Is it always safe to modify the `**kwargs` dictionary?

Question

Using the Python function syntax def f(**kwargs), in the function a keyword argument dictionary kwargs is created, and dictionaries are mutable, so the

Answer 1

For Python-level code, the kwargs dict inside a function will always be a new dict.

For C extensions, though, watch out. The C API version of kwargs will sometimes pass a dict through directly. In previous versions, it would even pass dict subclasses through directly, leading to the bug (now fixed) where

'{a}'.format(**collections.defaultdict(int))

would produce '0' instead of raising a KeyError.

If you ever have to write C extensions, possibly including Cython, don't try to modify the kwargs equivalent, and watch out for dict subclasses on old Python versions.