Prasad, Ramit wrote:
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.
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]
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)
--
Steven
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