Generator Expressions

List Comprehensions

• List comprehensions can be used to quickly build a new list from an existing list

Example

nums = list(range(10))
odds = [n for n in nums if n % 2 == 1]

print(odds)

Generator Expressions

• The expression inside a list comprehension is known as a generator expression
• These can be used in other contexts

Example

nums = list(range(10))
odds = (n for n in nums if n % 2 == 1)

print(odds)

Generators

• A generator object is an iterable that can only be iterated once
• The object cannot be index like a list
• Value are created just in time for their use

Example

squares = (n*n for n in range(2, 5))

print(squares)
print(next(squares))
print(next(squares))
print(next(squares))

for

• Generator expressions can be iterated using for

Example

words = "lorem ipsum dolor sit amet".split()

first_letters = (w[0] for w in words)

for letter in first_letters:
    print(letter)

Creating Dictionaries

• We can create dictionaries from pairs of values using the dict constructor
• This technique can be combined with generator expressions

Example

words = ["apple", "boy", "carrot"]

letter_words = dict((w[0], w) for w in words)

print(letter_words)

Dictionary Comprehension

• Dictionaries can be quickly created using specialized dictionary comprehension syntax

Example

words = ["apple", "boy", "carrot"]

letter_words = {w[0]: w for w in words}

print(letter_words)

Custom Generators

• The yield keyword can be used to emit values from generators
• Custom generators look like functions but maintain their state between yielding values

Example

def myrange(start, stop, step):
    i = start

    while i < stop:
        yield i
        i += step

for i in myrange(1, 10, 2):
    print(i)

Exercise

Create a generator that will yield every odd number that is not divisible by 5.