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

Tim Peters tim.peters at gmail.com
Sat Jul 1 10:08:39 CEST 2006

[Giovanni Bajo]

>>> 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.

[Greg Ewing]

This has nothing to do with the semantics of lambda! It's because Python's for-loop doesn't put its control variable in a new scope, the way Scheme's equivalent construct does.

I don't think I follow that. Scheme has no loops in Python's sense -- things like "do" are shorthand for expressing stylized recursion, where each conceptual iteration gets a fresh set of "loop variables". When people talk about giving a Python for-loop vrbl its own scope, they generally don't mean a new scope on each iteration, they just mean that, e.g.,

i = 5
for i in range(10):
     # do stuff
print i

prints 5 intead of 9, about the same as creating a nested block with its own autos in C. The Scheme way is more like:

i = 5
def step(i):
     # do stuff
     if i < 9:
         step(i+1)
step(0)
print i

except with tail-recursion elimination. That also prints 5, but does a hell of a lot more than just arrange for that.

That's what needs to be addressed to fix this problem. I've made a suggestion about that before, but Guido rejected it, so I won't repeat it here.

Don't recall what that was, but creating a new scope on each iteration sounds hard to explain in Python. If Giovanni wants the Scheme way ;-), it's available:

""" a = [] def step(i): a.append(lambda: i) if i < 9: step(i+1) step(0) print [x() for x in a] """

prints [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], although it's more sanely written in Python with a loop:

""" def make_lambda(i): return lambda: i a = [] for i in range(10): a.append(make_lambda(i)) print [x() for x in a] """

Abusing the default-argument machinery to capture current bindings is never necessary, and is abuse. Of course I do it too -- but rarely :-)

More information about the Python-Dev mailing list