Python yield keyword

The yield keyword is used in Python to create generator functions. It allows a function to produce a sequence of values lazily, pausing its execution and resuming where it left off when called again. Generators created with yield are more memory-efficient than returning a large list because values are generated one at a time.

Example 

def count_up_to(n):

    count = 1

    while count <= n:

        yield count

        count += 1

for number in count_up_to(5):

    print(number)

Output:

1

2

3

4

5

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Syntax

def generator_function():

    yield value

• yield: Pauses the function, saves its state, and returns the value to the caller.
• When the function is called again, it resumes execution from where it was paused.

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Why Use yield?

1. Memory Efficiency: Generators yield values one at a time instead of creating and returning large lists.
2. Lazy Evaluation: Values are generated only when needed, which saves computation time.
3. Infinite Sequences: Enables the creation of sequences that can be infinitely long without exhausting memory.
4. Simpler Code: Simplifies writing iterators by abstracting the logic of maintaining the internal state.

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Common Examples

1. Simple Generator Function

def generate_numbers():

    yield 1

    yield 2

    yield 3

for num in generate_numbers():

    print(num)

Output:

1

2

3

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2. Generator with a Loop

def even_numbers_up_to(n):

    for i in range(2, n + 1, 2):

        yield i

for even in even_numbers_up_to(10):

    print(even)

Output:

2

4

6

8

10

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3. Infinite Generator

def infinite_counter():

    count = 1

    while True:

        yield count

        count += 1

counter = infinite_counter()

print(next(counter))  # Output: 1

print(next(counter))  # Output: 2

print(next(counter))  # Output: 3

Why? yield allows creating infinite sequences where values are generated on demand.

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4. Using Generators with sum()

def squares_up_to(n):

    for i in range(1, n + 1):

        yield i ** 2

print(sum(squares_up_to(5)))  # Output: 55 (1^2 + 2^2 + 3^2 + 4^2 + 5^2)

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5. Sending Values to a Generator

Generators can receive values using the send() method.

def custom_counter():

    count = 0

    while True:

        step = yield count

        if step is None:

            step = 1

        count += step

counter = custom_counter()

print(next(counter))  # Output: 0

print(counter.send(5))  # Output: 5

print(counter.send(2))  # Output: 7