Invalid Syntax In Forward Annotation

What it does

Checks for string-literal annotations where the string cannot be parsed as a Python expression.

Why is this bad?

Type annotations are expected to be Python expressions that describe the expected type of a variable, parameter, attribute or return statement.

Type annotations are permitted to be string-literal expressions, in order to enable forward references to names not yet defined. However, it must be possible to parse the contents of that string literal as a normal Python expression.

Example

def foo() -> "intstance of C":  # error
    return 42


class C: ...

Use instead:

def foo() -> "C":
    return C()


class C: ...

References

• Typing spec: The meaning of annotations
• Typing spec: String annotations