How to use map() to call class methods on a list of objects

Question

I am trying to call object.method() on a list of objects.

I have tried this but can't get it to work properly

newList = map(method, objectList)

I get the error method is not defined but I know that is because it is a class method and not a local function.

Is there a way to do this with map(), or a similar built in function? Or will I have to use a generator/list comprehension?

edit Could you also explain the advantages or contrast your solution to using this list comprehension?

newList = [object.method() for object in objectList]

Answer 1

newList = map(method, objectList) would call method(object) on each object in objectlist.

The way to do this with map would require a lambda function, e.g.:

map(lambda obj: obj.method(), objectlist)

A list comprehension might be marginally faster, seeing as you wouldn't need a lambda, which has some overhead (discussed a bit here).

Answer 2

Use operator.methodcaller():

from operator import methodcaller

map(methodcaller('methodname'), object_list)

This works for any list of objects that all have the same method (by name); it doesn't matter if there are different types in the list.

Answer 3

If the contents of the list are all instances of the same class, you can prefix the method name with the class name.

class Fred:
    def __init__(self, val):
        self.val = val
    def frob(self):
        return self.val

freds = [Fred(4), Fred(8), Fred(15)]
print map(Fred.frob, freds)

Result:

[4, 8, 15]

This can also be done if the elements of the list are subclasses of the specified class. However, it will still call the specified implementation of the method, even if that method is overridden in the subclass. Example:

class Fred:
    def __init__(self, val):
        self.val = val
    def frob(self):
        return self.val

class Barney(Fred):
    def frob(self):
        return self.val * 2

freds = [Fred(4), Barney(8), Barney(15)]
#You might expect the barneys to return twice their val. ex. [4, 16, 30]
#but the actual output is [4, 8, 15]
print map(Fred.frob, freds)