[Tutor] __getitem__

[Kent Johnson]

Christopher Spears wrote:
> I understand that you can use __getitem__ as a hook to
> modify indexing behavoir in a class.  That's why
> __getitem__ not only affects [] but also for loops,
> map calls, list comprehension, etc.  For loops, etc.
> work by indexing a  sequences from zero to a higher
> index until out-of-bounds is reached.  But why does
> this work?
> 
> 
>>>>class stepper:
> 
> ...     def __getitem__(self, i):
> ...         return self.data[i]
> ...
> 
>>>>'p' in X
> 
> True
> 
> What does 'in' have to do with indexing?

How do you suppose 'in' works? To see if something is in a list, for 
example, you have to iterate over each element of the list and check if 
it is the item you expect.

Under the hood, Python will use the __contains__() or __getitem__() 
special method of a class to evaluate 'x in y'.

Loosely speaking, if y is an instance of a class that implements 
__getitem__() but not __contains__(), 'x in y' is more or less the same 
as this:

def in(x, y):
   for i in y:
     if i == x:
       return True
   return False

 From the language reference:
"For user-defined classes which do not define __contains__() and do 
define __getitem__(), x in y is true if and only if there is a 
non-negative integer index i such that x == y[i], and all lower integer 
indices do not raise IndexError exception. (If any other exception is 
raised, it is as if in raised that exception)."

http://docs.python.org/ref/comparisons.html#l2h-432

Kent