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Dynamic/runtime method creation (code generation) in Python

I need to generate code for a method at runtime. It's important to be able to run arbitrary code and have a docstring.

I came up with a solution combining exec and setattr, here's a dummy example:

class Viking(object):
 def __init__(self):
 code = '''
 def dynamo(self, arg):
 """ dynamo's a dynamic method!
 """
 self.weight += 1
 return arg * self.weight
 '''
 self.weight = 50
 d = {}
 exec code.strip() in d
 setattr(self.__class__, 'dynamo', d['dynamo'])
if __name__ == "__main__":
 v = Viking()
 print v.dynamo(10)
 print v.dynamo(10)
 print v.dynamo.__doc__

Is there a better / safer / more idiomatic way of achieving the same result?

Answer*

Python will let you declare a function in a function, so you don't have to do the `exec` trickery.

 def __init__(self):

 def dynamo(self, arg):
 """ dynamo's a dynamic method!
 """
 self.weight += 1
 return arg * self.weight
 self.weight = 50

 setattr(self.__class__, 'dynamo', dynamo)

If you want to have several versions of the function, you can put all of this in a loop and vary what you name them in the `setattr` function:

 def __init__(self):
 for i in range(0, 10):

 def dynamo(self, arg, i=i):
 """ dynamo's a dynamic method!
 """
 self.weight += i
 return arg * self.weight
 
 setattr(self.__class__, 'dynamo_' + i, dynamo)
 self.weight = 50

`i=i` is for solving a problem with closures, see https://stackoverflow.com/questions/233673/how-do-lexical-closures-work/235764#235764.

(I know this isn't great code, but it gets the point across). As far as setting the docstring, I know that's possible but I'd have to look it up in the documentation.

You can set the docstring via `dynamo.__doc__`, so you could do something like this in your loop body:

 dynamo.__doc__ = "Adds %s to the weight" % i

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‘i’ will be 10 in each instance of dynamo once the loop has finished. The variable is not rebound each time around the loop. This is one of the big gotchas about using closures in Python (and other similar languages).

bobince
– bobince

2009年02月10日 18:41:23 +00:00
Commented Feb 10, 2009 at 18:41
Ah, drat. Thanks for the clarification. Is there a technique that will work?

Justin Voss
– Justin Voss

2009年02月10日 23:55:23 +00:00
Commented Feb 10, 2009 at 23:55
1

Justin, for the solution of this gotcha see: stackoverflow.com/questions/233673/lexical-closures-in-python/…

Eli Bendersky
– Eli Bendersky

2009年02月11日 15:47:14 +00:00
Commented Feb 11, 2009 at 15:47

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Dynamic/runtime method creation (code generation) in Python

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