Python procedure return values [closed]

Answer 1

Heh. Coming at it from a Java perspective, your question is obvious. It should throw an error.

But if you came at it from a Perl perspective, it's not obvious at all. You're setting a variable to no value, big deal.

It's all about the typing. Like Java, Python is strongly typed, but unlike Java, it is also dynamically typed. Most dynamically typed languages allow shenanigans like this.

Answer 2

What compiler? The bytecode compiler checks your code for syntax on import, but syntax doesn't include "check all paths for a return". So when test() gets called at runtime, and the value of x isn't 0 and we just fall out of the function, what should we do? return 0? Why not -1, or "", or -infinity? So a nice neutral value should always be returned in the absence of an explicit return statement - and that value is None. This is a standard feature of the language - welcome to Python!

Answer 3

While every function might not have an explicit return it will have an implicit one, that is None, which incidentally is a normal Python object. Further, functions often return None explicitly.

I suppose returning None implicitly is just an ease-of-use optimisation.

P.S. I wonder what you propose compile would do in the following case:

def t():
    if True:
        return 'ham'

P.P.S. return statement indicates that something needs to be returned, be it None, 42 or 'spam'. Some functions, however, don't really return anything, like list.sort or __init__, therefore it would be an overhead to require a return None statement at the end of each __init__.

Answer 4

It's beautiful ...

It returns None in the case where val != 0 which to me make sense.

Python 2.6.1 (r261:67515, Jun 18 2009, 17:24:16)
[GCC 3.3.3 (SuSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def test(val):
...     if 0 == val:
...         return 8
...
>>> a = test(8)
>>> print a
None
>>> print test(0)
8
>>>

Do you have any arguments why you think the behavior is wrong?

You can use c++ if you need strong compile time type checking but for python it seems extremely flexible and I assume that's what you want when you selected python for your task.

Answer 5

It's because of the dynamic typing.

You don't have to make the distinction between, say, turbo pascal's procedure and function. They are all function, they return None by default, which is logically correct. If you don't say anything it returns nothing, None.

I hope it make more sense now-

Answer 6

Python never checks for human misbehavior. The language philosophy is to assume that programmers are grown up adults and know what they are doing.

Since :

• Python is an autodocumented language, so you'll know in a blink what returns a function;
• Python uses duck typing, so getting None by default can ease dynamic function call a lot.

Returning None make more senss than raising an error.

E.G :

function_stack = [func1, func2, func3]

for f in function_stack :
    a = f()
    if a is not None :
        # do something

No matter if f is f() is a function or a procedure. There is no such thing in Python. You can apply all the function, and process output if there is any.