Default Arguments, Keyword Arguments & *args/**kwargs

Real functions are rarely called with a single fixed set of arguments every time. Python gives you three complementary tools for flexibility: default values for parameters that are usually the same, keyword arguments for calling functions in a self-documenting way, and `*args`/`**kwargs` for functions that accept an arbitrary number of inputs.

A parameter can be given a default value in the `def` line. If the caller does not supply that argument, the default is used.

Here is one of the most notorious gotchas in the entire language: default argument values are evaluated exactly once, when the function is defined - not once per call. If you use a mutable object (a list, dict, or set) as a default, every call that relies on the default shares the *same* object.

The fix is a well-known idiom: default the parameter to `None`, then create a fresh mutable object inside the function body when the caller did not supply one.

Any argument can be passed by name instead of by position, which makes call sites self-documenting and lets you skip arguments that have defaults without worrying about order.

Sometimes you do not know in advance how many arguments a function will receive. `*args` collects any extra positional arguments into a tuple; `**kwargs` collects any extra keyword arguments into a dict.

• -`*args` and `**kwargs` are just conventional names - the `*` and `**` are what matter, not the word "args".
• -Order in a function signature is always: required positional params, then `*args`, then keyword-only/default params, then `**kwargs`.
• -You will see `*args, **kwargs` again when wrapping functions with decorators later in this course - it is exactly how a generic wrapper forwards "whatever was passed in" to the original function.