Incorrect inference on conditional inferred assignment

[xmo-odoo]

May be related to #192?

m = astroid.builder.parse('''
a = ''
if cond:
    a += thing()
print a
''')

m.body[-1].values[0].infer() will only contain a Const(value=''), completely ignoring the augmented assignment. I would have expected something similar to a regular concatenation-and-assignment:

m = astroid.builder.parse('''
a = ''
if cond:
    a = a + thing()
print a
''')
print list(m.body[-1].values[0].infer())

[<Const(str) l.2 [] at 0x1020d7550>, YES]

since augmented assignment will fallback on a normal operate-then-assign if not overridden, interpreting them as operate-then-assign would be a valid basic inference:

>>> class Foo(object):
...     def __radd__(self, other):
...             return self
... 
>>> a = 'a'
>>> a += Foo()
>>> a
<__main__.Foo object at 0x1020c0a10>

Note: while the inference is not incorrect on unconditional augmented assignment, it blows up entirely:

>>> m = astroid.builder.parse("""
... a = ''
... a += foo()
... print a
... """)
>>> list(m.body[-1].values[0].infer()) 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lib/python2.7/site-packages/astroid/bases.py", line 302, in wrapped
    for res in _func(node, context, **kwargs):
  File "lib/python2.7/site-packages/astroid/bases.py", line 108, in _infer_stmts
    raise exceptions.InferenceError(str(stmt))
astroid.exceptions.InferenceError: AssignName(a)

This is with astroid 1.4.3

[PCManticore]

Yeah, this could be nice to be improved, but probably it won't happen soon. I'd like to change the inference in the near future from evaluating all the states of variables during inference to an approach closer to interpretation, so that we could know at each line of code the potential values of variables.