Fix spurious possibly-undefined errors in for-else with break (#19696)

When a for loop contains branches with `break` and an `else` block,
variables declared inside those branches were incorrectly discarded from
further analysis, leading Mypy to incorrectly report a variable as
undefined after the loop or as used before declaration.

With this fix, when a for loop's `else` block is considered, variables
declared in every branch of the `for` loop body that called `break` are
now considered as defined within the body of the loop.

Fixes #14209
Fixes #19690

---------

Co-authored-by: Shantanu Jain <hauntsaninja@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Author: ambv

mypy/partially_defined.py

@@ -297,6 +297,8 @@ def is_undefined(self, name: str) -> bool:
 class Loop:
     def __init__(self) -> None:
         self.has_break = False
+        # variables defined in every loop branch with `break`
+        self.break_vars: set[str] | None = None
 
 
 class PossiblyUndefinedVariableVisitor(ExtendedTraverserVisitor):
@@ -476,6 +478,9 @@ def visit_for_stmt(self, o: ForStmt) -> None:
             has_break = loop.has_break
             if has_break:
                 self.tracker.start_branch_statement()
+                if loop.break_vars is not None:
+                    for bv in loop.break_vars:
+                        self.tracker.record_definition(bv)
                 self.tracker.next_branch()
             o.else_body.accept(self)
             if has_break:
@@ -508,6 +513,14 @@ def visit_break_stmt(self, o: BreakStmt) -> None:
         super().visit_break_stmt(o)
         if self.loops:
             self.loops[-1].has_break = True
+            # Track variables that are definitely defined at the point of break
+            if len(self.tracker._scope().branch_stmts) > 0:
+                branch = self.tracker._scope().branch_stmts[-1].branches[-1]
+                if self.loops[-1].break_vars is None:
+                    self.loops[-1].break_vars = set(branch.must_be_defined)
+                else:
+                    # we only want variables that have been defined in each branch
+                    self.loops[-1].break_vars.intersection_update(branch.must_be_defined)
         self.tracker.skip_branch()
 
     def visit_expression_stmt(self, o: ExpressionStmt) -> None:

test-data/unit/check-possibly-undefined.test

@@ -1055,6 +1055,158 @@ def foo(x: Union[int, str]) -> None:
     f  # OK
 [builtins fixtures/tuple.pyi]
 
+[case testForElseWithBreakInTryExceptContinue]
+# flags: --enable-error-code possibly-undefined
+# Test for issue where variable defined before break in try block
+# was incorrectly reported as undefined when except has continue
+def random() -> float: return 0.5
+
+if random():
+    for i in range(10):
+        try:
+            value = random()
+            break
+        except Exception:
+            continue
+    else:
+        raise RuntimeError
+
+    print(value)  # Should not error - value is defined if we broke
+else:
+    value = random()
+
+print(value)  # Should not error
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseWithBreakInTryExceptContinueNoIf]
+# flags: --enable-error-code possibly-undefined
+# Simpler version without if statement
+def random() -> float: return 0.5
+
+for i in range(10):
+    try:
+        value = random()
+        break
+    except Exception:
+        continue
+else:
+    raise RuntimeError
+
+print(value)  # Should not error
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseWithBreakInTryExceptPass]
+# flags: --enable-error-code possibly-undefined
+# Version with pass instead of continue - should also work
+def random() -> float: return 0.5
+
+if random():
+    for i in range(10):
+        try:
+            value = random()
+            break
+        except Exception:
+            pass
+    else:
+        raise RuntimeError
+
+    print(value)  # Should not error
+else:
+    value = random()
+
+print(value)  # Should not error
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseWithConditionalDefBeforeBreak]
+# flags: --enable-error-code possibly-undefined
+# Test that conditional definition before break still works correctly
+def random() -> float: return 0.5
+
+if random():
+    for i in range(10):
+        try:
+            if i > 10:
+                value = random()
+                break
+        except Exception:
+            continue
+    else:
+        raise RuntimeError
+
+    print(value)  # Should not error (though might be undefined at runtime)
+else:
+    value = random()
+
+print(value)  # Should not error
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseDefineInBothBranches]
+# flags: --enable-error-code possibly-undefined
+# Test that variable defined in both for break and else branches is not undefined
+for i in range(10):
+    if i:
+        value = i
+        break
+else:
+    value = 1
+
+print(value)  # Should not error - value is defined in all paths
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseWithWalrusInBreak]
+# flags: --enable-error-code possibly-undefined
+# Test with walrus operator in if condition before break
+def random() -> float: return 0.5
+
+if random():
+    for i in range(10):
+        if value := random():
+            break
+    else:
+        raise RuntimeError
+
+    print(value)  # Should not error - value is defined if we broke
+else:
+    value = random()
+
+print(value)  # Should not error
+[builtins fixtures/for_else_exception.pyi]
+
+[case testForElseWithWalrusInBreakConditionalRaise]
+# flags: --enable-error-code possibly-undefined
+# Test with walrus operator in if condition before break
+def random() -> float: return 0.5
+
+def f1() -> None:
+    if random():
+        for i in range(10):
+            if value := random():
+                break
+        else:
+            if random() > 0.5:  # E: Unsupported left operand type for > ("float")
+                raise RuntimeError
+
+        print(value)  # E: Name "value" may be undefined
+    else:
+        value = random()
+
+    print(value)  # Already emitted error
+
+
+def f2() -> None:
+    if random():
+        for i in range(10):
+            if value := random():
+                break
+        else:
+            if random() > 0.5:  # E: Unsupported left operand type for > ("float")
+                raise RuntimeError
+    else:
+        value = random()
+
+    print(value)  # E: Name "value" may be undefined
+[builtins fixtures/for_else_exception.pyi]
+
 [case testOmittedUnrequiredElse]
 # flags: --enable-error-code possibly-undefined
 from typing import Literal

