Functions, arguments, and scope

A function should be honest about what it touches

Python resolves a name by looking in the local scope, then any enclosing function, then the module, then the built-ins. Assigning to a name anywhere in a function makes it local for the whole function, which is why reading a global before assigning it raises an error.

Keyword arguments with defaults make a call site self-documenting, and are the difference between a call you can read and four bare booleans in a row.

A function that both returns a value and mutates its argument is hard to reuse and harder to test. Pick one.