They're both perfectly valid; but the expression style [0] is shorter and more obvious, especially compared to Optional. Even the documentation for Optional sta

They're both perfectly valid; but the expression style [0] is shorter and more obvious, especially compared to Optional. Even the documentation for Optional states [1]: Note that this is not the same concept as an optional argument, which is one that has a default. [0] https://docs.python.org/3.11/library/stdtypes.html#types-union https://docs.python.org/3.11/library/stdtypes.html#types-uni... [1] https://docs.python.org/3.11/library/typing.html#typing.Optional https://docs.python.org/3.11/library/typing.html#typing.Opti...