Use safe inference in unpacking-non-sequence checker

Unfortunately, this fix means that we won't be able to emit an error for
cases like this one:
```
def foo():
    if True:
        return [1, 2]
    return [3, 4, 5]
a, b = foo()
```
Well, not unless we get flow-sensitive inference. But we still need this fix to
reduce the number of false-positive errors.

Fixes issue #695.

1 files changed, 3 insertions, 1 deletions

diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 6f0f5a3..d813e1f 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -21,6 +21,7 @@ import re
 from copy import copy
 
 import astroid
+from astroid import helpers
 from astroid import modutils
 
 from pylint.interfaces import IAstroidChecker, INFERENCE, INFERENCE_FAILURE, HIGH
@@ -1033,7 +1034,8 @@ builtins. Remember that you should avoid to define new builtins when possible.'
 
         targets = node.targets[0].itered()
         try:
-            for infered in node.value.infer():
+            infered = helpers.safe_infer(node.value)
+            if infered is not None:
                 self._check_unpacking(infered, node, targets)
         except astroid.InferenceError:
             return