Python 2.x gotchas and landmines

Question

The purpose of my question is to strengthen my knowledge base with Python and get a better picture of it, which includes knowing its faults and surprises. To keep things sp

Answer 1

Unintentionally mixing oldstyle and newstyle classes can cause seemingly mysterious errors.

Say you have a simple class hierarchy consisting of superclass A and subclass B. When B is instantiated, A's constructor must be called first. The code below correctly does this:

class A(object):
    def __init__(self):
        self.a = 1

class B(A):
    def __init__(self):
        super(B, self).__init__()
        self.b = 1

b = B()

But if you forget to make A a newstyle class and define it like this:

class A:
    def __init__(self):
        self.a = 1

you get this traceback:

Traceback (most recent call last):
  File "AB.py", line 11, in 
    b = B()
  File "AB.py", line 7, in __init__
    super(B, self).__init__()
TypeError: super() argument 1 must be type, not classobj

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