Where is the data store for the python generator?

Question

I refer to Python : Using the map function It says "map returns a specific type of generator in Python 3 that is not a list (but rather a 'map object', as you can see). " That is my understanding too. Generator object do not contain the values but is able to give you the values when you call it (next()). So my question is where are those values store ?

I tried the following experiment.

1. create 2 tuple and check their size
2. create 2 map objects from the tuple
3. do a next() on the map objects to use up some of the values
4. delete one of the tuple
5. continue to do next()

I would assume that when I delete the tuple, there would be no more values to do a next() but that's not the case. So my question is where are those values coming from ? Where are they stored after I delete the tuple ?

Code:

t1 = tuple(range(1000))
t2 = tuple(range(10000))
print(f'{t1[:10]} len = {len(t1):5d} size = {getsizeof(t1):5d}')
print(f'{t2[:10]} len = {len(t2):5d} size = {getsizeof(t2):5d}')

Output:

(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) len =  1000 size =  8040
(0, 1, 2, 3, 4, 5, 6, 7, 8, 9) len = 10000 size = 80040

Code:

m1 = map(lambda y: print(y), t1)
m2 = map(lambda y: print(y), t2)
print(f'size of m1 = {getsizeof(m1)}')
print(f'size of m2 = {getsizeof(m2)}')

Output:

size of m1 = 48
size of m2 = 48

Do the following a number of times:

next(m1)
next(m2)

Output:

23
23

Delete the tuple:

import gc
del t1
gc.collect()
t1

Output:

168
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-336-df561a5cc277> in <module>
      2 del t1
      3 gc.collect()
----> 4 t1

NameError: name 't1' is not defined

Continue next()

next(m1)
next(m2)

Output:

31
31

I'm still able to get values from map after deleting the tuple.

Answer 1

With del t1 you delete the variable , not the object it references.

Before del t1 :

After del t1 :

So that's still all alive and well and functional. You just don't have the separate t1 variable referencing the tuple anymore.

Answer 2

del doesn't remove the tuple from memory, it just removes the variable.

The map object has its own reference to the tuple -- it's a class instance variable variable.

Garbage collection doesn't remove a the tuple from memory until all references to it are destroyed. This will happen when the generator reaches the end (it should delete its own reference to avoid a memory leak) or if you delete the reference to the generator.