Question

I am trying to dynamically create module level functions from the methods in a class. So for every method in a class, I want to create a function with the same name which creates an instance of the class and then calls the method.

The reason I want to do this is so I can take an object-oriented approach to creating Fabric files. Since Fabric will call module level functions but not methods of a class, this is my work-around.

I have used the following links to get me started

• How do I get list of methods in a Python class?
• dynamically adding functions to a Python module
• How do I call setattr() on the current module?
• The Python getattr Function
• Calling a function of a module by using its name (a string)
• How to modify the local namespace in python

And I have come up with the following code

import inspect
import sys
import types

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

    def method1(self, arg1):
        print 'method 1 %s' % arg1

    def method2(self):
        print 'method 2'

def fabric_class_to_function_magic(module_name):
    # get the module as an object
    print module_name
    module_obj = sys.modules[module_name]
    print dir(module_obj)

    # Iterate over the methods of the class and dynamically create a function
    # for each method that calls the method and add it to the current module
    for method in inspect.getmembers(TestClass, predicate=inspect.ismethod):
        print
        print method
        method_name, method_obj = method

        # create a new template function which calls the method
        def newfunc_template(*args, **kwargs):
            tc = TestClass()
            func = getattr(tc, method_name)
            return func(*args, **kwargs)

        # create the actual function
        print 'code: ', newfunc_template.func_code
        print 'method_name: ', method_name
        newfunc = types.FunctionType(newfunc_template.func_code,
                                     {'TestClass': TestClass,
                                      'getattr': getattr,
                                      'method_name': method_name,
                                      },
                                     name=method_name,
                                     argdefs=newfunc_template.func_defaults,
                                     closure=newfunc_template.func_closure,
                                     )

        # add the new function to the current module
        setattr(module_obj, method_name, newfunc)

# test the dynamically created module level function
thismodule = sys.modules[__name__]
print dir(thismodule)
fabric_class_to_function_magic(__name__)
print dir(thismodule)
method1('arg1')
method2()

And I get the following error

['TestClass', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'fabric_class_to_function_magic', 'inspect', 'sys', 'thismodule', 'types']
__main__
['TestClass', '__builtins__', '__doc__', '__file__', '__name__', '__package__', 'fabric_class_to_function_magic', 'inspect', 'sys', 'thismodule', 'types']

('__init__', <unbound method TestClass.__init__>)
code:  <code object newfunc_template at 0x7f8800a28d50, file "test.py", line 85>
method_name:  __init__

('method1', <unbound method TestClass.method1>)
code:  <code object newfunc_template at 0x7f8800a28d50, file "test.py", line 85>
method_name:  method1

('method2', <unbound method TestClass.method2>)
code:  <code object newfunc_template at 0x7f8800a28d50, file "test.py", line 85>
method_name:  method2
['TestClass', '__builtins__', '__doc__', '__file__', '__init__', '__name__', '__package__', 'fabric_class_to_function_magic', 'inspect', 'method1', 'method2', 'sys', 'thismodule', 'types']
Traceback (most recent call last):
  File "test.py", line 111, in <module>
    method1('arg1')
  File "test.py", line 88, in newfunc_template
    return func(*args, **kwargs)
TypeError: method2() takes exactly 1 argument (2 given)

It seems to be reusing the reference to the function? Any ideas?

UPDATE: Here is the working code with Ned Batchelder’s fix

def fabric_class_to_function_magic(module_name):
    # get the module as an object
    module_obj = sys.modules[module_name]

    # Iterate over the methods of the class and dynamically create a function
    # for each method that calls the method and add it to the current module
    for method in inspect.getmembers(TestClass, predicate=inspect.ismethod):
        method_name, method_obj = method

        # get the bound method
        tc = TestClass()
        func = getattr(tc, method_name)

        # add the function to the current module
        setattr(module_obj, method_name, func)

UPDATE 2: Here is my blog post on the subject: http://www.saltycrane.com/blog/2010/09/class-based-fabric-scripts-metaprogramming-hack/

Answer 1

You’re over-thinking your solution. Change the end of fabric_class_to_function_magic to be this:

    tc = TestClass()
    func = getattr(tc, method_name)

    # add the new function to the current module
    setattr(module_obj, method_name, func)

and it works fine. No need to make a new function object, you already have one returned by getattr on your object. The bound method returned by getattr is a callable thing. Just assign it to your module attribute, and you are good to go.