"Canonical" way of deleting elements from lists

[Hrvoje Niksic]

Robert Latest <boblatest at yahoo.com> writes:

> From a list of strings I want to delete all empty ones. This works:
>
>     while '' in keywords: keywords.remove('')

If you're looking for a quick (no quadratic behavior) and convenient
way to do it, you can do it like this:

keywords = [s for s in keywords if s != '']

But that creates a new list, which might not be wanted for long lists
with few empty elements (or for shared lists).  It also iterates over
every list element in a Python loop, which can take some time for long
lists.

> Is there a way to just walk the list once and throw out unwanted
> elements as one goes along?

I'd do it like this:

i = 0
while 1:
    try:
        i = keywords.index('', i)
    except ValueError:
        break
    del keywords[i]

Or even:

try:
    i = 0
    while 1:
        i = keywords.index('', i)      # throws ValueError and stops the loop
        del keywords[i]
except ValueError:
    pass

In both cases the search for the empty string is done in efficient C
code, and you only loop in Python over the actual matches.