[Tutor] special attributes naming confusion

[Prasad, Ramit]

> >> That is, for loops first try to build an iterator by calling __iter__, and
> if
> >> that fails they try the sequence protocol obj[0], obj[1], obj[2], ...
> >
> > So...I could instead write __getitem__ for the same effect?
> 
> 
> Er, no... __getitem__ and __iter__ do very different things. __getitem__
> returns individual items, and __iter__ returns an iterator object which, when
> passed to the next() builtin, returns the next item.
> 
> I trust you aren't calling __iter__ explicitly! Always call it from the
> built-in function iter(), or better still, don't call it at all and let the
> for loop do so.
> 

I never call __iter__ and rarely call iter; usually I just let
Python handle all that.

> For the record, here is pseudo-code emulating how Python for-loops work.
> "for x in obj: do_something(x)" becomes something similar to this:
> 
> try:
>      # Try the iterator protocol first.
>      it = iter(x)
> except TypeError:
>      # Fall back on the old-fashioned sequence protocol.
>      counter = 0
>      while True:
>          try:
>              x = obj[counter]

Does this access not use __getitem__()? Not that I would ever
use this in reality, but it is interesting to note.

>          except IndexError:
>              # We must be past the end of the sequence.
>              del counter
>              break
>          do_something(x)
>          counter += 1
> else:
>      # Iterator protocol.
>      while True:
>          try:
>              x = next(it)
>          except StopIteration:
>              # We're done.
>              del it
>              break
>          do_something(x)

Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
712 Main Street | Houston, TX 77002
work phone: 713 - 216 - 5423

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