exec

Jean-Michel Pichavant jeanmichel at sequans.com
Thu Mar 1 14:46:43 CET 2012


Rolf Wester wrote:
> Hi,
>
> I would like to define methods using exec like this:
>
> class A:
>     def __init__(self):
>         cmd = "def sqr(self, x):\n    return x**2\nself.sqr = sqr\n"
>         exec cmd
> a = A()
> print a.sqr(a, 2)
>
> This works, but I have to call sqr with a.sqr(a, 2), a.sqr(2) does not work
> (TypeError: sqr() takes exactly 2 arguments (1 given)).
>
> Is there a possibility to define methods using exec and getting normal behavior?
>
> I would be very appreciative for any help.
>
> With kind regards
> Rolf Wester
>
>
>   
I'll try to ignore you are using the exec statement which is an error :o)

a.sqr(2) would have worked only if sqr was a *bound* method.

print a.sqr
<function sqr at 0x94ecd14>


to oppose to
print a.__init__
<bound method A.__init__ of <__main__.A instance at 0x948734c>>


You cannot dynamically bind a method, however you can do the following.

cmd = "def sqr(x):\n    return x**2\nself.sqr = sqr\n"



http://docs.python.org/reference/datamodel.html : <http://docs.python.org/reference/datamodel.html>
"It is also important to note that user-defined functions which are attributes of a class instance are not converted bound methods; this only happens when the function is an attribute of the class. "

JM




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