This artical will talk about generator in Python coroutines. All the demo code below is from http://www.dabeaz.com/coroutines/ .
In Python, generator is used to produce a sequence of results. Everytime you call the generator, it will iterate the next value of the results, instead of returning the complete list of results and return them together. We use key word yield, to make a regular function as generator.
Let’s look at the sample code below:
# grep.py
#
# A very simple coroutine
def grep(pattern):
print "Looking for %s" % pattern
while True:
line = (yield)
if pattern in line:
print line,
# Example use
if __name__ == '__main__':
g = grep("python")
g.next()
g.send("Yeah, but no, but yeah, but no")
g.send("A series of tubes")
g.send("python generators rock!")
g.close()
In the __main__ function, we first create a generator using
g = grep("python")
So now g is a generator. We also see that there line of code’s grammar a bit weird:
line = (yield)
When yield is placed on the right hand side of equation, or say, when we ‘assign’ the yield to a variable, it means this generator becomes a coroutine. Note coroutine is a special case of generator but more powerful. Will provide how to make use of coroutine in next articles.
To make the coroutine work, it must be “primed” by first calling .next() (or send(None)), where this is the activation step. See code below:
g.next()
Then, we can send values to the function by calling the method send()
g.send("Yeah, but no, but yeah, but no")
g.send("A series of tubes")
g.send("python generators rock!")
From above, everytime when we make a call g.send(), we send the string data and the string value will be assigned to line. In this way, we are able to send data to generator. Traditionally, we will just iterate a generator and get the value. By using send(), we can now send the data to generator now.
Note that, there is a infinite while loop in the generator/coroutine definition:
while True:
...
...
To stop the coroutine from running, we need to explicitly call close() method:
g.close()
