Improve the detection of undefined variables in function arguments

This patch involves a lot of subtle changes:

* is_defined_before takes in consideration the fact that its argument
  node is a default value for a Lambda / Function.
  If it is a default value, then the search is left to the parent of
  the lambda. Basically for x in the following, the result of is_defined_before
  will be determined by foo:

  def foo():
      x = 42
      func = lambda x=x: x * x
      return func

* there's another special rule for undefined-variable which verifies that
  a node is used in Arguments.defaults and if the actual definition
  is happening before the Arguments statement, basically trying
  to understand the following:

  class A:
     x = 42
     foo = lambda x=x: x * x

* undefined variable is emitted if both scope types, for the node
  analyzed and for its actual definition, is they are both lambda
  scopes.

Closes issue #404.

1 files changed, 10 insertions, 0 deletions

diff --git a/pylint/checkers/utils.py b/pylint/checkers/utils.py
index 0dbf6cb..59708cc 100644
--- a/pylint/checkers/utils.py
+++ b/pylint/checkers/utils.py
@@ -160,6 +160,16 @@ def is_defined_before(var_node):
                     return True
         elif isinstance(_node, (astroid.Lambda, astroid.Function)):
             if _node.args.is_argument(varname):
+                # If the name is found inside a default value
+                # of a function, then let the search continue
+                # in the parent's tree.
+                if _node.args.parent_of(var_node):
+                    try:
+                        _node.args.default_value(varname)
+                        _node = _node.parent
+                        continue
+                    except astroid.NoDefault:
+                        pass
                 return True
             if getattr(_node, 'name', None) == varname:
                 return True