Handle inference ambiguity for invalid-format-index

Close #2752

ChangeLog

@@ -12,6 +12,10 @@ Release date: TBA
   This check warns when modules are imported from places other than a
   module toplevel, e.g. inside a function or a class.
 
+* Handle inference ambiguity for ``invalid-format-index``
+
+  Close #2752
+
 * Removed Python 2 specific checks such as ``relative-import``,
   ``invalid-encoded-data``, ``missing-super-argument``.
 

pylint/checkers/strings.py

@@ -471,10 +471,10 @@ def _check_new_format_specifiers(self, node, fields, named):
             if argname in (astroid.Uninferable, None):
                 continue
             try:
-                argument = next(argname.infer())
+                argument = utils.safe_infer(argname)
             except astroid.InferenceError:
                 continue
-            if not specifiers or argument is astroid.Uninferable:
+            if not specifiers or not argument:
                 # No need to check this key if it doesn't
                 # use attribute / item access
                 continue

tests/functional/s/string_formatting.py

@@ -2,9 +2,9 @@
 """
 # pylint: disable=too-few-public-methods, import-error, unused-argument, line-too-long, no-absolute-import, useless-object-inheritance
 import os
+import sys
 from missing import Missing
 
-__revision__ = 1
 
 class Custom(object):
     """ Has a __getattr__ """
@@ -186,3 +186,13 @@ def avoid_empty_attribute():
     """The following string is invalid, avoid crashing."""
 
     return "There are {.:2f} undiscovered errors.".format(1) # [bad-format-string]
+
+
+def invalid_format_index_on_inference_ambiguity():
+    """Test inference bug for invalid-format-index"""
+    options = []
+    if len(sys.argv) > 1:
+        options = [["Woof!"]]
+    else:
+        options = [["Barf!"]]
+    return 'Why is this bad? {options[0][0]}'.format(options=options)