To be expressive, a function should be free from the confusion created by side effects. Using pure functions can also allow some optimizations by changing evaluation order. The big win, however, stems from pure functions being conceptually simpler and much easier to test.
To write a pure function in Python, we have to write local-only code. This means we have to avoid the global statements. We need to look closely at any use of nonlocal; while it is a side effect in some ways, it's confined to a nested function definition. This is an easy standard to meet. Pure functions are a common feature of Python programs.
There isn't a trivial way to guarantee a Python function is free from side effects. It is easy to carelessly break ...
