Added new config switch to control reporting of uses of Optional type as an iterable.

client/schemas/pyrightconfig.schema.json

@@ -97,6 +97,12 @@
       "title": "Controls reporting of attempts to call a variable with Optional type",
       "default": "none"
     },
+    "reportOptionalIterable": {
+      "$id": "#/properties/reportOptionalIterable",
+      "$ref": "#/definitions/diagnostic",
+      "title": "Controls reporting of attempts to use an Optional type as an iterable value",
+      "default": "none"
+    },
     "reportUntypedFunctionDecorator": {
       "$id": "#/properties/reportUntypedFunctionDecorator",
       "$ref": "#/definitions/diagnostic",

docs/configuration.md

@@ -40,6 +40,8 @@ The following settings control pyright's diagnostic output (warnings or errors).
 
 **reportOptionalCall** [boolean or string, optional]: Generate or suppress diagnostics for an attempt to call a variable with an Optional type. The default value for this setting is 'none'.
 
+**reportOptionalIterable** [boolean or string, optional]: Generate or suppress diagnostics for an attempt to use an Optional type as an iterable value (e.g. within a `for` statement). The default value for this setting is 'none'.
+
 **reportUntypedFunctionDecorator** [boolean or string, optional]: Generate or suppress diagnostics for function decorators that have no type annotations. These obscure the function type, defeating many type analysis features.
 
 **reportUntypedClassDecorator** [boolean or string, optional]: Generate or suppress diagnostics for class decorators that have no type annotations. These obscure the class type, defeating many type analysis features.

server/src/analyzer/expressionEvaluator.ts

@@ -235,8 +235,11 @@ export class ExpressionEvaluator {
         const iterMethodName = isAsync ? '__aiter__' : '__iter__';
         const nextMethodName = isAsync ? '__anext__' : '__next__';
 
-        // TODO - tighten this up, perhaps with a configuration switch.
-        if (type instanceof UnionType) {
+        if (type instanceof UnionType && type.getTypes().some(t => t instanceof NoneType)) {
+            this._addDiagnostic(
+                this._configOptions.reportOptionalIterable,
+                `Object of type 'None' cannot be used as iterable value`,
+                errorNode);
             type = type.removeOptional();
         }
 

server/src/common/configOptions.ts

@@ -94,6 +94,9 @@ export class ConfigOptions {
     // Report attempts to call a Optional type?
     reportOptionalCall: DiagnosticLevel = 'none';
 
+    // Report attempts to use an Optional type as an iterable?
+    reportOptionalIterable: DiagnosticLevel = 'none';
+
     // Report untyped function decorators that obscure the function type?
     reportUntypedFunctionDecorator: DiagnosticLevel = 'none';
 
@@ -218,6 +221,10 @@ export class ConfigOptions {
         this.reportOptionalCall = this._convertDiagnosticLevel(
             configObj.reportOptionalCall, 'reportOptionalCall', 'none');
 
+        // Read the "reportOptionalIterable" entry.
+        this.reportOptionalIterable = this._convertDiagnosticLevel(
+            configObj.reportOptionalIterable, 'reportOptionalIterable', 'none');
+
         // Read the "reportUntypedFunctionDecorator" entry.
         this.reportUntypedFunctionDecorator = this._convertDiagnosticLevel(
             configObj.reportUntypedFunctionDecorator, 'reportUntypedFunctionDecorator', 'none');

server/src/tests/samples/optional1.py

@@ -36,3 +36,8 @@ if 1:
 # this should generate an error.
 c[2]
 
+
+# If "reportOptionalIterable" is enabled,
+# this should generate an error.
+for val in c:
+    pass

server/src/tests/typeAnalyzer.test.ts

@@ -198,15 +198,17 @@ test('Optional1', () => {
     configOptions.reportOptionalSubscript = 'warning';
     configOptions.reportOptionalMemberAccess = 'warning';
     configOptions.reportOptionalCall = 'warning';
+    configOptions.reportOptionalIterable = 'warning';
     analysisResults = TestUtils.typeAnalyzeSampleFiles(['optional1.py'], configOptions);
-    validateResults(analysisResults, 0, 3);
+    validateResults(analysisResults, 0, 4);
 
     // Turn on errors.
     configOptions.reportOptionalSubscript = 'error';
     configOptions.reportOptionalMemberAccess = 'error';
     configOptions.reportOptionalCall = 'error';
+    configOptions.reportOptionalIterable = 'error';
     analysisResults = TestUtils.typeAnalyzeSampleFiles(['optional1.py'], configOptions);
-    validateResults(analysisResults, 3);
+    validateResults(analysisResults, 4);
 });
 
 test('Tuples1', () => {