On Thu, Feb 15, 2018 at 10:13:37AM +0000, Jamie Willis wrote: > I'm not sure it does indicate a need for refactoring, I'd argue it's quite > a common pattern, at least in functional languages from which this > construct arises. > > In fact, in those languages, there are laws that govern interactions with > the comprehensions (though this comes from monads and monads perhaps don't > quite apply to pythons model). These laws define behaviour that is expected > equivalent by users;
Python is not a functional language, and the usual Pythonic solution to an overly complex or inefficient functional expression is to refactor into non-functional style. In my experience, most Python users have never even heard of monads, and those who have, few grok them. (I know I don't.) > [x for x in xs] = xs > [f(x) for x in [x]] = f(x) > [g(y) for y in [f(x) for x in xs]] = [g(y) for x in xs for y in f(x)] I think these should be: [x for x in xs] = list(xs) # not to be confused with [x] [f(x) for x in [x]] = [f(x)] [g(y) for y in [f(x) for x in xs]] = [g(y) for x in xs for y in [f(x)]] > Even though that last law isn't completely analogous to the given example > from the OP, It can be used though. He has: [f(x) + g(f(x)) for x in xs] which can be written as [y + g(y) for x in xs for y in [f(x)]] Here's an example: # calls ord() twice for each x py> [ord(x) + ord(x)**2 for x in "abc"] [9506, 9702, 9900] # calls ord() once for each x py> [y + y**2 for x in "abc" for y in [ord(x)]] [9506, 9702, 9900] And one last version: py> [y + y**2 for y in [ord(x) for x in "abc"]] [9506, 9702, 9900] In production, I'd change the last example to use a generator comprehension. > the transformation he wants to be able to do does arise from > the laws. So it could be argued that not being able to flatten the > comprehension down via law 3 is unexpected behaviour Law 3 does apply, and I'm not sure what you mean by the statement that we can't flatten the comprehension down. -- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
