false report of multiple definitions

[rbtcollins]

The following code in six fails to type check:

try:
    advance_iterator = next
except NameError:
    def advance_iterator(it):
        return it.next()
next = advance_iterator

.../lib/python3.4/site-packages/six.py, line 501: Name 'advance_iterator' already defined

But advance_iterator can't be already defined: for NameError to be thrown, the lookup of next must have failed.

Whats the right way to handle this? Just mask it?

[refi64]

This is a hack...but, whenever I encounter stuff like this, I do:

try:
    advance_iterator = next
except NameError:
    globals['advance_iterator'] = lambda it: it.next()
    next = advance_iterator

[JukkaL]

This is a known, long-standing issue. Conditional definitions are often rejected because mypy is too picky.

A potential fix would be to treat def more like an assignment, rather than a special 'function definition'.

[JukkaL]

Another, slightly different example:

if condition:
    from module import name
else:
    name = None

[JukkaL]

Merging with #212, which is similar.

[JukkaL]

Now some more conditional definitions are accepted by mypy. These are all okay:

try:
    from m import CONST
except ImportError:
    CONST = 'const'

try:
    from m import f
except ImportError:
    def f(): ...

def f(): ...
try:
    from m import f
except ImportError: pass

There's still a lot more work to do before mypy can deal with all interesting cases, but this should cover a good fraction of cases that used to fail.

[gvanrossum]

Thanks, it's a bit better. There's still this case that doesn't seem to
work right:

import sys
if sys.platform == 'darwin':
    import os
else:
    os = None

I get an error on each line mentioning os:

/Users/guido/mypy_tests/mypy_imp.py:3: error: Need type annotation for
variable
/Users/guido/mypy_tests/mypy_imp.py:5: error: Name 'os' already defined

On Fri, Nov 27, 2015 at 1:58 PM, Jukka Lehtosalo notifications@github.com
wrote:

Now some more conditional definitions are accepted by mypy. These are all
okay:

try:
from m import CONST
except ImportError:
CONST = 'const'

try:
from m import f
except ImportError:
def f(): ...

def f(): ...
try:
from m import f
except ImportError: pass

There's still a lot more work to do before mypy can deal with all
interesting cases, but this should cover a good fraction of cases that used
to fail.

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Reply to this email directly or view it on GitHub
#649 (comment).

--Guido van Rossum (python.org/~guido)

[JukkaL]

That should be pretty easy to support as well.

[gvanrossum]

OK, good to know, but right now I think there are higher priority items.

[gvanrossum]

OK, another case found in the wild:

if ignore_case:
    correct_case = None
else:
    def correct_case(args):  # E: error: Name 'correct_case' already defined
        ...

[JukkaL]

I also have a long, unprocessed list of related issues from the Python 2.7 std library that I should analyze one of these days.

[JukkaL]

These now work:

if x:
    def f(): ...
else:
    f = g

try:
    from m import f
except ...:
    f = None

These still don't work:

if x:
    f = g
else:
    def f(): ...

if x:
    import m
else:
    m = None

if x:
    m = None
else:
     import m