In Perl, one can do the following
for (@foo) {
# do something
next if $seen{$_}++;
}
I would like to be able to do the equivalent in Python, that is to skip a block if it has been executed once.
views:
152answers:
3
+6
Q:
What is the python equivalent of the Perl pattern to track if something has already been seen?
+11
A:
seen = set()
for x in foo:
if x in seen:
continue
seen.add(x)
# do something
See the set documentation for more information.
Also, the examples at the bottom of the itertools module documentation contains a unique_everseen generator that you can use like this:
for x in unique_everseen(foo):
# do something
Greg Hewgill
2010-02-03 00:35:25
Thanks. Looks like unique_everseen might fit the bill although it seems to be listed as an optional recipe instead of being part of the module.
jnman
2010-02-03 00:48:32
jnman, don't let anyone force Python on you. I mean, from the way you choose to write your comments, it's obvious that you find Python at best a necessary evil (perhaps it's a job requirement) and its library severely lacking (contrary to, say, the magnificent CPAN).
ΤΖΩΤΖΙΟΥ
2010-02-03 01:51:18
I am much more comfortable using perl than python. However, due to some stuff that I am currently working on, I have to use python instead hence the questions. It just seems that this particular perl(ism) is sufficiently useful (to me) that I am somewhat surprised that there is no equivalent "terse" version in python (to use your words).
jnman
2010-02-03 02:47:23
@jnman - Python and terse usually don't go together. If you want terse, stick with Perl. I bet money CPAN has a package to do whatever "stuff you're currently working on" that you're using Python for. (I like (and dislike) both languages, but I am a bit biased towards Perl since it's my native tongue, so to speak.)
Chris Lutz
2010-02-03 05:03:37
+2
A:
seen={}
for item in foo:
if seen.has_key(item):
seen[item]+=1
continue # continue is optional, just to illustrate the "next" in Perl
else:
seen[item]=1
ghostdog74
2010-02-03 00:36:09
Alternatively `from collections import defaultdict; seen = defaultdict(int); for item in foo: seen[item] += 1; if seen[item] > 1: continue; #rest of code` (with proper indentation, of course.)
Chris Lutz
2010-02-03 05:05:10
yes, +1 for that. But I guess i am still old school, so this works for version <2.4+
ghostdog74
2010-02-03 05:19:03
+1
A:
If you don't care about the order of the things in foo, and only that the unique items are iterated over, then the solution is much simpler.
for x in set(foo):
do something
Paul Hankin
2010-02-03 04:51:36