Python 代码改善

Python reversed list

列表倒序

old:

a = [1,2,3,4,5]
for i in range(len(a), 0, -1):
print i

new:

a = [1,2,3,4,5]
for i in reversed(a):
print i

Looping over a collection and indices

迭代两个数列，并且得到key ,value

old:

a = [1,2,3,4,5]
for i in range(len(a)):
print i '->' a[i]

new:

a = [1,2,3,4,5]
for key, value in enumerate(a):
print key, value

Looping over two collection;

同时循环两个数列

old:

a = ['a', 'b', 'c']
colors = ['red', 'blue', 'green', 'yellow', 'white']

length = min(a, colors)
for i in range(length):
print a[i], '->', colors[i]

new:

a = ['a', 'b', 'c']
colors = ['red', 'blue', 'green', 'yellow', 'white']

for a, color in zip(a, colors):
print a, '->', color

Custom sort order

自定义列表排序，比如按照内容长短

old:

colors = ['red', 'blue', 'green', 'yellow', 'white']
def compare_length(c1, c2):
if len(c1) > len(c2): return 1
if len(c1) < len(c2): return -1
return 0
print sorted(b, cmp=compare_length)

new:

print sorted(b, key=len)

call function until a sentinel value

调用函数直到某一个值

old:

while True:
with open('package-lock.json', 'r') as f:
block = f.read(32)
if block == '':
break
blocks.append(block)

new:

blocks = []
for block in iter(partial(f.read, 32), ''):
blocks.append(block)

distinguishing multiple exit point in loop

在循环中有多个退出点(for else)

old:

def find(seq, target):
found = False
for i, value in enumerate(seq):
if value == target:
found = True
break
if not found:
return -1
return 1

new:

def find(seq, target):
for i, value in enumerate(seq):
if value == target:
found = True
break
else:
return -1
return 1

looping over dictionary key

循环字段键值(python3)

old:

d = {'a':'b', 'c':'d'}
for k in d.keys():
if k.startswith('r'):
del d[k]

new:

d = [k : d[k] for k in d if not k.startswith('c')]

looping over dictionary key and value

循环迭代字典

old:

d = {'a':'b', 'c':'d'}
for k in d:
print k, '->', d[k]

new:

d = {'a':'b', 'c':'d'}
for k, value in d.items():
print k, '->', value

construct a dictionary from pairs

构建字典

# 两个列表构建字典
names = ['raymond', 'rachel', 'matthew']
colors = ['red', 'green', 'blue']
d = dict(zip(names, colors))

# 单个列表构建字典
d = dict(enumerate(numes))

counting with dictionaries

字典计数

old:

colors = ['red', 'green', 'red', 'blue', 'green', 'red']

# ex1
d = {}
for color in colors:
if color not in d:
d[color] = 0
d[color] += 1

# ex2
d = {}
for color in colors:
d[color] = d.get(color, 0) + 1

# ex3
from collections import defaultdict
d = defaultdict(int)
for color in colors:
d[color] += 1

new:

from collections import Counter
colors = ['red', 'green', 'red', 'blue', 'green', 'red']

d = Counter(colors)

grouping with dictionaries

字典分组统计

old:

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']
d = {}
for name in names:
key = len(name)
d.setdefault(key, []).append(name)

#In [1]: d
#Out[2]: {5: ['roger'], 6: ['rachel', 'judith'], 7: ['raymond', 'matthew', 'melissa']}

new:

from collections import defaultdict

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']
d = defaultdict(list)
for name in names:
key = len(name)
d[key].append(name)

is a dictionary popitem() atomic

字典pop原子操作

什么是原子操作:

原子操作就是不会因为进程并发或者线程并发而导致被中断的操作。原子操作的特点就是要么一次全部执行，要么全不执行。不存在执行了一半而被中断的情况。

d = {'judith': 'red', 'matthew': 'red', 'melissa': 'green', 'roger': 'blue'}
d.popitem()

Updating multiple state variables

更新多个变量状态

old:

def fibonacci(n):
x = 0
y = 1
for i in range(n):
print x
t = y
y = x + y
x = t

new:

def fibonacci(n):
x, y = 0, 1
for i in range(n):
yiled x
x, y = y, x + y

Concatenating strings

字符串拼接

old:

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']

s = names[0]
for name in names[1:]:
s += ', ' + name
print s

new:

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']

print ', '.join(names)

Updating sequences

列表更新

old:

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']

# delete
del names[0]
# pop
names.pop(0)
# insert
names.insert(0, 'eds')

new:

Deques对appendleft()和popleft()具有O(1)速度，list对插入和pop O(n)

Deques have O(1) speed for appendleft() and popleft() while lists have O(n) performance for insert(0, value) and pop(0).

from collections import deque

names = ['raymond', 'rachel', 'matthew', 'roger', 'melissa', 'judith']

del names[0]
names.popleft()
# append
names.appendleft('eds')

Using decorators to factor-out administrative logic

使用装饰器来分解管理函数逻辑

How to open and close files

使用with 来打开文件

new:

with open('./test.yml', 'r') as file:
pass

How to use locks

多线程如何使用锁

old:

import threading

lock = threading.Lock()
lock.acquire()
try:
print 'test1'
print 'test2'
finally:
lock.release()

new:

import threading

lock = threading.Lock()
with lock:
print 'test1'
print 'test2'