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reportPropertyTypeMismatch.md

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Overview

reportPropertyTypeMismatch is a diagnostic in Pylance and Pyright that warns you when the type of a property setter does not match the type returned by its getter. This helps ensure type consistency for properties, preventing subtle bugs and improving static analysis.

Representative Issues

  • #3102: Ensure that default argument types in functions match the annotated parameter types to avoid runtime errors and type checking issues.
  • #4163: Ensure consistency in the use of type stubs between Pyright's CLI and Pylance settings, especially with useLibraryCodeForTypes.
  • #5200: Provide a configuration setting to allow users to customize diagnostic rule severities based on the type checking mode, improving the granularity of error reporting.
  • #5887: Ensure that classes inheriting from abstract base classes implement all abstract methods.
  • #6300: When configuring file exclusions in a Python project, it's crucial to accurately exclude unnecessary folders like .venv to improve performance.
  • #990: Ensure all diagnostic severity rules in configuration files are accurately named to prevent misdirected diagnostics.
  • #2424: Ensure that property setters' value types are consistent with the getter return type to avoid mismatches that confuse static analysis tools.
  • #2721: When using iscoroutine or similar type guards to conditionally check and assign types, consider using isinstance for clearer control over the type narrowing process.
  • #2980: Ensure that configuration parameters default to "none" and are correctly applied in the project settings.
  • #2981: Ensure that configuration details for diagnostics and type checking are clearly documented, especially when different interfaces like VS Code settings or project-specific configurations might be used.
  • #4367: Ensure that comments in TOML files use the correct line endings and do not contain unsupported control characters to avoid parse errors.

Examples

Error:

class Widget:
    @property
    def color(self) -> str:
        return self._color

    @color.setter
    def color(self, value: int):  # Setter accepts int, getter returns str
        self._color = str(value)

Fix — make setter and getter types consistent:

class Widget:
    @property
    def color(self) -> str:
        return self._color

    @color.setter
    def color(self, value: str):  # Now matches getter return type
        self._color = value

Common Fixes & Workarounds

  1. Ensure the type of the property setter matches the getter's return type (or is a compatible union).
  2. Refactor property definitions to use consistent types for both getter and setter.
  3. Use Union types if a property can accept multiple types.
  4. Double-check type stubs and configuration for accuracy.
  5. Update your configuration files to use the correct diagnostic rule names.

For more details on configuring this diagnostic, see the Pyright configuration documentation.

See Also