How to build a basic iterator?

There are four ways to build an iterative function:

• create a generator (uses the yield keyword)
• use a generator expression (genexp)
• create an iterator (defines __iter__ and __next__ (or next in Python 2.x))
• create a class that Python can iterate over on its own (defines __getitem__)

Examples:

# generator def uc_gen(text): for char in text.upper(): yield char # generator expression def uc_genexp(text): return (char for char in text.upper()) # iterator protocol class uc_iter(): def __init__(self, text): self.text = text.upper() self.index = 0 def __iter__(self): return self def __next__(self): try: result = self.text[self.index] except IndexError: raise StopIteration self.index += 1 return result # getitem method class uc_getitem(): def __init__(self, text): self.text = text.upper() def __getitem__(self, index): return self.text[index] 

To see all four methods in action:

for iterator in uc_gen, uc_genexp, uc_iter, uc_getitem: for ch in iterator('abcde'): print(ch, end=' ') print() 

Which results in:

A B C D E A B C D E A B C D E A B C D E 

Note:

The two generator types (uc_gen and uc_genexp) cannot be reversed(); the plain iterator (uc_iter) would need the __reversed__ magic method (which, according to the docs, must return a new iterator, but returning self works (at least in CPython)); and the getitem iteratable (uc_getitem) must have the __len__ magic method:

 # for uc_iter we add __reversed__ and update __next__ def __reversed__(self): self.index = -1 return self def __next__(self): try: result = self.text[self.index] except IndexError: raise StopIteration self.index += -1 if self.index < 0 else +1 return result # for uc_getitem def __len__(self) return len(self.text) 

To answer Colonel Panic's secondary question about an infinite lazily evaluated iterator, here are those examples, using each of the four methods above:

# generator def even_gen(): result = 0 while True: yield result result += 2 # generator expression def even_genexp(): return (num for num in even_gen()) # or even_iter or even_getitem # not much value under these circumstances # iterator protocol class even_iter(): def __init__(self): self.value = 0 def __iter__(self): return self def __next__(self): next_value = self.value self.value += 2 return next_value # getitem method class even_getitem(): def __getitem__(self, index): return index * 2 import random for iterator in even_gen, even_genexp, even_iter, even_getitem: limit = random.randint(15, 30) count = 0 for even in iterator(): print even, count += 1 if count >= limit: break print 

Which results in (at least for my sample run):

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 

How to choose which one to use? This is mostly a matter of taste. The two methods I see most often are generators and the iterator protocol, as well as a hybrid (__iter__ returning a generator).

Generator expressions are useful for replacing list comprehensions (they are lazy and so can save on resources).

If one needs compatibility with earlier Python 2.x versions use __getitem__.

I see some of you doing return self in __iter__. I just wanted to note that __iter__ itself can be a generator (thus removing the need for __next__ and raising StopIteration exceptions)

class range: def __init__(self,a,b): self.a = a self.b = b def __iter__(self): i = self.a while i < self.b: yield i i+=1 

Of course here one might as well directly make a generator, but for more complex classes it can be useful.

First of all the itertools module is incredibly useful for all sorts of cases in which an iterator would be useful, but here is all you need to create an iterator in python:

yield

Isn't that cool? Yield can be used to replace a normal return in a function. It returns the object just the same, but instead of destroying state and exiting, it saves state for when you want to execute the next iteration. Here is an example of it in action pulled directly from the itertools function list:

def count(n=0): while True: yield n n += 1 

As stated in the functions description (it's the count() function from the itertools module...) , it produces an iterator that returns consecutive integers starting with n.

Generator expressions are a whole other can of worms (awesome worms!). They may be used in place of a List Comprehension to save memory (list comprehensions create a list in memory that is destroyed after use if not assigned to a variable, but generator expressions can create a Generator Object... which is a fancy way of saying Iterator). Here is an example of a generator expression definition:

gen = (n for n in xrange(0,11)) 

This is very similar to our iterator definition above except the full range is predetermined to be between 0 and 10.

I just found xrange() (suprised I hadn't seen it before...) and added it to the above example. xrange() is an iterable version of range() which has the advantage of not prebuilding the list. It would be very useful if you had a giant corpus of data to iterate over and only had so much memory to do it in.

This question is about iterable objects, not about iterators. In Python, sequences are iterable too so one way to make an iterable class is to make it behave like a sequence, i.e. give it __getitem__ and __len__ methods. I have tested this on Python 2 and 3.

class CustomRange: def __init__(self, low, high): self.low = low self.high = high def __getitem__(self, item): if item >= len(self): raise IndexError("CustomRange index out of range") return self.low + item def __len__(self): return self.high - self.low cr = CustomRange(0, 10) for i in cr: print(i) 

If you looking for something short and simple, maybe it will be enough for you:

class A(object): def __init__(self, l): self.data = l def __iter__(self): return iter(self.data) 

example of usage:

In [3]: a = A([2,3,4]) In [4]: [i for i in a] Out[4]: [2, 3, 4] 

Include the following code in your class code.

 def __iter__(self): for x in self.iterable: yield x 

Make sure that you replace self.iterablewith the iterable which you iterate through.

Here's an example code

class someClass: def __init__(self,list): self.list = list def __iter__(self): for x in self.list: yield x var = someClass([1,2,3,4,5]) for num in var: print(num) 

Output

1 2 3 4 5 

Note: Since strings are also iterable, they can also be used as an argument for the class

foo = someClass("Python") for x in foo: print(x) 

Output

P y t h o n 

All answers on this page are really great for a complex object. But for those containing builtin iterable types as attributes, like str, list, set or dict, or any implementation of collections.Iterable, you can omit certain things in your class.

class Test(object): def __init__(self, string): self.string = string def __iter__(self): # since your string is already iterable return (ch for ch in self.string) # or simply return self.string.__iter__() # also return iter(self.string) 

It can be used like:

for x in Test("abcde"): print(x) # prints # a # b # c # d # e 

This is an iterable function without yield. It make use of the iter function and a closure which keeps it's state in a mutable (list) in the enclosing scope for python 2.

def count(low, high): counter = [0] def tmp(): val = low + counter[0] if val < high: counter[0] += 1 return val return None return iter(tmp, None) 

For Python 3, closure state is kept in an immutable in the enclosing scope and nonlocal is used in local scope to update the state variable.

def count(low, high): counter = 0 def tmp(): nonlocal counter val = low + counter if val < high: counter += 1 return val return None return iter(tmp, None) 

Test;

for i in count(1,10): print(i) 1 2 3 4 5 6 7 8 9 

class uc_iter(): def __init__(self): self.value = 0 def __iter__(self): return self def __next__(self): next_value = self.value self.value += 2 return next_value 

Improving previous answer, one of the advantage of using class is that you can add __call__ to return self.value or even next_value.

class uc_iter(): def __init__(self): self.value = 0 def __iter__(self): return self def __next__(self): next_value = self.value self.value += 2 return next_value def __call__(self): next_value = self.value self.value += 2 return next_value 

c = uc_iter() print([c() for _ in range(10)]) print([next(c) for _ in range(5)]) # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18] # [20, 22, 24, 26, 28] 

Other example of a class based on Python Random that can be both called and iterated could be seen on my implementation here