Python Generator and Application

First. Definition

It can be understood as a data type, which automatically implements the iterator protocol (other data types need to call their own built-in __iter__ method), so the generator is an iterable object

Second, the two forms of generators (Python has two different ways to provide generators)

1. Generator function: regular function definition, however, use the yield statement instead of the return statement to return the result. The yield statement returns one result at a time, and in the middle of each result, suspend the state of the function so that it can continue to execute where it left next time.

  Yield's function:
    1 Make the result of the function as an iterator (encapsulate __iter__,__next__ in an elegant way)
     2 The state of function pause and resume operation is determined by yield



def func():
    print('first')
    yield 11111111
    print('second')
    yield 2222222
    print('third')
    yield 33333333
    print('fourth')


g=func()
print(g)
from collections import Iterator
print(isinstance(g,Iterator)) #Determine whether it is an iterator object

print(next(g))
print('======>')
print(next(g))
print('======>')
print(next(g))
print('======>')
print(next(g))

for i in g: #i=iter(g)
    print(i)
 

  

Note: Comparison between yield and return?
   Same: all have the function of return value
   Different: return can only return a value, and yield can return multiple values

2. Generator expression: similar to list comprehension, however, the generator returns an object that produces results on demand, rather than constructing a list of results at a time

 

g=('egg%s' %i for i in range(1000))
print(g)
print(next(g))
print(next(g))
print(next(g))

with open('a.txt',encoding='utf-8') as f:
    # res=max((len(line) for line in f))
    res=max(len(line) for line in f)
    print(res)

print(max([1,2,3,4,5,6]))

with open('a.txt',encoding='utf-8') as f:
    g=(len(line) for line in f)
    print(max(g))
    print(max(g))
    print(max(g))
 

Third. Application

 

# [{'name':'apple','price': 333,'count': 3}, ]File content
#Complete the reading and following operations of the file through the generator expression
with open('db.txt',encoding='utf-8') as f:
    info=[{'name':line.split()[0],
      'price':float(line.split()[1]),
      'count':int(line.split()[2])} for line in f if float(line.split()[1]) >= 30000]
    print(info)