How to get a tuple out of a generator? Best Practice

Question

I'm a neophyte programmer and I wanted to create a generator that would return me two values that I would use in another function as a tuple.

I don't understand why tuple(function_1(a,b)) returns ((1, 2),) while tuple(function_2(a,b)) will return a correct tuple.

I wonder what is going on here and what is the best syntax and eventually know if it is possible to retrieve a tuple from function_1.

Thanks in advance for any explanations!

>>> def function_1(a,b):
...     yield a,b
...
>>> def function_2(a,b):
...     yield a
...     yield b
...
>>> a = 1
>>> b = 2
>>>
>>> function_1(a,b)
<generator object function_1 at 0x1007931b0>
>>> function_2(a,b)
<generator object function_2 at 0x1007931f8>
>>> tuple(function_1(a,b))
((1, 2),)
>>> tuple(function_2(a,b))
(1, 2)
>>> for item in function_1(a,b):
...    print(item)
...
(1, 2)
>>> for item in function_2(a,b):
...    print(item)
...
1
2

Answer 1

Your first generator yields just once, and yields a tuple:

>>> gen = function_1(1, 2)
>>> gen = function_1(1, 2)
>>> next(gen)
(1, 2)
>>> next(gen)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

The next() function takes the next value from an iterator (a generator is a specific type of iterator), if available. The StopIteration exception shows the generator is done and cannot produce more values.

Your second generator yields two separate times, each an integer:

>>> gen = function_2(1, 2)
>>> next(gen)
1
>>> next(gen)
2
>>> next(gen)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

Calling tuple() on a generator will pull in all values the generator produces, so for function_1 you'll get a tuple with one element, another tuple. For the second generator you'll get a tuple with two elements, the two integers. Compare that to using list() on the two generators:

>>> list(function_1(1, 2))
[(1, 2)]
>>> list(function_2(1, 2))
[1, 2]

Now we have two lists, but the first contains just one element, a tuple, because that is all that the generator produced.

As such, the two generators have fundamentally different behaviour. Pick the behaviour you need; either yield complete tuples, or yield separate values, but don't try and treat the two types of output as the same thing.