Union[int, str] vs int | str - version compatibility

Question

Accepted Answer

The pipe operator syntax 'int | str' was introduced in Python 3.10 (PEP 604). Using it in a runtime expression on Python 3.8 or 3.9 raises a TypeError. Union[int, str] from the typing module works back to Python 3.5 and is the safe choice for libraries that support older versions. The trick is that from __future__ import annotations defers all annotations as strings, which lets you write 'int | str' in annotation positions on Python 3.8+ without error - but that only helps with annotations, not with runtime Union constructs like TypeVar bounds or isinstance checks. For data scientists working in controlled environments (conda, fixed Python version), int | str is fine and cleaner. For shared libraries, stick with Union from typing.