> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Fred Lionetti
> Sent: Wednesday, November 16, 2005 2:32 PM
> To: tutor@python.org
> Subject: [Tutor] lambda in a loop
>
> Hi everyone,
Hello,
> If I have this code:
>
> --------------------------------
> def doLambda(val):
> print "value 2:", val
>
> commands = []
> for value in range(5):
> print "value 1:", value
> commands.append(lambda:doLambda(value))
>
> for c in commands:
> c()
> ----------------------------------
>
> my output is:
> value 1: 0
> value 1: 1
> value 1: 2
> value 1: 3
> value 1: 4
> value 2: 4
> value 2: 4
> value 2: 4
> value 2: 4
> value 2: 4
>
> Obviously, the lambda is using "value" at the end of the loop (4),
> rather than what I want, "value" during the loop (0,1,2,3).
Right. I think the issue is that your lambda calls another funtion.
However, the function isn't called until the lambda is called later,
when value == 4.
> Is there
> any *simple* way around this? I'd prefer not to use a separate array
> with all the values ( i.e.
> commands.append(lambda:doLambda(values[commands.index(c)])) ) if
> possible.
I'd use a closure rather than a lambda.
def wrapper(val):
def inner():
print "value 2:", val
return inner
commands = []
for value in range(5):
print "value 1:", value
commands.append(wrapper(value))
for c in commands:
c()
That way each item in commands is an "inner" function that has its own
local copy of value. So it is really a variable scope issue.
> Thanks,
> Fred
HTH, sorry it isn't more clear.
Christian
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