Configuring warnings

The follow flags enable warnings for code that is sound but ispotentially problematic or redundant in some way.

• —warn-redundant-casts
• This flag will make mypy report an error whenever your code usesan unnecessary cast that can safely be removed.

• —warn-unused-ignores
• This flag will make mypy report an error whenever your code usesa # type: ignore comment on a line that is not actuallygenerating an error message.

This flag, along with the —warn-redundant-casts flag, are bothparticularly useful when you are upgrading mypy. Previously,you may have needed to add casts or # type: ignore annotationsto work around bugs in mypy or missing stubs for 3rd party libraries.

These two flags let you discover cases where either workarounds areno longer necessary.

• —no-warn-no-return

• By default, mypy will generate errors when a function is missingreturn statements in some execution paths. The only exceptionsare when:

  • The function has a None or Any return type
  • The function has an empty body or a body that is justellipsis (…). Empty functions are often used forabstract methods. Passing in —no-warn-no-return will disable these errormessages in all cases.

• —warn-return-any
• This flag causes mypy to generate a warning when returning a valuewith type Any from a function declared with a non-Any return type.

• —warn-unreachable
• This flag will make mypy report an error whenever it encounterscode determined to be unreachable or redundant after performing type analysis.This can be a helpful way of detecting certain kinds of bugs in your code.

For example, enabling this flag will make mypy report that the x > 7check is redundant and that the else block below is unreachable.

def process(x: int) -> None:
    # Error: Right operand of 'or' is never evaluated
    if isinstance(x, int) or x > 7:
        # Error: Unsupported operand types for + ("int" and "str")
        print(x + "bad")
    else:
        # Error: 'Statement is unreachable' error
        print(x + "bad")

To help prevent mypy from generating spurious warnings, the “Statement isunreachable” warning will be silenced in exactly two cases:

• When the unreachable statement is a raise statement, is anassert False statement, or calls a function that has the NoReturnreturn type hint. In other words, when the unreachable statementthrows an error or terminates the program in some way.
• When the unreachable statement was intentionally marked as unreachableusing Python version and system platform checks.

Note

Mypy currently cannot detect and report unreachable or redundant codeinside any functions using Type variables with value restriction.

This limitation will be removed in future releases of mypy.