Why would a function end with "return 0" instead of "return" in python?

Question

Could you please explain the difference between “return 0” and “return”?
For example:

do_1():
    for i in xrange(5):
        do_sth()
    return 0

do_2():
    for i in xrange(5):
        do_sth()
    return

What is the difference between two functions above?

Asked By: alwbtc

||

Source

Answer 1

def do_1():
    return 0

def do_2():
    return

# This is the difference
do_1 == 0 # => True
do_2 == 0 # => False

Answered By: user647772

Answer 2

it has nothing to do specifically with python.

whenever you perform a function you can optionally return a value.

the return keyword is what informs the function if it should return a value or not.

if no value is given to the return or no variable is assigned to be returned, then the return value is None

if you assign a value, in this case, 0 to be returned, then the value 0 will be returned by the function and the function will end when the return keyword and value is reached.

some more information on 0:
the reason a 0 would be used is because it is commonplace that functions which return 0 were “successful” and non-zero return values are either simply the value to be returned, or sometimes error codes if the function did not perform correctly.

Answered By: Inbar Rose

Answer 3

In python, a function would return None either explicitly or implicitly.

e.g.

# Explicit
def get_user(id):
    user = None
    try:
        user = get_user_from_some_rdbms_byId(id)
    except:
        # Our RDBMS raised an exception because the ID was not found.
        pass
    return user  # If it is None, the caller knows the id was not found.

# Implicit
def add_user_to_list(user):
    user_list.append(user)   # We don't return something, so implicitly we return None

A python function would return 0 either because of some computation:

def add_2_numbers(a,b):
    return a + b      # 1 -1 would return 0

Or because of a magic flag kind of thing, which is frowned upon.

But in python we don’t use 0 to denote success because this:

if get_user(id):

would not evaluate to True if we returned 0 therefore this if branch would not run.

In [2]: bool(0)
Out[2]: False

Answered By: rantanplan

Answer 4

Depends on usage:

>>> def ret_Nothing():
...     return
... 
>>> def ret_None():
...     return None
... 
>>> def ret_0():
...     return 0
... 
>>> ret_Nothing() == None
True
>>> ret_Nothing() is None  # correct way to compare values with None
True
>>> ret_None() is None
True
>>> ret_0() is None
False
>>> ret_0() == 0
True
>>> # and...
>>> repr(ret_Nothing())
'None'

And as mentioned by Tichodroma, 0 is not equal to None. However, in boolean context, they are both False:

>>> if ret_0():
...     print 'this will not be printed'
... else:
...     print '0 is boolean False'
... 
0 is boolean False
>>> if ret_None():
...     print 'this will not be printed'
... else:
...     print 'None is also boolean False'
... 
None is also boolean False

More on Boolean context in Python: Truth Value Testing

Answered By: aneroid

Answer 5

In Python, every function returns a return value, either implicitly or explicitly.

>>> def foo():
...     x = 42
... 
>>> def bar():
...     return
... 
>>> def qux():
...     return None
... 
>>> def zero():
...     return 0
... 
>>> print foo()
None
>>> print bar()
None
>>> print qux()
None
>>> print zero()
0

As you can see, foo, bar and qux return exactly the same, the built in constant None.

foo returns None because a return statement is missing and None is the default return value if a function doesn’t explicitly return a value.
bar returns None because it uses a return statement without an argument, which also defaults to None.
qux returns None because it explicitly does so.

zero however is entirely different and returns the integer 0.

If evaluated as booleans, 0 and None both evaluate to False, but besides that, they are very different (different types in fact, NoneType and int).

Answered By: Lukas Graf

Why would a function end with "return 0" instead of "return" in python?

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