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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