[kachayev]

This example was given to describe map/reduce pattern, not advice to use it instead of sum:

    reduce(operator.add, map(len, ss))

You know, it's hard to understand new concept. And it's twice harder to explain new concept with sophisticated code examples. So I used as simple code and situations as possible. The same for square_sum - it's easier "to see" functional pattern in map(f,l) than in (f(x) for x in l).

Regarding to debug function and partial function application, - it depends. As for me

    debug = partial(log, "debug")

is easier and meaningfully describe what's actually going on. But here we have only one function, what if you have to create 5-6 same functions? Would you prefer "def" syntax instead of partial? I have one more example of using this pattern in more sophisticated situation - code listing for dictionary binding to list of functions, "partial" looks really naturally.