Fix pylint-dev#85, pylint-dev#2615: Emit used-before-assignment in final or except blocks where try statements could have failed

Author: jacobtylerwalls

ChangeLog

@@ -16,6 +16,11 @@ Release date: TBA
 
   Closes #4761
 
+* ``used-before-assignment`` now considers that assignments in a try block
+  may not have occurred when the except or finally blocks are executed.
+
+  Closes #85, #2615
+
 * ``used-before-assignment`` now checks names in try blocks.
 
 * Some files in ``pylint.testutils`` were deprecated. In the future imports should be done from the

doc/whatsnew/2.13.rst

@@ -36,6 +36,11 @@ Other Changes
 
   Closes #4761
 
+* ``used-before-assignment`` now considers that assignments in a try block
+  may not have occurred when the except or finally blocks are executed.
+
+  Closes #85, #2615
+
 * ``used-before-assignment`` now checks names in try blocks.
 
 * Require Python ``3.6.2`` to run pylint.

pylint/checkers/variables.py

@@ -405,7 +405,8 @@ def _has_locals_call_after_node(stmt, scope):
     "E0601": (
         "Using variable %r before assignment",
         "used-before-assignment",
-        "Used when a local variable is accessed before its assignment.",
+        "Used when a local variable is accessed before its assignment or in "
+        "except or finally blocks without being defined before the try block.",
     ),
     "E0602": (
         "Undefined variable %r",
@@ -662,6 +663,49 @@ def get_next_to_consume(self, node):
             self.consumed_uncertain[node.name] += difference
             found_nodes = filtered_nodes
 
+        # 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(future=True).parent, nodes.TryFinally)
+            and node.statement(future=True)
+            in node.statement(future=True).parent.finalbody
+        ):
+            filtered_nodes = [
+                n
+                for n in found_nodes
+                if not (
+                    n.statement(future=True).parent
+                    is node.statement(future=True).parent
+                    and n.statement(future=True) in n.statement(future=True).parent.body
+                )
+            ]
+            filtered_nodes_set = set(filtered_nodes)
+            difference = [n for n in found_nodes if n not in filtered_nodes_set]
+            self.consumed_uncertain[node.name] += difference
+            found_nodes = filtered_nodes
+
+        # If this node is in an ExceptHandler,
+        # filter out assignments in the try portion, assuming they may fail
+        if found_nodes and isinstance(
+            node.statement(future=True).parent, nodes.ExceptHandler
+        ):
+            filtered_nodes = [
+                n
+                for n in found_nodes
+                if not (
+                    isinstance(n.statement(future=True).parent, nodes.TryExcept)
+                    and n.statement(future=True) in n.statement(future=True).parent.body
+                    and node.statement(future=True).parent
+                    in n.statement(future=True).parent.handlers
+                    and n.statement(future=True) in n.statement(future=True).parent.body
+                )
+            ]
+            filtered_nodes_set = set(filtered_nodes)
+            difference = [n for n in found_nodes if n not in filtered_nodes_set]
+            self.consumed_uncertain[node.name] += difference
+            found_nodes = filtered_nodes
+
         return found_nodes
 
 

tests/functional/u/use/used_before_assignment_issue2615.py

@@ -0,0 +1,9 @@
+"""https://github.com/PyCQA/pylint/issues/2615"""
+def main():
+    """When evaluating except blocks, assume try statements fail."""
+    try:
+        res = 1 / 0
+        res = 42
+    except ZeroDivisionError:
+        print(res)  # [used-before-assignment]
+    print(res)

tests/functional/u/use/used_before_assignment_issue2615.txt

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

tests/functional/u/use/used_before_assignment_issue85.py

@@ -0,0 +1,9 @@
+"""https://github.com/PyCQA/pylint/issues/85"""
+def main():
+    """When evaluating finally blocks, assume try statements fail."""
+    try:
+        res = 1 / 0
+        res = 42
+    finally:
+        print(res)  # [used-before-assignment]
+    print(res)

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