Python Yield

cheat_5.py

"""
CHEAT 5: yield

Generators are iterators, but you can only iterate over them once. It's because they do not
store all the values in memory, they generate the values on the fly.
Yield is a keyword that is used like return, except the function will return a generator.

http://freepythontips.wordpress.com/2013/09/29/the-python-yield-keyword-explained/
http://www.alvarohurtado.es/generadores-en-python/
"""

#example 1
def even_numbers_generator(start):
    a = start
    while a < 20:
        a += 2
        yield a
#test
for number in even_numbers_generator(8):
    print number
number = even_numbers_generator(8)
print number.next() # invoke next() method explicity
print number.next()

#result
"""
10
12
14
16
18
20
10
12
"""

#example 2
def next_number_generator(step):
    try:
        a = 0
        while True:
            yield a
            a += step
    finally:
        print "finish!"
        
#test
generator = next_number_generator(3)
print generator.send(None) # you're using send to "start" a generator (that is, execute the code from the
                           # first line of the generator function up to the first yield statement)
print generator.next()
print generator.next()
generator.close() # close method generates exception into next_number_generator method
print generator.next() # error

#result
"""
10
12
14
16
18
20
10
12
0
3
6
finish!

Traceback (most recent call last):
...
StopIteration
"""

#example 3
class Bank():
    crisis = False
    def create_atm(self):
        while not self.crisis:
            yield "$100"
            
#test
bank = Bank() 
corner_street_atm = bank.create_atm()
print(corner_street_atm.next())
print(corner_street_atm.next())
print([corner_street_atm.next() for cash in range(5)])
bank.crisis = True # no more money
print(corner_street_atm.next()) # error

#result
"""
$100
$100
['$100', '$100', '$100', '$100', '$100']

Traceback (most recent call last):
...
StopIteration
"""