Check if required variable has been passed

Question

I have a function which requires a parameter (whateverName(n)) to be passed in. I want to check if the parameter was indeed passed and if it wasn't, I want to display a prompt asking what the desired parameter should be (n = int(raw_input(...))). Any ideas how this can be done (note: I'm a rookie in Python)?

Answer 1

Give n a default value of None, and check for it in the function body.

>>> def frob(n=None):
...     if n is None:
...             n = int(raw_input("Please enter a value:"))
...     return n**2 + n
...
>>> frob(23)
552
>>> frob()
Please enter a value:42
1806

Of course, this means that the user will be unable to call frob(None) even if he's sure that's the value he wants n to have. But in this particular case, frob can only successfully handle integers anyway, so the user shouldn't need to call frob(None) anyway.