Dictionary of Functions

MRAB python at mrabarnett.plus.com
Thu Nov 15 17:27:28 CET 2012


On 2012-11-15 16:04, Kevin Gullikson wrote:
> Hi all,
>
> I am trying to make a dictionary of functions, where each entry in the
> dictionary is the same function with a few of the parameters set to
> specific parameters. My actual use is pretty complicated, but I managed
> to boil down the issue I am having to the following example:
>
> In [1]: def test_fcn(a, x):
>     ...:     return a*x
>     ...:
>
> In [2]: fcn_dict = {}
>
> In [3]: for i in [1,2,3]:
>     ...:     fcn_dict[i] = lambda x: test_fcn(i, x)
>     ...:
>
> In [4]: fcn_dict
> Out[4]:
> {1: <function <lambda> at 0x102b42c08>,
>   2: <function <lambda> at 0x102b42b18>,
>   3: <function <lambda> at 0x102b42c80>}
>
> In [5]: fcn_dict[1](5)
> Out[5]: 15
>
> In [6]: fcn_dict[2](5)
> Out[6]: 15
>
> In [7]: fcn_dict[3](5)
> Out[7]: 15
>
>
> As you can see, all of the functions are returning the value that I want
> for fcn_dict[3]. If I make separate functions for each case instead of a
> dictionary it works, but I would really prefer to use dictionaries if
> possible. Is there a way to make this work?
>
It's looking up 'i' at the time that the function is called, which is
after the 'for' loop has finished and 'i' has been left as 3.

What you need to do is capture the current value of 'i'. The usual way
is with a default argument:

for i in [1,2,3]:
     fcn_dict[i] = lambda x, i=i: test_fcn(i, x)



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