When to use yield instead of return in Python?

In Python, both yield and return are used to return values from a function. However, they work in different ways and serve different purposes.

Using return

The return statement is used to return a value from a function and terminate the function’s execution. When a function encounters a return statement, it immediately stops executing and returns the value specified in the statement. Here’s an example:

def sum_numbers(a, b):
    return a + b

result = sum_numbers(2, 3)
print(result) # Output: 5

In this example, the sum_numbers() function takes two arguments and returns their sum using the return statement. We then call the function with the arguments 2 and 3, and assign the result to a variable named result. Finally, we print out the value of result, which is 5.

Using yield

The yield statement is used to define a generator function. A generator function is a type of function that can be used to create an iterator, which can be used to iterate over a sequence of values. When a function encounters a yield statement, it pauses its execution and returns the value specified in the statement. The function can then be resumed from where it left off later, with the next value being returned by the next yield statement encountered. Here’s an example:

def generate_numbers(n):
    for i in range(n):
        yield i

numbers = generate_numbers(5)
for number in numbers:
    print(number)

In this example, the generate_numbers() function is a generator function that yields the numbers from 0 to n-1. We then create an iterator named numbers by calling generate_numbers(5), and iterate over it using a for loop. Each time we iterate over the iterator, the function is paused at the yield statement, and the current value is returned. This allows us to iterate over the sequence of numbers without having to generate the entire sequence upfront.

When to use yield instead of return

In general, you should use return when you want to return a single value from a function and terminate the function’s execution. You should use yield when you want to create a generator function that can be used to generate a sequence of values, without having to generate the entire sequence upfront.

Another use case for yield is when you want to implement a coroutine. A coroutine is a type of function that can be used to perform cooperative multitasking, where multiple functions can run concurrently and yield control to each other as needed. This can be useful in certain types of applications, such as network programming, where multiple connections may need to be handled at the same time.

In summary, return is used to return a single value from a function and terminate the function’s execution, while yield is used to define a generator function that can be used to generate a sequence of values or to implement a coroutine.