What is the most efficient way to compare similar contents in two lists?

From	Zachary Dziura <zcdziura@gmail.com>
Newsgroups	comp.lang.python
Subject	What is the most efficient way to compare similar contents in two lists?
Date	2011-06-13 07:58 -0700
Organization	http://groups.google.com
Message-ID	<3ff2f4cc-7c26-4f59-9463-d6f0874174d8@dn9g2000vbb.googlegroups.com> (permalink)

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similar_headers = 0
different_headers = 0
source_headers = sorted(source_mapping.headers)
target_headers = sorted(target_mapping.headers)

# Check if the headers between the two mappings are the same
if set(source_headers) == set(target_headers):
    similar_headers = len(source_headers)
else:
    # We're going to do two run-throughs of the tables, to find the
    # different and similar header names. Start with the source
    # headers...
    for source_header in source_headers:
        if source_header in target_headers:
            similar_headers += 1
        else:
            different_headers += 1
    # Now check target headers for any differences
    for target_header in target_headers:
        if target_header in source_headers:
            pass
        else:
            different_headers += 1

As you can probably tell, I make two iterations: one for the
'source_headers' list, and another for the 'target_headers' list.
During the first iteration, if a specific header (mapped to a variable
'source_header') exists in both lists, then the 'similar_headers'
variable is incremented by one. Similarly, if it doesn't exist in both
lists, 'different_headers' is incremented by one. For the second
iteration, it only checks for different headers.

My code works as expected and there are no bugs, however I get the
feeling that I'm not doing this comparison in the most efficient way
possible. Is there another way that I can make this same comparison
while making my code more Pythonic and efficient? I would prefer not
to have to install an external module from elsewhere, though if I have
to then I will.

Thanks in advance for any and all answers!

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