Yield

[Carsten Haese]

On Wed, 2006-11-15 at 09:13 -0800, Mateuszk87 wrote:
> Hi.
> 
> may someone explain "yield" function, please. how does it actually work
> and when do you use it?

[There is probably a much better explanation of this somewhere on the
net already, but I feel like writing this out myself.]

"yield" is a keyword. In simple terms, if "yield" appears inside a
function, the function becomes a generator that can return multiple
results.

Consider the following trivial example of a function that calculates the
elements of some sequence and prints each one.

def squares(n):
  for i in range(n):
    print i**2

Now suppose it turns out that in addition to--or instead of--printing
the elements of this sequence, the caller wants to do something
different with them. You could handle this by building a list, and
return the list to the caller to loop over and operate on as they
choose. But if the list is very big, that's not very memory-efficient if
the caller only needs to operate on one element at a time.

This is where yield comes in. If you write this:

def squares(n):
  for i in range(n):
    yield i**2

squares is now a generator function. If you call squares(100), it won't
calculate any sequence elements immediately. It will instead return an
iterator the caller can loop over:

for el in squares(100):
  print el # or do anything else the caller decides

The beauty of this is that you achieve physical separation between
generating elements of a sequence and using them. The generator function
only cares about how to produce the elements, and the caller only cares
about how to use them.

Hope this helps,

Carsten.