send and throw
- Python course
- Intermediate
- Lesson 103/116
- 5 min
Learn generator interaction with send() and throw().
The send method
A generator can receive values from outside. The g.send(value) call sends a value to the awaited yield expression and advances to the next yield. This lets generators act as coroutines.
Example: v = yield total. The call g.send(5) assigns 5 to v. Before the first send, you must call next(g) to advance the generator to the first yield; this is called priming.
The throw method
The g.throw(Exception) call raises an exception at the awaited yield point in the generator. The generator can catch it with try/except or let it propagate up.
Example: if the generator body is try:\n v = yield total\nexcept ValueError:\n print("error"), the call g.throw(ValueError) runs the except block. This is used to signal errors into the generator from outside; it is practical for cancellation or state changes in long-running tasks.
Practice acc generator and send
Define the function def acc():. In the body write total = 0; open a while True: loop; inside write v = yield total and if v is not None: total += v. Create an instance with g = acc(), prime with next(g). Send two values and print the result with print(g.send(5)) and print(g.send(10)).
Starter code
# acc generator and accumulate with sendSample solution
# acc generator and accumulate with send
def acc():
total = 0
while True:
v = yield total
if v is not None:
total += v
g = acc()
next(g)
print(g.send(5))
print(g.send(10))Expected output
5 15
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the send and throw parts
| Term | Meaning |
|---|---|
send |
Sends a value |
throw |
Raises an error |
next(g) |
Priming |
v = yield |
Receive value |
2 What happens before send is called?
- The generator is deleted
- It is primed with next(g) Correct answer
- throw is called
- close is called
Explanation: This is known as priming.
3 send call
g = acc()
next(g)
print(g.____(5))g = acc()
next(g)
print(g.send(5))Explanation: g.send(5) sends 5 into the generator.