了解Python中的生成器函数

Question

Why it outputs None ?

def hello():
    print("hello")

def gen():
    yield hello();
    yield hello();
    yield hello();

for x in gen():
    print(x)

result is:

hello 
None 
hello 
None 
hello 
None

why None is printed?? not hello * 3 ?

Answer 1

Why it prints None :

Here when you print() but doesnt return anything, python will add a return automatically at the end.

Let's see an example using dis

import dis

def hello():
  print('hello')

dis.dis(hello)

output:

      0 LOAD_CONST               1 ('hello')
      3 PRINT_ITEM          
      4 PRINT_NEWLINE       
      5 LOAD_CONST               0 (None)
      8 RETURN_VALUE    

Let's see now when you return an explicit values:

import dis

def hello():
  return 'hello'

dis.dis(hello)

output:

    0 LOAD_CONST               1 ('hello')
    3 RETURN_VALUE  

See, the LOAD_CONST 0 (None) was not called the second times. Therefore, the first value being loaded was return.

What you should do to improve your code :

Do this, if you just need to print "hello" inside the hello function.

def hello():
    print("hello")

def gen():
    yield hello()
    yield hello()
    yield hello()

for x in gen():
    x

OR, use return value, in that case you need to return instead of printing.

def hello():
    return "hello"
    ^ Here I return rather than print hello

def gen():
    yield hello()
    yield hello()
    yield hello()

for x in gen():
    print(x)

But it is weird to call several yield , better do a loop to avoid a StopIteration

eg

def hello():
    return "hello"

def gen():
    while True:
        yield hello()

x = gen()

for i in range(3):
  print(x.next())

Answer 2

By default functions return None type.

def hello():
    pass

def gen():
    yield hello();
    yield hello();
    yield hello();

for x in gen():
    print(x)

Output:

None
None
None

What are generators?

The main difference between generators and functions is that you can obtain values on the fly. And after one value is yield 'ed by the generator. and next value is returned the old one is not stored in the memory .

Iterating over a generator

def hello():
    for x in range(3):
        yield x*x


for i in hello():
    print(i)

Output:

0
1
4

Now using next()

def hello():
    for x in range(3):
        yield x*x

gen = hello()

for i in range(3):
    print(next(gen))

Output:

0
1
4

So far so good. Right? gen = hello() here gen becomes a generator object.

Iterating over a list

my_list = [x*x for x in range(3)]
for i in my_list:
    print(i)

Output:

0
1
4

Same output? Yes same output. But only difference here is that I can use the my_list iterable any number of times I want,

my_list = [x*x for x in range(3)]
for i in my_list:
    print(i)
print("\n")
for i in my_list:
    print(i)

Output:

0
1
4

0
1
4

However if I try to use the Generator when it is exhausted.

def hello():
    for x in range(3):
        yield x*x

gen = hello()

for i in range(3):
    print(next(gen))
next(gen)

Output

0
1
4
Traceback (most recent call last):
  File "/home/mr/sadas.py", line 12, in <module>
    print(next(gen))
StopIteration

How to overcome this? Create a new generator object again and use.

def hello():
    for x in range(3):
        yield x*x

gen = hello()

for i in range(3):
    print(next(gen))

gen = hello()

for i in range(3):
    print(next(gen))

Output:

0
1
4
0
1
4

You see the difference? Hope I was clear.