[SOLVED] python: lambda problem

[Flimm]

Why won't this code work?


#!/usr/bin/env python

if __name__ == "__main__":
my_list = ["l0", "l1", "l2"]
functions = []

for item in my_list:
functions.append(lambda: item)

# this prints l2 l2 l2, how can I make it print l0 l1 l2?
print functions[0](), functions[1](), functions[2]()



How can I make the lambda function save item's value?

[CptPicard]

Forget what I said here at first, it was nonsense...

...


I think the reason why it doesn't work is that Python's lambdas are not real closures so you can't capture values like that.

[fred.reichbier]

Hello,

it works with an optional argument with a default value which is the desired value (ouch. understandable?):


for item in my_list:
functions.append(lambda item=item: item)


But that's not really nice :D

[Flimm]

Actually, I think I've found a solution:


#!/usr/bin/env python

if __name__ == "__main__":
my_list = ["l0", "l1", "l2"]
functions = []

for item in my_list:
exec "functions.append(lambda: \"%s\")" % item

# this now prints l0 l1 l2
print functions[0](), functions[1](), functions[2]()


I don't think this is the best solution though.

[Flimm]

Hello,

it works with an optional argument with a default value which is the desired value (ouch. understandable?):


for item in my_list:
functions.append(lambda item=item: item)


But that's not really nice :D
Wow, thanks for that. It almost makes sense.
Is this a bad hack, or is this an official way to do it? In other words, is it likely to stop working in future python versions?
Actually, strike that, I think I remember reading that python 3000 will drop support for lambda altogethor.

[fred.reichbier]

Hello,

I haven't heard anything about dropping lambda support in Python 3000, but maybe you're right anyway ;) That is (imho) not perfect coding style, but not a hack, and sometimes useful.
It's just a lambda with one argument which has a default value. The following would look less like a hack:

lambda blah=item: blah :D
So I suggest it will work in Py3K if it supports lambdas ;)

Greetings,

Fred

[pointone]

At one point lambda was slated for removal in Python 3000. Unfortunately no one was able to come up with a better way of providing anonymous functions. And so lambda is here to stay.

:o

[fred.reichbier]

Good to know, thanks :D

[days_of_ruin]

And even if they did it would be in the functools module they are making,
which is going probably going to have map and reduce in it.

[Can+~]

But why are you creating a list of functions that just return the value? I don't see any benefit from that approach.

[Flimm]

But why are you creating a list of functions that just return the value? I don't see any benefit from that approach.
I'm not :D It's just a simplified version of the problem I was facing.
What I really wanted to do was to connect several gtk CellRenderers of several TreeView widgets to just one function, passing which TreeView as an argument.
Like this:

cell_renderer[which_tree_view].connect("toggled", lambda cell, path, which_tree_view=which_tree_view: self.customise_toggled(cell, path, which_tree_view))
Come to think of it, I could have just done:

cell_renderer[which_tree_view].connect("toggled", self.customise_toggled, which_tree_view)
*slaps himself on the forehead*

[pmasiar]

another cool way to create a function which manipulates another function in some consistent way is decorator (http://www.ibm.com/developerworks/linux/library/l-cpdecor.html)