What you are pointing to is Generator in Python. Take a look at: –
- http://wiki.python.org/moin/Generators
- http://www.python.org/dev/peps/pep-0255/
- http://docs.python.org/whatsnew/2.5.html#pep-342-new-generator-features
See the documentation: – Generator Expression which contains exactly the same example you have posted
From the documentation: –
Generators are a simple and powerful tool for creating iterators. They
are written like regular functions but use the yield statement
whenever they want to return data. Each time next() is called, the
generator resumes where it left-off (it remembers all the data values
and which statement was last executed)
Generators are similar to List Comprehension that you use with square brackets instead of brackets, but they are more memory efficient. They don’t return the complete list of result at the same time, but they return generator object. Whenever you invoke next() on the generator object, the generator uses yield to return the next value.
List Comprehension for the above code would look like: –
[x * x for x in range(10)]
You can also add conditions to filter out results at the end of the for.
[x * x for x in range(10) if x % 2 != 0]
This will return a list of numbers multiplied by 2 in the range 1 to 5, if the number is not divisible by 2.
An example of Generators depicting the use of yield can be: –
def city_generator():
yield("Konstanz")
yield("Zurich")
yield("Schaffhausen")
yield("Stuttgart")
>>> x = city_generator()
>>> x.next()
Konstanz
>>> x.next()
Zurich
>>> x.next()
Schaffhausen
>>> x.next()
Stuttgart
>>> x.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
So, you see that, every call to next() executes the next yield() in generator. and at the end it throws StopIteration.