2018-03-06 13:18 GMT+03:00 Chris Angelico <ros...@gmail.com>: > The identity function is: > > filter(lambda x: x, range(10)) > > How is it consistent with truthiness? Exactly the same way the > underlying object is. There's no requirement for the predicate > function to return True or False - it's perfectly acceptable, for > instance, to do this: > > filter(lambda x: x % 3, range(10)) > > to eliminate all multiples of three. >
Yes there is no reason to return True and False, but in the case of `None` and `bool` under the hood there will be no difference and the form with `bool` is much more readable. > > That said, though, any use of filter() that involves a lambda function > should probably become list comps or genexps, so filter itself should > only be used when there really IS a pre-existing function that does > the job. Filter is generally faster than list comprehension or generators. %timeit [*filter(lambda x: x % 3, range(1000))] 100 µs ± 16.4 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each) f = lambda x: x % 3 %timeit [*(f(i) for i in range(1000))] 132 µs ± 73.5 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each) %timeit [f(i) for i in range(1000)] 107 µs ± 179 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each) > So, for instance, you could strip out every occurrence of the > string "0" with: > > filter(int, list_of_strings) > > And that still depends on the normal Python rules for boolification. > If that's valid, then it should be just as viable to say > "filter(identity-function, ...)", which is spelled "filter(None, > ...)". > > ChrisA > -- > https://mail.python.org/mailman/listinfo/python-list > -- https://mail.python.org/mailman/listinfo/python-list
