python reduce 函数使用详解

reduce(...)
reduce(function, sequence[, initial]) -> value

Apply a function of two arguments cumulatively to the items of a sequence,
from left to right, so as to reduce the sequence to a single value.
For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
((((1+2)+3)+4)+5). If initial is present, it is placed before the items
of the sequence in the calculation, and serves as a default when the
sequence is empty.

> def add(x, y):
...   return x+y
...
> from functools import reduce
> reduce(add, [1,2,3,4])
10
>

> sum([1,2,3,4])
10
>

> from functools import reduce
> reduce(lambda x, y: x * 10 + y, [1 , 2, 3, 4, 5])
12345
>

 from functools import reduce
 scientists =({'name':'Alan Turing', 'age':105},
       {'name':'Dennis Ritchie', 'age':76},
       {'name':'John von Neumann', 'age':114},
       {'name':'Guido van Rossum', 'age':61})
 def reducer(accumulator , value):
   sum = accumulator['age'] + value['age']
   return sum
 total_age = reduce(reducer, scientists)
 print(total_age)

 from functools import reduce
 scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'},
       {'name':'Dennis Ritchie', 'age':76, 'gender':'male'},
       {'name':'Ada Lovelace', 'age':202, 'gender':'female'},
       {'name':'Frances E. Allen', 'age':84, 'gender':'female'})
 def reducer(accumulator , value):
   sum = accumulator + value['age']
   return sum
 total_age = reduce(reducer, scientists, 0)
 print(total_age)

from functools import reduce
scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'},
       {'name':'Dennis Ritchie', 'age':76, 'gender':'male'},
       {'name':'Ada Lovelace', 'age':202, 'gender':'female'},
       {'name':'Frances E. Allen', 'age':84, 'gender':'female'})
def group_by_gender(accumulator , value):
  accumulator[value['gender']].append(value['name'])
  return accumulator
grouped = reduce(group_by_gender, scientists, {'male':[], 'female':[]})
print(grouped)

import itertools
scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'},
       {'name':'Dennis Ritchie', 'age':76, 'gender':'male'},
       {'name':'Ada Lovelace', 'age':202, 'gender':'female'},
       {'name':'Frances E. Allen', 'age':84, 'gender':'female'})
grouped = {item[0]:list(item[1])
      for item in itertools.groupby(scientists, lambda x: x['gender'])}
print(grouped)

from functools import reduce
scientists =({'name':'Alan Turing', 'age':105, 'gender':'male'},
       {'name':'Dennis Ritchie', 'age':76, 'gender':'male'},
       {'name':'Ada Lovelace', 'age':202, 'gender':'female'},
       {'name':'Frances E. Allen', 'age':84, 'gender':'female'})
grouped = reduce(lambda acc, val: {**acc, **{val['gender']: acc[val['gender']]+ [val['name']]}}, scientists, {'male':[], 'female':[]})
print(grouped)