Fixed a bug that led to a false positive error about an "unknown type" in certain cases when using an assignment expression (walrus operator) in a loop. This addresses https://github.com/microsoft/pyright/issues/4557.

packages/pyright-internal/src/analyzer/typeEvaluator.ts

@@ -1113,7 +1113,7 @@ export function createTypeEvaluator(importLookup: ImportLookup, evaluatorOptions
                 assignTypeToExpression(
                     node.leftExpression,
                     typeResult.type,
-                    /* isTypeIncomplete */ false,
+                    !!typeResult.isIncomplete,
                     node.rightExpression,
                     /* ignoreEmptyContainers */ true,
                     /* allowAssignmentToFinalVar */ true
@@ -1126,7 +1126,7 @@ export function createTypeEvaluator(importLookup: ImportLookup, evaluatorOptions
                 assignTypeToExpression(
                     node.name,
                     typeResult.type,
-                    /* isTypeIncomplete */ false,
+                    !!typeResult.isIncomplete,
                     node.rightExpression,
                     /* ignoreEmptyContainers */ true
                 );

packages/pyright-internal/src/tests/samples/codeFlow8.py

@@ -0,0 +1,27 @@
+# This sample tests the case where an assignment expression
+# is used within a looping construct such that the assigned
+# value is initially unknown.
+
+# pyright: strict
+
+from typing import Iterator
+
+for _ in ["1"]:
+    old_lines: Iterator[str] = iter(["2", "3"])
+
+    try:
+        while True:
+            line = next(old_lines)
+            count = 1
+            if count:
+                while True:
+                    if not (line := next(old_lines)):
+                        pass
+                    elif line.startswith(""):
+                        print(line.removeprefix(""))
+                    else:
+                        old_lines = iter([line] + list(old_lines))
+                        break
+
+    except StopIteration:
+        pass

packages/pyright-internal/src/tests/typeEvaluator1.test.ts

@@ -926,6 +926,12 @@ test('CodeFlow7', () => {
     TestUtils.validateResults(analysisResults, 0);
 });
 
+test('CodeFlow8', () => {
+    const analysisResults = TestUtils.typeAnalyzeSampleFiles(['codeFlow8.py']);
+
+    TestUtils.validateResults(analysisResults, 0);
+});
+
 test('CapturedVariable1', () => {
     const analysisResults = TestUtils.typeAnalyzeSampleFiles(['capturedVariable1.py']);