python标准库

setdefault和defaultdict

#setdefault
periodic_table = {'Hydrogen': 1, 'Helium': 2}
carbon = periodic_table.setdefault('Carbon', 12)
print(carbon)
print(periodic_table)
helium = periodic_table.setdefault('Helium', 947)
print(helium)
print(periodic_table)

#defaultdict
print(int())
from collections import defaultdict
periodic_table2 = defaultdict(int)
periodic_table2['Hydrogen'] = 1
print(periodic_table2['Lead'])
print(periodic_table2)
def no_idea():
    return 'Hub?'
bestiary = defaultdict(no_idea)
bestiary['A'] = 'Abominable Snowman'
print(bestiary['B'])

输出

12
{'Carbon': 12, 'Helium': 2, 'Hydrogen': 1}
2
{'Carbon': 12, 'Helium': 2, 'Hydrogen': 1}
0
0
defaultdict(<class 'int'>, {'Lead': 0, 'Hydrogen': 1})
Hub?

pprint 

itertools

import itertools

count = 0
for item in itertools.chain([1, 2, 3], ['a', 'b']):
    count += 1
    print('count', count)
    print(item)

print('-------------')
count = 0
for item in itertools.cycle([1, 2]):
    count += 1
    print('count', count)
    if count < 5:
        print(item)
    else:
        break

print('-------------')
#累积的值，默认累积和
for item in itertools.accumulate([1, 2, 3, 4]):
    print(item)

print('-------------')
def multiply(a, b):
    return a * b

for item in itertools.accumulate([1, 2, 3, 4], multiply):
    print(item)

输出

count 1
1
count 2
2
count 3
3
count 4
a
count 5
b
-------------
count 1
1
count 2
2
count 3
1
count 4
2
count 5
-------------
1
3
6
10
-------------
1
2
6
24

deque 

#deque是一种双端队列，同时具有栈和队列的特征
def palindrome(word):
    from collections import deque
    dq = deque(word)
    while len(dq) > 1:
        if dq.popleft() != dq.pop():
            return False
    return True

palindrome('racecar')

 OrderedDict

Counter计数