python - Difference between a 'for' loop and map

Question

From the title, yes there is a difference. Now applied to my scenario: let's consider a class Dummy:

class Dummy:
    def __init__(self):
        self.attached = []

    def attach_item(self, item):
        self.attached.append(item)

If I use this:

D = Dummy()
items = [1, 2, 3, 4]
for item in items:
    D.attach_item(item)

I indeed get D.attached = [1, 2, 3, 4]. But if I map the function attach_item to the items, D.attached remains empty.

map(D.attach_item, items)

What is it doing?

question from:https://stackoverflow.com/questions/51671132/difference-between-a-for-loop-and-map

Answer 1

A very interesting question which has an interesting answer.

The map function returns a Map object which is iterable. map is performing its calculation lazily so the function wouldn't get called unless you iterate that object.

So if you do:

x = map(D.attach_item, items)
for i in x:
    continue

The expected result will show up.