A few things to remember while coding in Python
    halve_evens_only = lambda nums: map(lambda i: i/2, filter(lambda i: not i%2, nums))

I still find it rather silly that python doesn't supper a nice list map/filter; it could be so much nicer    nums.filter(lambda i: i%2 == 0).map(lambda i: i/2)

If they did, even including the annoyingly long-to-type "lambda". List comprehensions are cool and all, but do not really scale visually (i.e. get rather messy) when you have more than one map and filter step.These arbitrary break-away from OO method style into module+data style (len(L) is another!) are one of the things I hate most about Python. There are some reasons for doing so, but a pure-OO (like Scala) or pure-method+data (like F#) would have saved me many a runtime error.