Yield, Generators & Iterables, non separable concepts in Python

Iterables

To understand what yield does, you must understand what generators are. And before generators come iterables. When you create a list, you can read its items one by one, and it's called iteration :

>>> mylist = [1, 2, 3]
>>> for i in mylist :
...    print(i)
1
2
3

Mylist is an iterable. When you use a comprehension list, you create a list, and so an iterable :

>>> mylist = [x*x for x in range(3)]
>>> for i in mylist :
...    print(i)
0
1
4

Everything you can use "for... in..." on is an iterable : lists, strings, files... These iterables are handy because you can read them as much as you wish, but you store all the values in memory and it's not always what you want when you have a lot of values.

Generators

Generators are iterables, but you can only read them once. It's because they do not store all the values in memory, they generate the values on the fly :

>>> mygenerator = (x*x for x in range(3))
>>> for i in mygenerator :
...    print(i)
0
1
4

It just the same except you used () instead of []. BUT, you can not perform for i in mygenerator a second time since generators can only be used once: they calculate 0, then forget about it and calculate 1 and ends calculating 4, one by one.

Yield

Yield is a keyword that is used like return, except the function will return a generator.

>>> def createGenerator() :
...    mylist = range(3)
...    for i in mylist :
...        yield i*i...
>>> mygenerator = createGenerator() # create a generator
>>> print(mygenerator) # mygenerator is an object !
<generator object createGenerator at 0xb7555c34>
>>> for i in mygenerator:
...     print(i)
0
1
4

Here it's a useless example, but it's handy when you know your function will return a huge set of values that you will only need to read once.

To master yield, you must understand that when you call the function, the code you have written in the function body does not run. The function only returns the generator object, this is bit tricky :-)

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