Test your knowledge: Python Functions

Review

1. A function's body ends without a return statement. What does a call to it hand back?

   A function with no return, or one whose return carries nothing, hands back None. That is why a forgotten return prints None instead of stopping the program: the value is the absence of one, not a failure.
2. What is wrong with def add(order, cart=[])?

   Default values are evaluated once, when the function is defined, so a mutable default is one list shared by every call that leans on it. The standard escape is defaulting to None and building the container inside the body.
3. What can a function do with a variable defined outside it?

   A function sees the names of its enclosing scopes and can read them, but assigning inside the body creates a new local name instead, shadowing the outer one. Reaching out to rebind takes an explicit global or nonlocal statement, which is usually a sign the design wants a parameter instead.
4. What did the call tax(total, rate=0.05) change?

   A keyword argument overrides the default for that single call, by name, while every other call keeps the default as written. One line changed its behavior and the signature kept its promise.
5. The lesson says composition reads a certain way. Which is it?

   Composition reads top-down as a sentence, each function a clause built from the ones below it. Changing behavior meant changing one function, and every call site followed, which is the whole dividend.