Support conditional Python version and platform checks

[JukkaL]

Mypy should support some common kinds of Python version checks and platform checks (e.g. Windows vs. Posix). We should ignore code paths that won't be run on the targeted Python version or platform. This way mypy can more effectively type check code that supports Python 2 and 3 and multiplatform code.

We first need to decide which checks to support. These examples are from PEP 484 and should be supported:

import sys

if sys.version_info[0] >= 3:
    # Python 3 specific definitions
    ...
else:
    # Python 2 specific definitions
    ...

if sys.platform == 'win32':
    # Windows specific definitions
    ...
else:
    # Posix specific definitions
    ...

When type checking code as above, always only if or the else block would be analyzed, never both, since on any given program run only one them can be evaluated (we assume that nobody does anything crazy like modifying sys.platform at runtime). We'd detect the check expressions during semantic analysis and wouldn't semantically analyze (or type check) the skipped blocks, similar to how mypy currently deals with PY2/PY3 conditions in if statements.

We already have Python 2 and Python 3 modes, and we should also implement Windows and non-Windows (Posix) modes. Initially, we can just use the platform on which the type checker is being run, but more generally this should be configurable (e.g., mypy --platform win32 ...).

[JukkaL]

We should also support code that assigns the result of a check to a variable. Example:

import sys

PY3_OR_LATER = sys.version_info[0] >= 3

if PY3_OR_LATER:
    ...
else:
    ...

[methane]

How about type comment?

def quote(s):
    # if PY2:
    #     type: (AnyStr) -> AnyStr
    # else:
    #     type: (str) -> str
    return '"' + s + '"'

or

def quote(s):
    if PY2:
        # type: (AnyStr) -> AnyStr
        ...
    else:
        # type: (str) -> str
        ...
    return '"' + s + '"'

[gvanrossum]

Please no new proposals.

[rwbarton]

Cases like the one above could be handled already by defining a type variable or synonym MyStr conditionally on the python version and then declaring the type of quote to be (MyStr) -> MyStr.