Match mypy Error Codes to Actual Error Type

Rule

Match mypy error codes to the actual error type when using type: ignore comments. Use import-untyped for packages missing type stubs, import-not-found for missing packages. Mismatched codes cause mypy to report both 'Unused type: ignore comment' and the original error.

Context

Mypy requires specific error codes in type: ignore comments to suppress warnings. Using the wrong code (e.g., import-not-found when the package exists but lacks stubs) results in the comment being ignored, producing dual errors. The better solution is often to fix the root cause: install missing packages or add type stub dependencies.

Detection

Mypy reports 'Unused "type: ignore" comment' alongside the original error
type: ignore[import-not-found] used but actual error is 'Cannot find implementation or library stub'
type: ignore[import-untyped] used but package is actually missing from dependencies

Pattern

Common error codes:

import-untyped: Package exists but has no type stubs
import-not-found: Package is completely missing
arg-type: Wrong argument type
return-value: Wrong return type
attr-defined: Attribute not found

# ❌ Wrong: Using import-not-found for installed package without stubs
import requests  # type: ignore[import-not-found]

# ✅ Correct: Using import-untyped for package without stubs
import requests  # type: ignore[import-untyped]

# ✅ Better: Add type stubs to dependencies
# In pyproject.toml: "types-requests"

Outcome

Single error resolution: No more dual error messages
Clear intent: Error code documents why ignore is needed
Better fixes: Often reveals need for type stub installation

Configure pytest pythonpath - Related Python import configuration