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DetachHead merged 5 commits into from
Feb 18, 2026
Merged

Allow TYPE_CHECKING guards in match statements #1726

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c6320d0
working
grahamcracker1234 Feb 10, 2026
3cfec8b
passing missing test
grahamcracker1234 Feb 10, 2026
fa0a411
minor duplication refactor
grahamcracker1234 Feb 10, 2026
cfcf9eb
addressed feedback
grahamcracker1234 Feb 13, 2026
4d95898
addressed comments
grahamcracker1234 Feb 17, 2026
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86 changes: 48 additions & 38 deletions packages/pyright-internal/src/analyzer/binder.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1375,36 +1375,18 @@ export class Binder extends ParseTreeWalker {
const elseLabel = this._createBranchLabel();
const postIfLabel = this._createBranchLabel(preIfFlowNode);

const notTypeCheckingNode: FlowNode = {
flags: FlowFlags.NotTypeChecking | FlowFlags.Unreachable,
id: getUniqueFlowNodeId(),
};

const isTypeCheckingNode = (node: ExpressionNode): node is NameNode =>
node.nodeType === ParseNodeType.Name && node.d.value === 'TYPE_CHECKING';

postIfLabel.affectedExpressions = this._trackCodeFlowExpressions(() => {
// Determine if the test condition is always true or always false. If so,
// we can treat either the then or the else clause as unconditional.
const constExprValue = StaticExpressions.evaluateStaticBoolLikeExpression(
node.d.testExpr,
this._fileInfo.executionEnvironment,
this._fileInfo.definedConstants,
this._typingImportAliases,
this._sysImportAliases
);
const constExprValue = this._evaluateStaticBoolExpr(node.d.testExpr);

this._bindConditional(node.d.testExpr, thenLabel, elseLabel);

// Handle the if clause.
if (constExprValue === false) {
this._currentFlowNode =
node.d.testExpr.nodeType === ParseNodeType.UnaryOperation &&
node.d.testExpr.d.operator === OperatorType.Not &&
isTypeCheckingNode(node.d.testExpr.d.expr) &&
constExprValue === false
? notTypeCheckingNode
: Binder._unreachableFlowNode;
this._currentFlowNode = this._isNotTypeCheckingExpr(node.d.testExpr)
? this._createNotTypeCheckingFlowNode()
: Binder._unreachableFlowNode;
} else {
this._currentFlowNode = this._finishFlowLabel(thenLabel);
}
Expand All @@ -1415,8 +1397,8 @@ export class Binder extends ParseTreeWalker {
// are chained "else if" statements, they'll be handled
// recursively here.
if (constExprValue === true) {
this._currentFlowNode = isTypeCheckingNode(node.d.testExpr)
? notTypeCheckingNode
this._currentFlowNode = this._isTypeCheckingExpr(node.d.testExpr)
? this._createNotTypeCheckingFlowNode()
: Binder._unreachableFlowNode;
} else {
this._currentFlowNode = this._finishFlowLabel(elseLabel);
Expand All @@ -1440,13 +1422,7 @@ export class Binder extends ParseTreeWalker {

// Determine if the test condition is always true or always false. If so,
// we can treat either the while or the else clause as unconditional.
const constExprValue = StaticExpressions.evaluateStaticBoolLikeExpression(
node.d.testExpr,
this._fileInfo.executionEnvironment,
this._fileInfo.definedConstants,
this._typingImportAliases,
this._sysImportAliases
);
const constExprValue = this._evaluateStaticBoolExpr(node.d.testExpr);
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const preLoopLabel = this._createLoopLabel();
this._addAntecedent(preLoopLabel, this._currentFlowNode!);
Expand Down Expand Up @@ -2389,6 +2365,17 @@ export class Binder extends ParseTreeWalker {

this._currentFlowNode = this._finishFlowLabel(preSuiteLabel);

// If the guard is always false, mark the case body as unreachable.
// If the guard is `not TYPE_CHECKING`, suppress unreachable diagnostics.
if (caseStatement.d.guardExpr) {
const constExprValue = this._evaluateStaticBoolExpr(caseStatement.d.guardExpr);
if (constExprValue === false) {
this._currentFlowNode = this._isNotTypeCheckingExpr(caseStatement.d.guardExpr)
? this._createNotTypeCheckingFlowNode()
: Binder._unreachableFlowNode;
}
}

