[Python-Dev] 2.5 and beyond (original) (raw)

Tim Peters tim.peters at gmail.com
Sat Jul 1 02:15:51 CEST 2006

[Giovanni Bajo]

Yes but:

>>> a = [] >>> for i in range(10): ... a.append(lambda: i) ... >>> print [x() for x in a] [9, 9, 9, 9, 9, 9, 9, 9, 9, 9] This subtle semantic of lambda is quite confusing, and still forces people to use the "i=i" trick.

So stay away from excruciating abuses of lexical scoping you don't understand ;-) What do you expect i to refer to? "Oh, it should guess that I didn't really mean to defer evaluation of the lambda body at all, but instead evaluate the lambda body at the time I define the lambda and then synthesize some other function that captures the specific outer bindings in effect at lambda-definition time" doesn't really cut it.

Try spelling what you think you want here in Scheme. Before it works, you'll probably end up with some equally "atrocious" (let ((i i)) ...) gimmick to force capturing each binding for i as it flies by. Else Scheme will also use the outer binding for i in effect at the time the lambdas are executed.

This isn't typical use for lambda, and I don't think it's what Andrew had in mind.

More information about the Python-Dev mailing list