Generators and for loops in Python

Question

So I have a generator function, that looks like this.

def generator():     while True:         for x in range(3):             for j in range(5):                 yield x 

After I load up this function and call "next" a bunch of times, I'd expect it to yield values

0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 0 0 0 0 0 ...

But instead it just yields 0 all the time. Why is that?

>>> execfile("test.py") >>> generator <function generator at 0x10b6121b8> >>> generator().next() 0 >>> generator().next() 0 >>> generator().next() 0 >>> generator().next() 0 >>> generator().next() 0 >>> generator().next() 0 >>> generator().next() 0 

Answer 1

generator() initializes new generator object:

In [4]: generator() is generator() # Creating 2 separate objects Out[4]: False 

Then generator().next() gets the first value from the newly created generator object (0 in your case).

You should call generator once:

In [5]: gen = generator() # Storing new generator object, will reuse it  In [6]: [gen.next() for _ in range(6)] # Get first 6 values for demonstration purposes Out[6]: [0, 0, 0, 0, 0, 1] 

Note: generator.next was removed from Python 3 (PEP 3114) - use the next function instead:

In [7]: next(gen) Out[7]: 1 

Answer 2

With each call of generator you are creating a new generator object:

generator().next() # 1st item in 1st generator  generator().next() # 1st item in 2nd generator 

Create one generator, and then call the next for subsequent items:

g = generator()  g.next() # 1st item in 1st generator g.next() # 2nd item in 1st generator