What is the purpose of the “send” function on Python generators?

It’s used to send values into a generator that just yielded. Here is an artificial (non-useful) explanatory example:

>>> def double_inputs():
...     while True:
...         x = yield
...         yield x * 2
...
>>> gen = double_inputs()
>>> next(gen)       # run up to the first yield
>>> gen.send(10)    # goes into 'x' variable
20
>>> next(gen)       # run up to the next yield
>>> gen.send(6)     # goes into 'x' again
12
>>> next(gen)       # run up to the next yield
>>> gen.send(94.3)  # goes into 'x' again
188.5999999999999

You can’t do this just with yield.

As to why it’s useful, one of the best use cases I’ve seen is Twisted’s @defer.inlineCallbacks. Essentially it allows you to write a function like this:

@defer.inlineCallbacks
def doStuff():
    result = yield takesTwoSeconds()
    nextResult = yield takesTenSeconds(result * 10)
    defer.returnValue(nextResult / 10)

What happens is that takesTwoSeconds() returns a Deferred, which is a value promising a value will be computed later. Twisted can run the computation in another thread. When the computation is done, it passes it into the deferred, and the value then gets sent back to the doStuff() function. Thus the doStuff() can end up looking more or less like a normal procedural function, except it can be doing all sorts of computations & callbacks etc. The alternative before this functionality would be to do something like:

def doStuff():
    returnDeferred = defer.Deferred()
    def gotNextResult(nextResult):
        returnDeferred.callback(nextResult / 10)
    def gotResult(result):
        takesTenSeconds(result * 10).addCallback(gotNextResult)
    takesTwoSeconds().addCallback(gotResult)
    return returnDeferred

It’s a lot more convoluted and unwieldy.