Difference between a 'for' loop and map

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.

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.

Quoting the documentation

Return an iterator that applies function to every item of iterable, yielding the results.

which means you have to collect the iterator, e.g.

list(map(D.attach_item, items))> [None, None, None, None]

Hmmm, strange. Why None, None, ...

Yes, you can convert any loop in a map statement, but it's not always useful.Map takes a parameter and does something with it (in most cases) an returns it, without side effects! Here's an example:

def add(a):    return a + 3list(map(add, items))> [4, 5, 6, 7]

The true power comes, when you combine it with other functions like filter

def add(a):    return a + 3def odd(a):    return a % 2 == 1list(map(add, filter(odd, items)))> [4, 6]