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?

Asked By: Mathieu

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Source

Answer 1

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.

Answered By: Lev Zakharov

Answer 2

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 + 3
list(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 + 3
def odd(a):
    return a % 2 == 1
list(map(add, filter(odd, items)))

> [4, 6]

Answered By: hellow

Answer 3

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.

Answered By: Farbod Shahinfar

Difference between a 'for' loop and map

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