Comparison chaining

[spkersten]

This fixes #309

I've added a new AST node, ComparisonExpr, for (chained) comparison expressions. This node is type checked as were it a boolean-and expression of pair-wise comparisons of all consecutive operands.

[JukkaL]

Thanks for the fix! I don't have time to review it right now, but I will do it later this week (I have time dedicated to mypy on Sunday).

Can you have a look at the Travis CI failure (see Details above). Here is an extract -- it might be a mypy parser issue:

In module imported in mypy/pprinter.py, line 3,
in mypy/test/testdyncheck.py, line 16,
in tests.py, line 17:
mypy/output.py: In member "visit_comparison_expr" of class "OutputVisitor":
mypy/output.py, line 385: Parse error before ,
mypy/output.py, line 385: Parse error before :
mypy/output.py, line 386: Inconsistent indentation
mypy/output.py: At top level:
mypy/output.py, line 525: Inconsistent indentation
mypy/output.py, line 525: Parse error before :
mypy/output.py, line 526: Inconsistent indentation
mypy/output.py, line 614: Inconsistent indentation

[spkersten]

It think the errors at line 385 are caused by an existing bug in the mypy parser, which can be reproduced.

mpf.py:

ts = [(1,2), (3,4)]
xs = ['a', 'b']

for (t1, t2), x in zip(ts, xs):
    print(t1, t2, x)

Python runs fine:

$ python3 mpf.py 
1 2 a
3 4 b

But mypy fails to parse it:

$ python3 scripts/mypy mpf.py 
mpf.py, line 5: Parse error before ,
mpf.py, line 5: Parse error before :
mpf.py, line 6: Inconsistent indentation
mpf.py, line 9: Inconsistent indentation

I've created issue #441 for this.

Lines 525 and 614 where not touched by me at all.

[JukkaL]

Yeah, the mypy parser is to blame. For the time being, could you refactor it to use code that is supported by mypy, since otherwise mypy can't type check itself. For example, consider this:

for (t1, t2), x in zip(ts, xs):
    print(t1, t2, x)

It can be written like this (though uglier):

for t, x in zip(ts, xs):
    t1, t2 = t
    print(t1, t2, x)

[JukkaL]

Thanks for the fix -- looks great!