undefined-name (F821)

Derived from the Pyflakes linter.

What it does

Checks for uses of undefined names.

Why is this bad?

An undefined name is likely to raise NameError at runtime.

Example

def double():
    return n * 2  # raises `NameError` if `n` is undefined when `double` is called

Use instead:

def double(n):
    return n * 2

References

• Python documentation: Naming and binding