Performance problem with filtering

[Gerhard Häring]

I have two lists of files (approx. 50000 entries). Now I want to have all the
entries of list b, that are not in list a. However, the primitive:

results = []
for entry in b:
    if entry not in a:
        results.append(entry)

is terribly slow. I mean *really* slow. Any recommendations on how to optimize
this? Wouldn't it be nice if I could simply do b.removeall(a)?

Gerhard
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