substitute of function pointers in python

Question

I have worked in low level C programming for years and I don't have enough exposure to Object oriented approaches. In C if I was developing some layered architecture then each layer has interfaces defined by function pointers. The advantage of that the whole layer can be replaced by just setting those function pointers at initialization to another layer.

I want the same thing but this time in Python. What is the coolest way to achieve that. To give a little background to my problem, I have a data generator which can output records to different mediums. The medium is specified at the configuration time. I don't want to use if or switch statements here. The best way was to use function pointers in C but what are the options available here in Python. Any Object oriented approaches are also appreciated.

Thanks

take a look at polymorphism en.wikipedia.org/wiki/…

Facundo Casco
– Facundo Casco

2013-01-18 20:40:22 +00:00
Commented Jan 18, 2013 at 20:40 — Facundo Casco
– Facundo Casco, Commented Jan 18, 2013 at 20:40

Greg Hewgill · Accepted Answer · 2013-01-18 20:48:14Z

16

Python supports functions as a first-class data type. So you can do something like:

def foo(x):
    print("foo: " + x)

def bar(x):
    print("bar: " + x)

f = foo
f("one")
f = bar
f("ten")

prints

foo: one
bar: ten

This is very similar to your experience with function pointers in C. Although Python certainly supports more elaborate object-oriented programming styles, you are under no obligation to use them.

An example using classes, where you can group related functions together:

class Dog:
    def noise(self, x):
        print("bark! " + x)
    def sleep(self):
        print("sleeping on floor")

class Cat:
    def noise(self, x):
        print("meow! " + x)
    def sleep(self):
        print("sleeping on keyboard")

a = Dog()
a.noise("hungry")
a.sleep()

a = Cat()
a.noise("hungry")
a.sleep()

This version prints:

bark! hungry
sleeping on floor
meow! hungry
sleeping on keyboard

edited Jan 18, 2013 at 20:48

answered Jan 18, 2013 at 20:37

Greg Hewgill

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3 Comments

abarnert Over a year ago

+1. It might be worth adding an example showing how easily you can port the "table of function pointers" C idiom (which is where I think the OP is going) to a simple dict of functions.

Kanwar Saad Over a year ago

Thanks, this looks nice ... but can you elaborate other options or programming styles to achieve this goal.

Greg Hewgill Over a year ago

@KanwarSaad: I have added an example using classes.

John La Rooy · Accepted Answer · 2013-01-18 20:48:25Z

5

You can simply put the functions in a dict

{"type1": function1,
 "type2": function2,
 "type3": function3,
}.get(config_option, defaultfunction)(parameters, go, here)

default_function is called if none of the keys match

If you wish you can separate out the selection an the calling

selected_function = {"type1": function1,
                     "type2": function2,
                     "type3": function3,
                     }.get(config_option, defaultfunction)

some_instance = SomeClass(selected_function)

answered Jan 18, 2013 at 20:48

John La Rooy

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Comments

argentage · Accepted Answer · 2013-01-18 20:39:36Z

3

in python functions are first-class data types.

def z(a):
    print(a)
def x(a):
    print "hi"

functions = [z,x]
y = functions[0]
y("ok") # prints "ok"
y = functions[1]
y("ok") # prints "hi"

answered Jan 18, 2013 at 20:39

argentage

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1 Comment

user395760 Over a year ago

Note that "first class value" means much more than function pointers: For example, new functions can be created at runtime.

Facundo Casco · Accepted Answer · 2013-01-18 20:46:00Z

3

Maybe something alog this lines:

class MediumA:
    def one_method(self):
        return

    def another_method(self):
        return

class MediumB:
    def one_method(self):
        return

    def another_method(self):
        return


class DataGenerator:
    # here you can read this from some configuration
    medium = MediumA()  # this will be shared between all instances of DataGenerator

    def generate(self):
        # all mediums responds to this methods
        medium.one_method()
        medium.another_method()

generator = DataGenerator()
# you can even change the medium here
generator.medium = MediumB()

Hope it helps

answered Jan 18, 2013 at 20:46

Facundo Casco

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