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The Python yield keyword explained

Can someone explain to me what the yield statement actually does in this bit of code here:

 def fibonacci():
     a, b = 0, 1
     while True:
         yield a
         a, b = b, a+b

for number in fibonacci(): # Use the generator as an iterator; print number

What I understand so far is, we are defining a function finonacci(), with no parameters?
inside the function we are defining a and b equal to 0 and 1, next, while this is true, we are yielding a. What is this actually doing? Furthermore, while yielding a? a is now equal to b, while b is now equal to a + b.

Next question, for number in fibonacci(), does this mean for every number in the function or what? I’m equally stumped on what yield and ‘for number’ are actually doing. Obviously I am aware that it means for every number in fibonacci() print number. Am I actually defining number without knowing it?

Thanks, sorry if I’m not clear. BTW, it’s for project Euler, if I knew how to program well this would be a breeze but I’m trying to learn this on the fly.

Answer 1

Using yield makes the function a generator.
The generator will continue to yield the a variable on each loop, waiting until the generator’s next() method is called to continue on to the next loop iteration.

Or, until you return or StopIteration is raised.

Slightly modified to show use of StopIteration:

>>> def fib():
...     a = 0
...     b = 1
...     while True:
...         yield a
...         a = b
...         b += a
...         if a > 100:
...             raise StopIteration
...
>>>
>>> for value in fib():
...     print value
...
0
1
2
4
8
16
32
64
>>>

>>> # assign the resulting object to 'generator'
>>> generator = fib()
>>> generator.next()
0
>>> generator.next()
1
>>> for value in generator:
...     print value
...
2
4
8
16
32
64
>>>