Question

I am getting my head around Python for the first time and I am stuck here:

class A:
    def __init__(self):
        a = foo("baa")

class B(A):
    b = foo("boo")

def foo(string):
    return string

At this point I load the above file (named classes) and this happens:

$ python
Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from classes import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "classes.py", line 5, in <module>
    class B(A):
  File "classes.py", line 6, in B
    b = foo("boo")
NameError: name 'foo' is not defined

Notice how the error is in class B, where foo is called directly and not from __init__. Notice also how I have not yet instantiated the class B.

First question:

• Why is it returning an error? I am not instantiating a class.

Moving on. The ‘problem’ is solved by moving the definition of foo() a few lines above:

def foo(string):
    return string

class A:
    def __init__(self):
        a = foo("baa")

class B(A):
    b = foo("boo")

Now I can do

>>> x = B()
>>> x.b
'boo'

but I can’t do

>>> y = A()
>>> y.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'a'

Further questions:

• What is that I have not understood about __init__?

I don’t think this is the same as a forward declaration, hence I hope this question is not a duplicate.

On a side note, I am aiming towards implementing a DSL, but that’s mostly an excuse to get myself to learn python.

Answer 1

First, in class B, the function foo() is called before being declared. A does not have this problem because foo() is only called when the class is instantiated–after the function foo is defined.

For your second question, y.a will not work because you did not say self.a = foo('stirng'). a = foo('stirng') only creates the variable a in the scope of __ init __.

An ideal __ init __ function assigns variables to the instance self.