Removing an element from a list based on a predicate

Question

I want to remove an element from list, such that the element contains \'X\' or \'N\'. I have to apply for a large genome. Here is an example:

Answer 1

It is (asympotically) faster to use a regular expression than searching many times in the same string for a certain character: in fact, with a regular expression the sequences is only be read at most once (instead of twice when the letters are not found, in gnibbler's original answer, for instance). With gnibbler's memoization, the regular expression approach reads:

import re
remove = re.compile('[XN]').search

codon = ['AAT','XAC','ANT','TTA']
def pred(s,memo={}):
    if s not in memo:
        memo[s]= not remove(s)
    return memo[s]

print filter(pred,codon)

This should be (asymptotically) faster than using the "in s" or the "set" checks (i.e., the code above should be faster for long enough strings s).

I originally thought that gnibbler's answer could be written in a faster and more compact way with dict.setdefault():

codon = ['AAT','XAC','ANT','TTA']
def pred(s,memo={}):
    return memo.setdefault(s, not any(y in s for y in "XN"))

print filter(pred,codon)

However, as gnibbler noted, the value in setdefault is always evaluated (even though, in principle, it could be evaluated only when the dictionary key is not found).