// Bind the body of the case statement.
this.walk(caseStatement.d.suite);
this._addAntecedent(postMatchLabel, this._currentFlowNode);
Expand Down Expand Up @@ -3034,13 +3021,7 @@ export class Binder extends ParseTreeWalker {
if (antecedent.flags & FlowFlags.Unreachable) {
return antecedent;
}
const staticValue = StaticExpressions.evaluateStaticBoolLikeExpression(
expression,
this._fileInfo.executionEnvironment,
this._fileInfo.definedConstants,
this._typingImportAliases,
this._sysImportAliases
);
const staticValue = this._evaluateStaticBoolExpr(expression);
if (
(staticValue === true && flags & FlowFlags.FalseCondition) ||
(staticValue === false && flags & FlowFlags.TrueCondition)
Expand Down Expand Up @@ -4346,6 +4327,35 @@ export class Binder extends ParseTreeWalker {
return getUniqueFlowNodeId();
}

private _evaluateStaticBoolExpr(expr: ExpressionNode): boolean | undefined {
return StaticExpressions.evaluateStaticBoolLikeExpression(
expr,
this._fileInfo.executionEnvironment,
this._fileInfo.definedConstants,
this._typingImportAliases,
this._sysImportAliases
);
}

private _isTypeCheckingExpr(expr: ExpressionNode): boolean {
return expr.nodeType === ParseNodeType.Name && expr.d.value === 'TYPE_CHECKING';
}

private _isNotTypeCheckingExpr(expr: ExpressionNode): boolean {
return (
expr.nodeType === ParseNodeType.UnaryOperation &&
expr.d.operator === OperatorType.Not &&
this._isTypeCheckingExpr(expr.d.expr)
);
}

private _createNotTypeCheckingFlowNode(): FlowNode {
return {
flags: FlowFlags.NotTypeChecking | FlowFlags.Unreachable,
id: this._getUniqueFlowNodeId(),
};
}

private _addDiagnostic(rule: DiagnosticRule, message: string, textRange: TextRange) {
const diagLevel = this._fileInfo.diagnosticRuleSet[rule] as DiagnosticLevel;

Expand Down
9 changes: 9 additions & 0 deletions packages/pyright-internal/src/tests/samples/unreachable1.py
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Expand Up @@ -125,3 +125,12 @@ def func12(obj: str) -> list:

# This should be marked as unreachable.
return obj


def func13(value: int | str) -> None:
match value:
case int(): ...
case str(): ...
case _:
# This should be marked as unreachable.
...
8 changes: 7 additions & 1 deletion packages/pyright-internal/src/tests/samples/unreachable2.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,4 +6,10 @@
if TYPE_CHECKING:
...
else:
...
...

def func(value: int | str) -> None:
match value:
case int(): ...
case str(): ...
case _ if not TYPE_CHECKING: ...
2 changes: 1 addition & 1 deletion packages/pyright-internal/src/tests/typeEvaluator1.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -27,7 +27,7 @@ import * as TestUtils from './testUtils';
test('Unreachable1', () => {
const analysisResults = TestUtils.typeAnalyzeSampleFiles(['unreachable1.py']);

TestUtils.validateResults(analysisResults, 0, 0, 2, 7);
TestUtils.validateResults(analysisResults, 0, 0, 2, 8);
});

test('Builtins1', () => {
Expand Down
5 changes: 3 additions & 2 deletions packages/pyright-internal/src/tests/typeEvaluatorBased.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@ test('reportUnreachable', () => {
configOptions.diagnosticRuleSet.reportUnreachable = 'error';
const analysisResults = typeAnalyzeSampleFiles(['unreachable1.py'], configOptions);
validateResultsButBased(analysisResults, {
errors: [78, 89, 106, 110, 118, 126].map((line) => ({ code: DiagnosticRule.reportUnreachable, line })),
errors: [78, 89, 106, 110, 118, 126, 135].map((line) => ({ code: DiagnosticRule.reportUnreachable, line })),
infos: [{ line: 95 }, { line: 98 }],
hints: [{ line: 102 }],
});
Expand Down Expand Up @@ -50,7 +50,7 @@ test('default typeCheckingMode=recommended', () => {
const analysisResults = typeAnalyzeSampleFiles(['unreachable1.py'], configOptions);
validateResultsButBased(analysisResults, {
warnings: [
...[78, 89, 106, 110, 118, 126].map((line) => ({ code: DiagnosticRule.reportUnreachable, line })),
...[78, 89, 106, 110, 118, 126, 135].map((line) => ({ code: DiagnosticRule.reportUnreachable, line })),
{ line: 19, code: DiagnosticRule.reportUnknownParameterType },
{ line: 33, code: DiagnosticRule.reportUnknownParameterType },
{ line: 33, code: DiagnosticRule.reportInvalidAbstractMethod },
Expand All @@ -61,6 +61,7 @@ test('default typeCheckingMode=recommended', () => {
{ line: 121, code: DiagnosticRule.reportUnknownParameterType },
{ line: 122, code: DiagnosticRule.reportUnnecessaryIsInstance },
{ line: 123, code: DiagnosticRule.reportUnknownVariableType },
{ line: 133, code: DiagnosticRule.reportUnnecessaryComparison },
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],
errors: [
{ line: 16, code: DiagnosticRule.reportUninitializedInstanceVariable },
Expand Down