Here are some short examples that have provided unexpected results
for intermediate-level Python programmers. 

Add new examples by copying and editing the following sample. 
''Edit your code so it is short and does nothing more than illustrate
your problem.''  Often programming problems are solved by 
making the effort to create simple examples that reproduce them.
This page is meant for problems that appear in a few lines of code
after the extraneous fluff has been removed.  Things that may appear
normal to experienced Python programmers but appear weird: at least
when one first encounters them.

If you can answer your own question, do so. Otherwise leave the 
answer part for somebody else to edit.

=== Sample (copy and edit me) ===

'''Bold'''
{{{
print("edit this")
}}}

'''Usage'''
{{{
>>> edit this
}}}

'''Question'''

Edit this question.

'''Answer'''

Edit this answer.

=== Metaclass Instantiation ===

'''Bold'''
{{{
class A(object):
    
    def __init__(self):
        self.__setattr__('x',1)
        
a = A()

class M(type):
    
    def __init__(cls,cls_name,bases,cls_dict):
        super(M,cls).__init__(cls_name,bases,cls_dict)
        cls.__setattr__('y',1)

class B:
    __metaclass__ = M
    pass
}}}

'''Usage'''
{{{
>>> import Play
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "Play.py", line 14, in ?
    class B:
  File "Play.py", line 12, in __init__
    cls.__setattr__('y',1)
TypeError: Error when calling the metaclass bases
     expected 2 arguments, got 1
}}}

'''Question'''

I thought creating the class B was analogous to creatig the object a.
Clearly, my use of {{{__setattr__()}}} has stretched the analogy too
far but ...  I don't know what's going on.

'''Answer'''

While it is true that - conceptually -  a class is an instance of its metaclass, there is still a big difference
between class objects and instance objects (Yes, it's confusing). 

A call to {{{instance.func(a, b)}}} gets translated into {{{instance.__class.__.func(instance, a, b)}}} 
-- it's a bound method.
A call to {{{cls.func(a, b)}}} does '''not''' get translated into {{{cls.__class__.func(cls, a, b)}}}, because it is an '''unbound method'''. It won't get translated into anything and that's why 
{{{__setattr__()}}} complains, it expects the object whose attribute should get set as its first
argument.
You could have written {{{M.__setattr__(cls, 'y', 1)}}} instead.


'''Hex values passed to external code'''
{{{
ioctl(fd, 0xc0107307)
}}}

'''Usage'''
{{{
>>> fd = open("/dev/null")
>>> import fcntl
>>> fcntl.ioctl(fd,0xc0107307)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to int

}}}

'''Question'''

How do I pass in a hex cmd to ioctl, or some other external function that is expecting an int, not a long?

'''Answer'''

There doesn't seem to be anyway to let python know that you are trying to build an unsigned value, so:

def convert32(number):
   return int(-(number ^ 0xffffffff)-1)

ioctl(fd, convert32(0xc0107307))