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

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

def do_1():
    return 0

def do_2():
    return

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

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