10个良好习惯

1. Make a script both importable and executable

def main():
    print('Doing stuff in module', __name__)

if __name__ == '__main__':
    print('Executed from the command line')
    main()
$ python mymodule.py
Executed from the command line
Doing stuff in module __main__

$ python
>>> import mymodule
>>> mymodule.main()
Doing stuff in module mymodule

2. Test for “truthy” and “falsy” values

GOOD

name = 'Safe'
pets = ['Dog', 'Cat', 'Hamster']
owners = {'Safe': 'Cat', 'George': 'Dog'}
if name and pets and owners:
    print('We have pets!')

NOT SO GOOD

if name != '' and len(pets) > 0 and owners != {}:
    print('We have pets!')

● Checking for truth doesn't tie the conditional expression to the type of object being checked.

(检测对象真实不应该使用和对象类型相应的表达式)

● Checking for truth clearly shows the sxh's intention rather than drawing attention to a specific outcome.

(检测对象时应当清楚的表达代码的意向,而不是为了得到一个结果)

What is truth?

True False
Non-empty string Empty string
Number not 0 Number 0
Non-empty container: len(x) > 0 Empty container: len(x) == 0
- None
True False
nonzero (2.x) / bool (3.x) nonzero (2.x) / bool (3.x)

3. Use in where possible

GOOD(contains)

name = 'Safe Hammad'
if 'H' in name:
    print('This name has an H in it!')

NOT SO GOOD(contains)

name = 'Safe Hammad'
if name.find('H') != -1:
    print('This name has an H in it!')

GOOD(iteration)

pets = ['Dog', 'Cat', 'Hamster']

for pet in pets:
    print('A', pet, 'can be very cute!')

NOT SO GOOD(iteration)

pets = ['Dog', 'Cat', 'Hamster']
i = 0
while i < len(pets):
    print('A', pets[i], 'can be very cute!')
    i += 1
● Using in to check if an item is in a sequence is clear and concise.

用于检测元素在不在序列中显得意图明显且简洁

● Can be used on lists, dicts (keys), sets, strings, and your own classes by implementing the contains special method.

可以适用于多种数据类型,并且可以通过定义contains方法来适用于自定义类

4. Swap values without temp variable

GOOD

a, b = 5, 6
print(a, b) # 5, 6

a, b = b, a
print(a, b) # 6, 5

NOT SO GOOD

a, b = 5, 6
print(a, b) # 5, 6

temp = a
a = b
b = temp
print(a, b) # 6, 5

● Avoids polluting namespace with temp variable used only once

避免因为仅仅使用一次的temp变量污染命名空间

5. Build strings using sequence

GOOD

chars = ['S', 'a', 'f', 'e']
name = ''.join(chars)
print(name) # Safe

NOT SO GOOD

chars = ['S', 'a', 'f', 'e']
name = ''
for char in chars:
    name += char
    print(name) # Safe

● The join method called on a string and passed a list of strings takes linear time based on length of list.

join()可以用于字符串和字符串的list,并且只使用一次方的遍历事件

● Repeatedly appending to a string using '+' takes quadratic time!

使用+ 将会使用二次方的时间

6. EAFP is preferable to LBYL

It's Easier to Ask for Forgiveness than Permission.

Look Before You Leap.

请求原谅比请求许可要简单的多,所以三思而后行。

GOOD

d = {'x': '5'}
try:
    value = int(d['x'])
except (KeyError, TypeError, ValueError):
    value = None

NOT SO GOOD

d = {'x': '5'}
if 'x' in d and \
   isinstance(d['x'], str) and \
   d['x'].isdigit():
    value = int(d['x'])
else:
    value = None
● Throwing exceptions is not “expensive” in Python unlike e.g. Java.

抛出异常并不像java这样的软件一样重

● Rely on duck typing rather than checking for a specific type.

尽量不要以来数据的类型

7. Enumerate

GOOD

names = ['Safe', 'George', 'Mildred']
    for i, name in enumerate(names):
    print(i, name) # 0 Safe, 1 George etc.

NOT SO GOOD

names = ['Safe', 'George', 'Mildred']
count = 0
for name in names:
    print(i, name) # 0 Safe, 1 George etc.
    count += 1

● Available since Python 2.3!

python 2.3 之后可以使用

● Use the start parameter available since Python 2.6 to start at a number other than 0.

8. Build lists using list comprehensions

GOOD

data = [7, 20, 3, 15, 11]
result = [i * 3 for i in data if i > 10]
print(result) # [60, 45, 33]
</xsh>
=== NOT SO GOOD (MOST OF THE TIME) ===
<sxh python>
data = [7, 20, 3, 15, 11]
result = []
for i in data:
    if i > 10:
    result.append(i * 3)
print(result) # [60, 45, 33]
● Very concise syntax. 非常简洁的语法 ● Be careful it doesn't get out of hand (in which case the second formcan be clearer). 要小心不要脱离控制(部分情况下,第二种方法可能会更好一些)

9. Create dict from keys and values using zip

GOOD

keys = ['Safe', 'Bob', 'Thomas']
values = ['Hammad', 'Builder', 'Engine']
d = dict(zip(keys, values))
print(d) # {'Bob': 'Builder','Safe': 'Hammad','Thomas': 'Engine'}

NOT SO GOOD

keys = ['Safe', 'Bob', 'Thomas']
values = ['Hammad', 'Builder', 'Engine']
d = {}
for i, key in enumerate(keys):
    d[keys] = values[i]
    print(d) # {'Bob': 'Builder', 'Safe': 'Hammad', 'Thomas': 'Engine'}
● There are several ways of constructing dicts! 其实有很多方法来创建一个字典

10 .And the rest ... !

● while True:

      break # This will spark discussion!!!

● Generators and generator expressions.

● Avoid from module import *

  Prefer: import numpy as np; import pandas as pd

● Use _ for “throwaway” variables e.g.:

  for k, _ in [('a', 1), ('b', 2), ('c', 3)]

● dict.get() and dict.setdefault()

● collections.defaultdict

● Sort lists using l.sort(key=key_func)

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