Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> writes: > Suppose you have an expensive calculation that gets used two or more times in > a > loop. The obvious way to avoid calculating it twice in an ordinary loop is > with > a temporary variable: > > result = [] > for x in data: > tmp = expensive_calculation(x) > result.append((tmp, tmp+1)) > > > But what if you are using a list comprehension? Alas, list comps don't let > you > have temporary variables, so you have to write this: > > > [(expensive_calculation(x), expensive_calculation(x) + 1) for x in data] > > > Or do you? ... no, you don't! > > > [(tmp, tmp + 1) for x in data for tmp in [expensive_calculation(x)]] > > > I can't decide whether that's an awesome trick or a horrible hack...
It is a poor man's 'let'. It would be nice if python had a real 'let' construction. Or for example: [(tmp, tmp + 1) for x in data with tmp = expensive_calculation(x)] Alas! -- Piet van Oostrum <pie...@pietvanoostrum.com> WWW: http://pietvanoostrum.com/ PGP key: [8DAE142BE17999C4] -- https://mail.python.org/mailman/listinfo/python-list
