On Jan 5, 2010, at 1:47 PM, Jason Grout wrote:
Dag Sverre Seljebotn wrote:
Jason Grout wrote:
I'm trying to generate a list of functions where each function
returns its place in a list. Here is my code:
cc=[(lambda: x) for x in [1..2]]
However, I have:
cc[0]() returns 2 (but I want it to return 1)
cc[1]() returns 2 (correctly)
Does anyone know what is going on here? It seems like all the
functions just end up being the last function!
Py> x
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined
Py> cc=[(lambda: x) for x in range(2)]
Py> x=10
Py> cc[0]()
10
x is declared by Python as a variable in the enclosing scope (the
function containing that line you wrote). So all the functions
returns the same variable.
Declare a new function which returns the lambda and you're good.
I vaugely remember this behaviour being changed in Python 3,
although it might only have been for generator expressions..
Ah, I see. I was confused by scoping rules in python.
I just found this post, which discusses the problem in detail:
http://lackingrhoticity.blogspot.com/2009/04/python-variable-binding-semantics-part.html
I like the double-lambda solution:
sage: cc=[(lambda y: lambda: y)(x) for x in [1..5]]
sage: [c() for c in cc]
[1, 2, 3, 4, 5]
- Robert
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