Dynamic/runtime method creation (code generation) in Python

Question

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?

Asked By: Eli Bendersky

||

Source

Answer 1

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.

Answered By: Justin Voss

Answer 2

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

Answered By: Theran

Answer 3

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

Answered By: John Montgomery

Answer 4

Pardon me for my bad English.

I recently need to generate dynamic function to bind each menu item to open particular frame on wxPython. Here is what i do.

first, i create a list of mapping between the menu item and the frame.

menus = [(self.menuItemFile, FileFrame), (self.menuItemEdit, EditFrame)]

the first item on the mapping is the menu item and the last item is the frame to be opened. Next, i bind the wx.EVT_MENU event from each of the menu item to particular frame.

for menu in menus:
    f = genfunc(self, menu[1])
    self.Bind(wx.EVT_MENU, f, menu[0])

genfunc function is the dynamic function builder, here is the code:

def genfunc(parent, form):
    def OnClick(event):
        f = form(parent)
        f.Maximize()
        f.Show()
    return OnClick

Answered By: desdulianto

Answer 5

class Dynamo(object):
    def __init__(self):
        pass

    @staticmethod
    def init(initData=None):
        if initData is not None:
            dynamo= Dynamo()
            for name, value in initData.items():
                code = '''
def %s(self, *args, **kwargs):
%s
                            ''' % (name, value)
                result = {}
                exec code.strip() in result
                setattr(dynamo.__class__, name, result[name])

            return dynamo

        return None

service = Dynamo.init({'fnc1':'pass'})
service.fnc1()

Answered By: Alexander

Answer 6

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__

Answered By: Markus Dutschke

Dynamic/runtime method creation (code generation) in Python

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