Python

Generator

Iterable Objects

#!/usr/bin/python
 
#list
l = range(10);
for i in l:
    print (i)
 
#tuple
t = (1, 2, 3, 4);
for i in t:
    print (i)
 
#dict
d = {'x':1, 'y':2};
for k in d:
    print (k, d[k])
 
#string
s = 'Hello World!';
for i in s:
    print (i)

Iterator

#!/usr/bin/python
 
l = iter(range(10)); #iterator
 
try:
    while True:
        print(next(l)) #if no more element, raise StopIteration
except StopIteration:
    pass

Implement Iterator as Classes

#!/usr/bin/python
 
class yrange:
    def __init__(self, n):
        self.i = 0
        self.n = n
 
    def __iter__(self): # make an object iterable
        #print('Call __iter__ ...')
        self.i = 0
        return self # return an iterator

    def __next__(self):
        #print('Call __next__ ...')
        if self.i < self.n:
            i = self.i
            self.i += 1
            return i
        else:
            raise StopIteration()
 
y = yrange(10);
 
l = list(y); # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(l)
l = list(y); # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(l)

Iterable and iterator are same, it is consumed in a single iteration

#!/usr/bin/python
 
 
class zrange:
    def __init__(self, n):
        self.n = n
 
    def __iter__(self):
        return zrange_iter(self.n)
 
class zrange_iter:
    def __init__(self, n):
        self.i = 0
        self.n = n

    def __iter__(self):
	# Iterators are iterables too.
	# Adding this functions to make them so.
        return self
    
    def __next__(self):
        if self.i < self.n:
            i = self.i
            self.i += 1
            return i
        else:
            raise StopIteration()
 
l = zrange(10);
 
print(list(l)) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(list(l)) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Iterable and iterator are not same, every time a new iterator is created, is able to be re-used, same as xrange

Generator

#!/usr/bin/python
 
def yrange(n):
    i = 0;
    while i < n:
        yield i;
        i += 1

l = yrange(10);
for i in l:
	print(i)
 
for i in l: # do not print anything, l has been consumed
	print(i)

Generator is iterator, can be consumed in a single iteration

Generator Expression

#!/usr/bin/python
 
# list
l = [x*x for x in range(4)]; # [0, 1, 4, 9]
print(l)
 
# generator
g = (x*x for x in range(4));
for i in g:
	print(i)

for i in g: # do nothing, g has been consumed in the above for loop
	print(i)

Reference