Overload __iter__ method to create Iterator
Overload __iter__
The magic method, __iter__, is the basis of the iterator protocol.
The __iter__ method returns an iterator, which is any object with a method called next,
which is callable without any arguments.
When you call the next method, the iterator should return its "next value." If the method is called, and the iterator has no more values to return, it should raise a StopIteration exception.
class Fibs: # from www. jav a2 s . c o m
def __init__(self):
self.a = 0
self.b = 1
def next(self):
self.a, self.b = self.b, self.a+self.b
return self.a
def __iter__(self):
return self
fibs = Fibs()
for f in fibs:
if f > 1000:
print f
break
The code above generates the following result.
An object that implements the __iter__ method is iterable, while the object implementing next is the iterator.
__iter__ method provides protocol for iterator.
We can provide custom implementation for __iter__
to create our own iterator.
The code above creates a Fibonacci numbers iterator.
class FileList:# ww w .j a v a 2 s .c om
def __init__(self):
self.ptr = 0
self.file_list = ["file1", "file2", "file3", "file4" ]
def __iter__(self):
self.ptr = 0
return self
def next(self):
if self.ptr == len(self.file_list):
raise StopIteration
s = self.file_list[self.ptr]
self.ptr = self.ptr + 1
return s
fl = FileList()
for f in fl :
print f
The code above generates the following result.
Making Sequences from Iterators
We can convert iterators to sequences.
class TestIterator:
value = 0 # w w w . j a v a 2s . co m
def next(self):
self.value += 1
if self.value > 10: raise StopIteration
return self.value
def __iter__(self):
return self
ti = TestIterator()
print list(ti)
The code above generates the following result.
Add iterator behavior to your classes
The following code
defines a __iter__() method which returns an
object with a next() method.
If the class defines next(),
then __iter__() can just
return self:
class Reverse:# w ww .ja v a 2s . c om
"Iterator for looping over a sequence backwards"
def __init__(self, data):
self.data = data
self.index = len(data)
def __iter__(self):
return self
def next(self):
if self.index == 0:
raise StopIteration
self.index = self.index - 1
return self.data[self.index]
for char in Reverse('spam'):
print char
The code above generates the following result.