test-data/unit/fixtures/for_else_exception.pyi

@@ -0,0 +1,54 @@
+# Fixture for for-else tests with exceptions
+# Combines needed elements from primitives.pyi and exception.pyi
+
+from typing import Generic, Iterator, Mapping, Sequence, TypeVar
+
+T = TypeVar('T')
+V = TypeVar('V')
+
+class object:
+    def __init__(self) -> None: pass
+class type:
+    def __init__(self, x: object) -> None: pass
+class int:
+    def __init__(self, x: object = ..., base: int = ...) -> None: pass
+    def __add__(self, i: int) -> int: pass
+    def __rmul__(self, x: int) -> int: pass
+    def __bool__(self) -> bool: pass
+    def __eq__(self, x: object) -> bool: pass
+    def __ne__(self, x: object) -> bool: pass
+    def __lt__(self, x: 'int') -> bool: pass
+    def __le__(self, x: 'int') -> bool: pass
+    def __gt__(self, x: 'int') -> bool: pass
+    def __ge__(self, x: 'int') -> bool: pass
+class float:
+    def __float__(self) -> float: pass
+    def __add__(self, x: float) -> float: pass
+    def hex(self) -> str: pass
+class bool(int): pass
+class str(Sequence[str]):
+    def __add__(self, s: str) -> str: pass
+    def __iter__(self) -> Iterator[str]: pass
+    def __contains__(self, other: object) -> bool: pass
+    def __getitem__(self, item: int) -> str: pass
+    def format(self, *args: object, **kwargs: object) -> str: pass
+class dict(Mapping[T, V]):
+    def __iter__(self) -> Iterator[T]: pass
+class tuple(Generic[T]):
+    def __contains__(self, other: object) -> bool: pass
+class ellipsis: pass
+
+class BaseException:
+    def __init__(self, *args: object) -> None: ...
+class Exception(BaseException): pass
+class RuntimeError(Exception): pass
+
+class range(Sequence[int]):
+    def __init__(self, __x: int, __y: int = ..., __z: int = ...) -> None: pass
+    def count(self, value: int) -> int: pass
+    def index(self, value: int) -> int: pass
+    def __getitem__(self, i: int) -> int: pass
+    def __iter__(self) -> Iterator[int]: pass
+    def __contains__(self, other: object) -> bool: pass
+
+def print(x: object) -> None: pass