• Intermediate Conundrums

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))