# 5.13 Generator Expressions¶

- Like list comprehensions, but create iterable generator objects that produce values on demand.
- Known as lazy evaluation.
- For large numbers of items, creating lists can take substantial memory and time.
- Generator expressions can reduce memory consumption and improve performance if the whole list is not needed at once.

In [1]:

    numbers = [10, 3, 7, 1, 9, 4, 2, 8, 5, 6]

In [2]:

    for value in (x ** 2 for x in numbers if x % 2 != 0):
        print(value, end='  ')

    9  49  1  81  25

In [3]:

    squares_of_odds = (x ** 2 for x in numbers if x % 2 != 0)

In [4]:

    squares_of_odds

Out[4]:

    <generator object <genexpr> at 0x110430408>

- Output indicates that `square_of_odds` is a generator object that was created from a generator expression (`<genexpr>`).
- Built-in function `next` receives a generator or iterator and returns the next item.

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