How exactly does a generator comprehension work?

A generator comprehension is the lazy version of a list comprehension.

It is just like a list comprehension except that it returns an iterator instead of the list ie an object with a next() method that will yield the next element.

If you are not familiar with list comprehensions see here and for generators see here.

Do you understand list comprehensions? If so, a generator expression is like a list comprehension, but instead of finding all the items you're interested and packing them into list, it waits, and yields each item out of the expression, one by one.

>>> my_list = [1, 3, 5, 9, 2, 6]
>>> filtered_list = [item for item in my_list if item > 3]
>>> print(filtered_list)
[5, 9, 6]
>>> len(filtered_list)
3
>>> # compare to generator expression
... 
>>> filtered_gen = (item for item in my_list if item > 3)
>>> print(filtered_gen)  # notice it's a generator object
<generator object <genexpr> at 0x7f2ad75f89e0>
>>> len(filtered_gen) # So technically, it has no length
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'generator' has no len()
>>> # We extract each item out individually. We'll do it manually first.
... 
>>> next(filtered_gen)
5
>>> next(filtered_gen)
9
>>> next(filtered_gen)
6
>>> next(filtered_gen) # Should be all out of items and give an error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>> # Yup, the generator is spent. No values for you!
... 
>>> # Let's prove it gives the same results as our list comprehension
... 
>>> filtered_gen = (item for item in my_list if item > 3)
>>> gen_to_list = list(filtered_gen)
>>> print(gen_to_list)
[5, 9, 6]
>>> filtered_list == gen_to_list
True
>>> 

Because a generator expression only has to yield one item at a time, it can lead to big savings in memory usage. Generator expressions make the most sense in scenarios where you need to take one item at a time, do a lot of calculations based on that item, and then move on to the next item. If you need more than one value, you can also use a generator expression and grab a few at a time. If you need all the values before your program proceeds, use a list comprehension instead.