Emit `used-before-assignment` in final or except blocks where try statements could have failed (#5384)

* Emit `used-before-assignment` in final or except blocks where try statements could have failed

Fix #85, #2615

7 files changed, 77 insertions, 7 deletions

diff --git a/ChangeLog b/ChangeLog
index 738d8acda..a90ade1cc 100644
--- a/ChangeLog
+++ b/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
diff --git a/doc/whatsnew/2.13.rst b/doc/whatsnew/2.13.rst
index 001b6b976..b3372278d 100644
--- a/doc/whatsnew/2.13.rst
+++ b/doc/whatsnew/2.13.rst
@@ -48,6 +48,11 @@ 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.
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 13b8589af..0a4a21c18 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -405,7 +405,10 @@ MSGS = {
     "E0601": (
         "Using variable %r before assignment",
         "used-before-assignment",
-        "Used when a local variable is accessed before its assignment.",
+        "Emitted when a local variable is accessed before its assignment took place. "
+        "Assignments in try blocks are assumed not to have occurred when evaluating "
+        "associated except/finally blocks. Assignments in except blocks are assumed "
+        "not to have occurred when evaluating statements outside the block.",
     ),
     "E0602": (
         "Undefined variable %r",
@@ -580,12 +583,10 @@ scope_type : {self._atomic.scope_type}
     def consumed_uncertain(self) -> DefaultDict[str, List[nodes.NodeNG]]:
         """
         Retrieves nodes filtered out by get_next_to_consume() that may not
-        have executed, such as statements in except blocks. Checkers that
-        want to treat the statements as executed (e.g. for unused-variable)
-        may need to add them back.
-
-        TODO: A pending PR will extend this to nodes in try blocks when
-        evaluating their corresponding except and finally blocks.
+        have executed, such as statements in except blocks, or statements
+        in try blocks (when evaluating their corresponding except and finally
+        blocks). Checkers that want to treat the statements as executed
+        (e.g. for unused-variable) may need to add them back.
         """
         return self._atomic.consumed_uncertain
 
@@ -617,6 +618,7 @@ scope_type : {self._atomic.scope_type}
         name = node.name
         parent_node = node.parent
         found_nodes = self.to_consume.get(name)
+        node_statement = node.statement(future=True)
         if (
             found_nodes
             and isinstance(parent_node, nodes.Assign)
@@ -662,6 +664,44 @@ scope_type : {self._atomic.scope_type}
             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.parent, nodes.TryFinally)
+            and node_statement in node_statement.parent.finalbody
+        ):
+            filtered_nodes = [
+                n
+                for n in found_nodes
+                if not (
+                    n.statement(future=True).parent is node_statement.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.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.parent
+                    in n.statement(future=True).parent.handlers
+                )
+            ]
+            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
 
 
diff --git a/tests/functional/u/use/used_before_assignment_issue2615.py b/tests/functional/u/use/used_before_assignment_issue2615.py
new file mode 100644
index 000000000..912c71387
--- /dev/null
+++ b/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)
diff --git a/tests/functional/u/use/used_before_assignment_issue2615.txt b/tests/functional/u/use/used_before_assignment_issue2615.txt
new file mode 100644
index 000000000..ce6e4b9d0
--- /dev/null
+++ b/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
diff --git a/tests/functional/u/use/used_before_assignment_issue85.py b/tests/functional/u/use/used_before_assignment_issue85.py
new file mode 100644
index 000000000..58d8e38d8
--- /dev/null
+++ b/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)
diff --git a/tests/functional/u/use/used_before_assignment_issue85.txt b/tests/functional/u/use/used_before_assignment_issue85.txt
new file mode 100644
index 000000000..ce6e4b9d0
--- /dev/null
+++ b/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