Generators

Generator expressions are actually a sort of comprehension too; they compress the more advanced (this time it really is more advanced!) generator syntax into one line. The greater generator syntax looks even less object-oriented than anything we've seen, but we'll discover that once again, it is a simple syntax shortcut to create a kind of object.

Let's take the log file example a little further. If we want to delete the WARNING column from our output file (since it's redundant: this file contains only warnings), we have several options, at various levels of readability. We can do it with a generator expression:

import sys
inname, outname = sys.argv[1:3]

with open(inname) as infile:
    with open(outname, "w") as outfile:
        warnings = (l.replace('\tWARNING', '')
                for l in infile if 'WARNING' in l)
        for l in warnings:
            outfile.write(l)

That's perfectly readable, though I wouldn't want to make the expression much more complicated than that. We...