Dynamic/runtime method creation (code generation) in Python

eliben_8206 0
Viewed 53979

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?

exec metaprogramming python
6 Answers

Based on Theran's code, but extending it to methods on classes:



class Dynamo(object):
    pass

def add_dynamo(cls,i):
    def innerdynamo(self):
        print "in dynamo %d" % i
    innerdynamo.__doc__ = "docstring for dynamo%d" % i
    innerdynamo.__name__ = "dynamo%d" % i
    setattr(cls,innerdynamo.__name__,innerdynamo)

for i in range(2):
    add_dynamo(Dynamo, i)

d=Dynamo()
d.dynamo0()
d.dynamo1()

Which should print:


in dynamo 0
in dynamo 1
John_Montgomery_58680

Function docstrings and names are mutable properties. You can do anything you want in the inner function, or even have multiple versions of the inner function that makedynamo() chooses between. No need to build any code out of strings.

Here's a snippet out of the interpreter:

>>> def makedynamo(i):
...     def innerdynamo():
...         print "in dynamo %d" % i
...     innerdynamo.__doc__ = "docstring for dynamo%d" % i
...     innerdynamo.__name__ = "dynamo%d" % i
...     return innerdynamo

>>> dynamo10 = makedynamo(10)
>>> help(dynamo10)
Help on function dynamo10 in module __main__:

dynamo10()
    docstring for dynamo10
Theran_Cochran_401800

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 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.

Edit: 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

Another Edit: With help from @eliben and @bobince, the closure problem should be solved.

Justin_Voss_56160

A bit more general solution:

You can call any method of an instance of class Dummy. The docstring is generated based on the methods name. The handling of any input arguments is demonstrated, by just returning them.

Code

class Dummy(object):

    def _mirror(self, method, *args, **kwargs):
        """doc _mirror"""
        return args, kwargs

    def __getattr__(self, method):
        "doc __getattr__"

        def tmp(*args, **kwargs):
            """doc tmp"""
            return self._mirror(method, *args, **kwargs)
        tmp.__doc__ = (
                'generated docstring, access by {:}.__doc__'
                .format(method))
        return tmp

d = Dummy()    
print(d.test2('asd', level=0), d.test.__doc__)
print(d.whatever_method(7, 99, par=None), d.whatever_method.__doc__)

Output

(('asd',), {'level': 0}) generated docstring, access by test.__doc__
((7, 99), {'par': None}) generated docstring, access by whatever_method.__doc__
user_71281540
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