There are some sticking points: * types.SimpleNamespace() sorts attributes, so this would get in the way of issue #39058. * argparse.Namespace() supports a __contains__() method that isn't offered by types.SimpleNamespace(): >>> 'abc' in Namespace(abc=10) True * Argparse is sensitive to start-up time so we mostly want to avoid adding new dependencies. Start-up time recently got worse when the re module added a number of dependencies. * The __repr__ for SimpleNamespace() doesn't round-trip and isn't what we have now with Namespace. That potentially breaks doctests and diverges from published examples: >>> Namespace(abc=10) Namespace(abc=10) >>> SimpleNamespace(abc=10) namespace(abc=10) * Ironically, the class name "Namespace" is simpler than "SimpleNamespace" ;-) * Much of the code in argparse.Namespace() inherits from _AttributeHolder, so switching to types.SimpleNamespace() doesn't really save us much code. Are there any upsides to switching? Attribute lookup is almost equally fast using either approach, so there is no speed benefit: $ python3.8 -m timeit -r 11 -s 'from argparse import Namespace as NS' -s 'args=NS(abc=10)' 'args.abc' 10000000 loops, best of 11: 32.7 nsec per loop $ python3.8 -m timeit -r 11 -s 'from types import SimpleNamespace as NS' -s 'args=NS(abc=10)' 'args.abc' 10000000 loops, best of 11: 32.4 nsec per loop