How does python implement mutual recursion?

The line B(n-1) says "when this statement is executed, lookup some function B in the module scope, then call it with parameters n-1". Since the lookup happens when the function is executed, B can be defined later.

(Additionally, you can completely overwrite B with a different function, and A will call the new B afterwards. But that can lead to some confusing code.)

If you're worried about not catching calls to nonexistent functions, you can try using static analysis tools. Other than that, be sure you're testing your code.

A SyntaxError will be caught at compile time, but most other errors (NameError, ValueError, etc.) will be caught only at runtime, and then only if that function is called.

"if I have written a function, if its not called in my test.." - and that is why you should test everything.

Some IDEs will raise warnings in various situations, but the best option is still to conduct thorough testing yourself. This way, you can also check for errors that arise through factors like user input, which an IDE's automated checks won't cover.