How to Send Values into a Python Generator Coroutine

Use the .send() method to pass values into a running generator coroutine and capture them.

Medium Python 3.9+ Aug 9, 2026 Comprehensions & generators 13 views 0 copies

generators coroutines yield send

Python code

21 lines

Python 3.9+

def coroutine():
    received = []
    while True:
        value = yield
        received.append(value)
        print(f"Coroutine received: {value}")
        if value == "stop":
            break
    return received

if __name__ == "__main__":
    gen = coroutine()
    next(gen)  # Prime the generator
    gen.send("hello")
    gen.send(42)
    gen.send([1, 2, 3])
    
    try:
        gen.send("stop")
    except StopIteration as e:
        print(f"Coroutine finished. Received values: {e.value}")

Output

stdout

Coroutine received: hello
Coroutine received: 42
Coroutine received: [1, 2, 3]
Coroutine received: stop
Coroutine finished. Received values: ['hello', 42, [1, 2, 3]]

How it works

A generator becomes a coroutine when it contains a yield statement without an expression. Calling next(gen) primes the generator, moving execution to the first yield. Each gen.send(value) resumes the generator, assigns value to the yield expression, and continues until the next yield. When 'stop' is sent, the generator breaks out of the loop and returns a list, which is caught in the StopIteration exception's value attribute. This pattern allows two-way communication between the caller and the coroutine.

Common mistakes

Forgetting to prime the generator with `next(gen)` before sending values
Sending to a generator that hasn't started results in TypeError
Not catching StopIteration when the generator exits cleanly

Variations

Use `yield from` to delegate to a subgenerator
Use `gen.close()` to terminate a generator without handling StopIteration

Real-world use cases

Implementing a data stream processor that receives chunks and aggregates stats
Building a state machine that transitions based on incoming events
Creating a logging pipeline that accepts log entries and formats them in real time

How to Send Values into a Python Generator Coroutine

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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Real-world use cases

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