符合语言习惯的Python编程

Jun 2013 · He, Xiaocong

Programs must be written for people to read, and only incidentally for machines to execute.

程序必须先让人读懂，然后才能让计算机执行。

- Abelson & Sussman

PEP 8: Style Guide for Python Code

http://www.python.org/dev/peps/pep-0008/

交换赋值

• 不推荐

temp = a
a = b
b = a  

• 推荐

a, b = b, a  #  先生成一个元组(tuple)对象，然后unpack  

Unpacking

• 不推荐

l = ['David', 'Pythonista', '+1-514-555-1234']
first_name = l[0]
last_name = l[1]
phone_number = l[2]  

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l = ['David', 'Pythonista', '+1-514-555-1234']
first_name, last_name, phone_number = l

# Python 3 Only
first, *middle, last = another_list  

使用操作符in

• 不推荐

if fruit == "apple" or fruit == "orange" or fruit == "berry":
    # 多次判断  

• 推荐

if fruit in ["apple", "orange", "berry"]:
    # 使用 in 更加简洁  

字符串操作

• 不推荐

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

result = ''
for s in colors:
    result += s  #  每次赋值都丢弃以前的字符串对象, 生成一个新对象  

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colors = ['red', 'blue', 'green', 'yellow']
result = ''.join(colors)  #  没有额外的内存分配  

字典键值列表

• 不推荐

for key in my_dict.keys():
    #  my_dict[key] ...  

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for key in my_dict:
    #  my_dict[key] ...

# 只有当循环中需要更改key值的情况下，我们需要使用 my_dict.keys()
# 生成静态的键值列表。  

字典键值判断

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if my_dict.has_key(key):
    # ...do something with d[key]  

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if key in my_dict:
    # ...do something with d[key]  

字典 get 和 setdefault 方法

• 不推荐

navs = {}
for (portfolio, equity, position) in data:
    if portfolio not in navs:
            navs[portfolio] = 0
    navs[portfolio] += position * prices[equity]

• 推荐

navs = {}
for (portfolio, equity, position) in data:
    # 使用 get 方法
    navs[portfolio] = navs.get(portfolio, 0) + \
                      position * prices[equity]
    # 或者使用 setdefault 方法
    navs.setdefault(portfolio, 0)
    navs[portfolio] += position * prices[equity]

判断真伪

• 不推荐

if x == True:
    # ....
if len(items) != 0:
    # ...
if items != []:
    # ...  

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if x:
    # ....
if items:
    # ...  

遍历列表以及索引

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items = 'zero one two three'.split()
# method 1
i = 0
for item in items:
    print i, item
    i += 1
# method 2
for i in range(len(items)):
    print i, items[i]

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items = 'zero one two three'.split()
for i, item in enumerate(items):
    print i, item

列表推导

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new_list = []
for item in a_list:
    if condition(item):
        new_list.append(fn(item))  

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new_list = [fn(item) for item in a_list if condition(item)]  

列表推导-嵌套

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for sub_list in nested_list:
    if list_condition(sub_list):
        for item in sub_list:
            if item_condition(item):
                # do something...  

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gen = (item for sl in nested_list if list_condition(sl) \
            for item in sl if item_condition(item))
for item in gen:
    # do something...  

循环嵌套

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for x in x_list:
    for y in y_list:
        for z in z_list:
            # do something for x & y  

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from itertools import product
for x, y, z in product(x_list, y_list, z_list):
    # do something for x, y, z  

尽量使用生成器代替列表

• 不推荐

def my_range(n):
    i = 0
    result = []
    while i < n:
        result.append(fn(i))
        i += 1
    return result  #  返回列表

• 推荐

def my_range(n):
    i = 0
    result = []
    while i < n:
        yield fn(i)  #  使用生成器代替列表
        i += 1

^*尽量用生成器代替列表，除非必须用到列表特有的函数。

中间结果尽量使用imap/ifilter代替map/filter

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reduce(rf, filter(ff, map(mf, a_list)))

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from itertools import ifilter, imap
reduce(rf, ifilter(ff, imap(mf, a_list)))

^*lazy evaluation 会带来更高的内存使用效率，特别是当处理大数据操作的时候。

使用any/all函数

• 不推荐

found = False
for item in a_list:
    if condition(item):
        found = True
        break
if found:
    # do something if found...  

• 推荐

if any(condition(item) for item in a_list):
    # do something if found...  

属性(property)

• 不推荐

class Clock(object):
    def __init__(self):
        self.__hour = 1
    def setHour(self, hour):
        if 25 > hour > 0: self.__hour = hour
        else: raise BadHourException
    def getHour(self):
        return self.__hour

• 推荐

class Clock(object):
    def __init__(self):
        self.__hour = 1
    def __setHour(self, hour):
        if 25 > hour > 0: self.__hour = hour
        else: raise BadHourException
    def __getHour(self):
        return self.__hour
    hour = property(__getHour, __setHour)

使用 with 处理文件打开

• 不推荐

f = open("some_file.txt")
try:
    data = f.read()
    # 其他文件操作..
finally:
    f.close()

• 推荐

with open("some_file.txt") as f:
    data = f.read()
    # 其他文件操作...

使用 with 忽视异常(仅限Python 3)

• 不推荐

try:
    os.remove("somefile.txt")
except OSError:
    pass

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from contextlib import ignored  # Python 3 only

with ignored(OSError):
    os.remove("somefile.txt")

使用 with 处理加锁

• 不推荐

import threading
lock = threading.Lock()

lock.acquire()
try:
    # 互斥操作...
finally:
    lock.release()

• 推荐

import threading
lock = threading.Lock()

with lock:
    # 互斥操作...

参考

• Idiomatic Python: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html

• Python Code Style: http://docs.python-guide.org/en/latest/writing/style.html

谢谢 !