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?

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.

Answer 2

map only creates an iterator. You should iterate through it to add items into D.attached. Like this:

D = Dummy() items = [1, 2, 3, 4] list(map(D.attach_item, items)) 

Yep, don't do it in your code:) But the example is just useful for understanding.