python 生成器

一 

# Initialize the list
my_list = [1, 3, 6, 10]

# square each term using list comprehension
# Output: [1, 9, 36, 100]
[x**2 for x in my_list]

# same thing can be done using generator expression
# Output: <generator object <genexpr> at 0x0000000002EBDAF8>
(x**2 for x in my_list)

二

# Intialize the list
my_list = [1, 3, 6, 10]

a = (x**2 for x in my_list)
# Output: 1
print(next(a))

# Output: 9
print(next(a))

# Output: 36
print(next(a))

# Output: 100
print(next(a))

# Output: StopIteration
next(a)

例子1

#!/usr/bin/python3
#coding=utf-8

def my_gen():
    n = 1
    print('This is printed first, n= ', n)
    # Generator function contains yield statements
    yield n

    n += 1
    print('This is printed second, n= ', n)
    yield n

    n += 1
    print('This is printed at last, n= ', n)
    yield n

输出

>>> # It returns an object but does not start execution immediately.
>>> a = my_gen()

>>> # We can iterate through the items using next().
>>> next(a)
This is printed first, n = 1
1
>>> # Once the function yields, the function is paused and the control is transferred to the caller.

>>> # Local variables and theirs states are remembered between successive calls.
>>> next(a)
This is printed second, n = 2
2

>>> next(a)
This is printed at last, n = 3
3

>>> # Finally, when the function terminates, StopIteration is raised automatically on further calls.
>>> next(a)
Traceback (most recent call last):
...
StopIteration
>>> next(a)
Traceback (most recent call last):
...
StopIteration

例子2

# A simple generator function
def my_gen():
    n = 1
    print('This is printed first')
    # Generator function contains yield statements
    yield n

    n += 1
    print('This is printed second')
    yield n

    n += 1
    print('This is printed at last')
    yield n

# Using for loop
for item in my_gen():
    print(item)

输出

This is printed first
1
This is printed second
2
This is printed at last
3

例子3

反转字符串的生成器

def rev_str(my_str):
    length = len(my_str)
    for i in range(length - 1,-1,-1):
        yield my_str[i]

for char in rev_str("hello"):
     print(char)

输出

o
l
l
e
h