The following produces unexpected behavior.
I think that it should produce a list of six different lambda expressions,
but after creation, all six lambda expressions produce the same output.
It's possible that I'm missing something about Python.

##### begin example #######
from math import *

mm = []

for n in range(6):
    f = lambda x: sin(n*x)
    print f, f(1.0)
    mm.append(f)
    
print '***'

for m in mm:
    print m, m(1.0)
###### end example ####

This is due to the fact that Python evaluates the variable 'n' when the function is called, not when it is created. As such, the variable holds the latest value for all functions, and they exhibit identical behaviour.

Workaround:

...
f = lambda x, n=n: sin(n*x)
...

And this should work as you expect. More information is available at https://docs.python.org/3/faq/programming.html#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result

I eventually figured that out that source of the problem;

thanks for the coding fix; that's useful to know.

-John

On Thu, Aug 11, 2016 at 8:17 AM, Emanuel Barry <report@bugs.python.org>
wrote:

>
> Emanuel Barry added the comment:
>
> This is due to the fact that Python evaluates the variable 'n' when the
> function is called, not when it is created. As such, the variable holds the
> latest value for all functions, and they exhibit identical behaviour.
>
> Workaround:
>
> ...
> f = lambda x, n=n: sin(n*x)
> ...
>
> And this should work as you expect. More information is available at
> https://docs.python.org/3/faq/programming.html#why-do-
> lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result
>
> ----------
> nosy: +ebarry
> resolution:  -> not a bug
> stage:  -> resolved
> status: open -> closed
> versions:  -Python 2.7
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue27738>
> _______________________________________
>