A list comprehension in Python is probably one of the first things a Python coder will learn (usually right after arithmetic and boolean logic). But in my own experience, not a lot of people talk about a generator expression. While I have been coding in Python for over 5 years, I personally have learned about it not so long ago.

Instead of using a list comprehension to get something like

[i for i in range(10) if i % 2 == 0]

we can do something like,

(i for i in range(10) if i % 2 == 0)

The latter line of code is a generator expression and returns a generator object which is stored in memory. The generator then only generates the values only when called, so we don’t have to allocate a lot of memory to store something in a list. Compared to a list, a generator takes up much less space. Moreover, it is much more efficient (faster) than building a list.

The above line of code is equivalent to writing a generator function:

def gen(bound):
    for i in range(bound):
        if i % 2 == 0:
            yield i

Some simple examples of what you can use generator expressions for:

sum(1 for i in some_list if condition)  # sum conditional elements in a list

for i in irange(1000000): do_something(i)
# irange() is a range generator and is more efficient than range() which builds the whole list

For further examples and explanations, see https://wiki.python.org/moin/Generators.