How to call same method for a list of objects?

Question:

Suppose code like this:

class Base:
    def start(self):
        pass
    def stop(self)
        pass

class A(Base):
    def start(self):
        ... do something for A
    def stop(self)
        .... do something for A

class B(Base):
    def start(self):

    def stop(self):

a1 = A(); a2 = A()
b1 = B(); b2 = B()

all = [a1, b1, b2, a2,.....]

Now I want to call methods start and stop (maybe also others) for each object in the list all. Is there any elegant way for doing this except of writing a bunch of functions like

def start_all(all):
    for item in all:
        item.start()

def stop_all(all):
Asked By: Dmitry

||

Answers:

maybe map, but since you don’t want to make a list, you can write your own…

def call_for_all(f, seq):
    for i in seq:
        f(i)

then you can do:

call_for_all(lamda x: x.start(), all)
call_for_all(lamda x: x.stop(), all)

by the way, all is a built in function, don’t overwrite it 😉

Answered By: fortran

The *_all() functions are so simple that for a few methods I’d just write the functions. If you have lots of identical functions, you can write a generic function:

def apply_on_all(seq, method, *args, **kwargs):
    for obj in seq:
         getattr(obj, method)(*args, **kwargs)

Or create a function factory:

def create_all_applier(method, doc=None):
    def on_all(seq, *args, **kwargs):
        for obj in seq:
            getattr(obj, method)(*args, **kwargs)
    on_all.__doc__ = doc
    return on_all

start_all = create_all_applier('start', "Start all instances")
stop_all = create_all_applier('stop', "Stop all instances")
...
Answered By: Ants Aasma

The approach

for item in all:
    item.start()

is simple, easy, readable, and concise. This is the main approach Python provides for this operation. You can certainly encapsulate it in a function if that helps something. Defining a special function for this for general use is likely to be less clear than just writing out the for loop.

Answered By: Mike Graham

Taking @Ants Aasmas answer one step further, you can create a wrapper that takes any method call and forwards it to all elements of a given list:

class AllOf:
    def __init__(self, elements):
        self.elements = elements
    def __getattr__(self, attr):
        def on_all(*args, **kwargs):
            for obj in self.elements:
                getattr(obj, attr)(*args, **kwargs)
        return on_all

That class can then be used like this:

class Foo:
    def __init__(self, val="quux!"):
        self.val = val
    def foo(self):
        print "foo: " + self.val

a = [ Foo("foo"), Foo("bar"), Foo()]
AllOf(a).foo()

Which produces the following output:

foo: foo
foo: bar
foo: quux!

With some work and ingenuity it could probably be enhanced to handle attributes as well (returning a list of attribute values).

Answered By: Joachim Sauer

It seems like there would be a more Pythonic way of doing this, but I haven’t found it yet.

I use “map” sometimes if I’m calling the same function (not a method) on a bunch of objects:

map(do_something, a_list_of_objects)

This replaces a bunch of code that looks like this:

 do_something(a)
 do_something(b)
 do_something(c)
 ...

But can also be achieved with a pedestrian “for” loop:

  for obj in a_list_of_objects:
       do_something(obj)

The downside is that a) you’re creating a list as a return value from “map” that’s just being throw out and b) it might be more confusing that just the simple loop variant.

You could also use a list comprehension, but that’s a bit abusive as well (once again, creating a throw-away list):

  [ do_something(x) for x in a_list_of_objects ]

For methods, I suppose either of these would work (with the same reservations):

map(lambda x: x.method_call(), a_list_of_objects)

or

[ x.method_call() for x in a_list_of_objects ]

So, in reality, I think the pedestrian (yet effective) “for” loop is probably your best bet.

Answered By: abonet

This will work

all = [a1, b1, b2, a2,.....]

map(lambda x: x.start(),all)    

simple example

all = ["MILK","BREAD","EGGS"]
map(lambda x:x.lower(),all)
>>>['milk','bread','eggs']

and in python3

all = ["MILK","BREAD","EGGS"]
list(map(lambda x:x.lower(),all))
>>>['milk','bread','eggs']
Answered By: Mark Essel

Starting in Python 2.6 there is a operator.methodcaller function.

So you can get something more elegant (and fast):

from operator import methodcaller

map(methodcaller('method_name'), list_of_objects)
Answered By: notbad.jpeg

If you would like to have a generic function while avoiding referring to method name using strings, you can write something like that:

def apply_on_all(seq, method, *args, **kwargs):
    for obj in seq:
         getattr(obj, method.__name__)(*args, **kwargs)

# to call:
apply_on_all(all, A.start)

Similar to other answers but has the advantage of only using explicit attribute lookup (i.e. A.start). This can eliminate refactoring errors, i.e. it’s easy to rename the start method and forget to change the strings that refer to this method.

Answered By: Aleksei Petrenko
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