List performance and CSV

[jepler at unpythonic.net]

You'll probably see a slight speed increase with something like
    for a in CustomersToMatch:
        for b in Customers:
            if a[2] == b[2]:
                a[1] = b[1]
                break
But a really fast approach is to use a dictionary or other structure
that turns the inner loop into a fast lookup, not a slow loop through
the 'Customers' list.  Preparing the dictionary would look like
    custmap = {}
    for c in Customers:
        k = c[2]
        if k in custmap: continue
        custmap[k] = c
and the loop to update would look like
    for a in customerstomatch:
        try:
            a[1] = custmap[a[2]][1]
        except KeyError:
            continue

(all code is untested)

In "big-O" terms, I believe this changes the complexity from O(m*n) to O(m+n).

Jeff
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