Python yield

The yield keyword is used in a function to make it a generator function. Generator functions return an iterator that produces one value per call instead of all values at once. To get a value from a generator, call the next() function on it. The code within the function is executed up to the yield statement. When yield is encountered, the current value is returned. The function maintains its state between calls, resuming from where it left off. This enhances memory efficiency for large datasets and allows better resource management, such as processing streams or database results.

Syntax

def generatorFunction():
  # ...
  yield value
  # ...

Parameters:

• value: The object to be produced by the generator. If omitted, None is yielded.

Return value:

yield returns the next value in a generator sequence to the caller.

Example

In this example, the generator yields numbers from 1 to 5, producing values one at a time:

# Define a generator function that yields numbers from 1 to 5
def count_up_to_five():
  for i in range(1, 6):
    yield i

# Create the generator
gen = count_up_to_five()

# Iterate over the generator to get values
for number in gen:
  print(number)

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The output of this code is:

1
2
3
4
5

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Note: for loop implicitly calls next() method on the generator gen.

In this code:

• count_up_to_five is defined as a generator function.
• count_up_to_five() is called and it returns a generator object without executing the function body.
• for loop calls next() on gen, which starts executing count_up_to_five.
• count_up_to_five runs until it hits the yield statement, returning the current value of i.
• for loop calls next() again, count_up_to_five function resumes right after the yield statement.

Codebyte Example: Printing a list of words with return vs. yield

This Codebyte example shows the difference between using return and yield in a function:

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Note: Because yield produces an iterator, we need to loop through the iterator to get the values to print. If not, we’ll see the generator object memory address printed.

In this code:

• The print_list_by_return function prints only "Before return" because it exits immediately.
• The print_list_by_yield function prints "Before yield" and "After yield" for each item, pausing at yield and resuming when iterated.