument for the `list.sort` or `sorted builtin` is encountered, since it was removed in Python 3. This is part of the issue #376.