Generators
Generators simplifies creation of iterators. A generator is a
function that produces a sequence of results instead of a single value.
def yrange(n):
i = 0
while i < n:
yield i
i += 1
Each time the
yield statement is executed the function generates a new value.
>>> y = yrange(3)
>>> y
<generator object yrange at 0x401f30>
>>> y.next()
0
>>> y.next()
1
>>> y.next()
2
>>> y.next()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
So a generator is also an iterator. You don’t have to worry about the iterator protocol.
The word “generator” is confusingly used to mean both the function that
generates and what it generates. In this chapter, I’ll use the word “generator”
to mean the genearted object and “generator function” to mean the function that
generates it.
Can you think about how it is working internally?
When a generator function is called, it returns an generator object without
even beginning execution of the function. When
next` method is called for the
first time, the function starts executing until it reaches
yield statement.
The yielded value is returned by the
next call.
The following example demonstrates the interplay between
yield and call to
next method on generator object.
>>> def foo():
... print "begin"
... for i in range(3):
... print "before yield", i
... yield i
... print "after yield", i
... print "end"
...
>>> f = foo()
>>> f.next()
begin
before yield 0
0
>>> f.next()
after yield 0
before yield 1
1
>>> f.next()
after yield 1
before yield 2
2
>>> f.next()
after yield 2
end
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
>>>
Lets see an example:
def integers():
"""Infinite sequence of integers."""
i = 1
while True:
yield i
i = i + 1
def squares():
for i in integers():
yield i * i
def take(n, seq):
"""Returns first n values from the given sequence."""
seq = iter(seq)
result = []
try:
for i in range(n):
result.append(seq.next())
except StopIteration:
pass
return result
print take(5, squares()) # prints [1, 4, 9, 16, 25]
