Note: this post assumes Python 3.x syntax.†
A generator is simply a function which returns an object on which you can call
next, such that for every call it returns some value, until it raises a
StopIteration exception, signaling that all values have been generated. Such an object is called an iterator.
Normal functions return a single value using
return, just like in Java. In Python, however, there is an alternative, called
yield anywhere in a function makes it a generator. Observe this code:
>>> def myGen(n): ... yield n ... yield n + 1 ... >>> g = myGen(6) >>> next(g) 6 >>> next(g) 7 >>> next(g) Traceback (most recent call last): File "<stdin>", line 1, in <module> StopIteration
As you can see,
myGen(n) is a function which yields
n + 1. Every call to
next yields a single value, until all values have been yielded.
for loops call
next in the background, thus:
>>> for n in myGen(6): ... print(n) ... 6 7
Likewise there are generator expressions, which provide a means to succinctly describe certain common types of generators:
>>> g = (n for n in range(3, 5)) >>> next(g) 3 >>> next(g) 4 >>> next(g) Traceback (most recent call last): File "<stdin>", line 1, in <module> StopIteration
Note that generator expressions are much like list comprehensions:
>>> lc = [n for n in range(3, 5)] >>> lc [3, 4]
Observe that a generator object is generated once, but its code is not run all at once. Only calls to
next actually execute (part of) the code. Execution of the code in a generator stops once a
yield statement has been reached, upon which it returns a value. The next call to
next then causes execution to continue in the state in which the generator was left after the last
yield. This is a fundamental difference with regular functions: those always start execution at the “top” and discard their state upon returning a value.
There are more things to be said about this subject. It is e.g. possible to
send data back into a generator (reference). But that is something I suggest you do not look into until you understand the basic concept of a generator.
Now you may ask: why use generators? There are a couple of good reasons:
- Certain concepts can be described much more succinctly using generators.
- Instead of creating a function which returns a list of values, one can write a generator which generates the values on the fly. This means that no list needs to be constructed, meaning that the resulting code is more memory efficient. In this way one can even describe data streams which would simply be too large to fit in memory.
Generators allow for a natural way to describe infinite streams. Consider for example the Fibonacci numbers:
>>> def fib(): ... a, b = 0, 1 ... while True: ... yield a ... a, b = b, a + b ... >>> import itertools >>> list(itertools.islice(fib(), 10)) [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
This code uses
itertools.isliceto take a finite number of elements from an infinite stream. You are advised to have a good look at the functions in the
itertoolsmodule, as they are essential tools for writing advanced generators with great ease.
† About Python <=2.6: in the above examples
next is a function which calls the method
__next__ on the given object. In Python <=2.6 one uses a slightly different technique, namely
o.next() instead of
next(o). Python 2.7 has
.next so you need not use the following in 2.7:
>>> g = (n for n in range(3, 5)) >>> g.next() 3