Fix pylint-dev#85: Emit used-before-assignment in final blocks where try statements could have failed

Author: jacobtylerwalls

ChangeLog

@@ -9,10 +9,15 @@ Release date: TBA
 ..
   Put new features here and also in 'doc/whatsnew/2.13.rst'
 
+* ``used-before-assignment`` now considers that assignments in the try block
+  of a try/finally clause may not have occurred when the final block is
+  executed.
+
+  Closes #85
 
 ..
   Insert your changelog randomly, it will reduce merge conflicts
-  (Ie. not necessarilly at the end)
+  (Ie. not necessarily at the end)
 
 
 What's New in Pylint 2.12.1?
@@ -24,7 +29,7 @@ Release date: TBA
 
 ..
   Insert your changelog randomly, it will reduce merge conflicts
-  (Ie. not necessarilly at the end)
+  (Ie. not necessarily at the end)
 
 
 What's New in Pylint 2.12.0?

doc/whatsnew/2.13.rst

@@ -19,3 +19,9 @@ Extensions
 
 Other Changes
 =============
+
+* ``used-before-assignment`` now considers that assignments in the try block
+  of a try/finally clause may not have occurred when the final block is
+  executed.
+
+  Closes #85

pylint/checkers/variables.py

@@ -564,6 +564,8 @@ def mark_as_consumed(self, name, consumed_nodes):
         If all of the nodes for the name were consumed, delete the name from
         the to_consume dictionary
         """
+        if name not in self.to_consume:
+            return
         unconsumed = [n for n in self.to_consume[name] if n not in set(consumed_nodes)]
         self.consumed[name] = consumed_nodes
 
@@ -606,6 +608,25 @@ def get_next_to_consume(self, node):
                 or n.statement().parent_of(node)
             ]
 
+        # If this node is in a Finally block of a Try/Finally,
+        # filter out assignments in the try portion, assuming they may fail
+        if (
+            found_nodes
+            and isinstance(node.statement().parent, nodes.TryFinally)
+            and node.statement() in node.statement().parent.finalbody
+        ):
+            filtered_nodes = [
+                n
+                for n in found_nodes
+                if not (
+                    isinstance(n.statement().parent, nodes.TryFinally)
+                    and n.statement() in n.statement().parent.body
+                )
+            ]
+            if not filtered_nodes:
+                self.mark_as_consumed(name, found_nodes)
+            return filtered_nodes
+
         return found_nodes
 
 

tests/functional/u/use/used_before_assignment_issue85.py

@@ -0,0 +1,8 @@
+# pylint: disable=missing-module-docstring, missing-function-docstring
+def main():
+    try:
+        # throw exception here to make res undefined
+        res = 1 / 0
+        res = 42
+    finally:
+        print(res)  # [used-before-assignment]

tests/functional/u/use/used_before_assignment_issue85.txt

@@ -0,0 +1 @@
+used-before-assignment:8:14:8:17:main:Using variable 'res' before assignment:UNDEFINED