Global Warming Can Be Fun

So one of the things that’s always bugged me about Python and other languages with ‘foreach’ sorts of iteration are those times when you iterate over a list and need to do something that requires an index, or that requires you to iterate it back-to-front, or that requires you to iterate multiple lists simultaneously.I used to write such loops like this:

for n in range(len(mylist)):
     print n, mylist[i] 

The disadvantage of that is that you have to take the length of the list, and generate a range object, etc. It’s wasteful.

But today I was reading PEP 322 about the reversed() function, and I came across this sentence:

“Like its cousins, zip() and enumerate(), [reversed()] needs to be directly accessible in daily programming. Each solves a basic looping problem: lock-step iteration, loop counting, and reverse iteration.”

And I suddenly went “DOH!”

I’ve used zip() before, and reversed(), but I didn’t even know about enumerate(). Upon reading that sentence, a couple of annoying idiosyncrasies I thought Python had vanished into thin air.

So that code I wrote before should properly be written as:

for n, item in enumerate(mylist):
    print n, item

That’s the magic of python’s generators. I presume that enumerate is written as a generator, almost as simply as this:

def enumerate(iterable):
    i = 0
    for item in iterable:
        yield i, item
        i += 1

The result is that it’s compact, obvious in its usage, and it doesn’t use items from a generator until it needs one. It can even cope with infinite generators (generators that never stop and don’t have a “length”).

The more I use this language (and it’s been 5 years now), the more I like it.