使用python的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

• 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)

name = 'Safe'
pets = ['Dog', 'Cat', 'Hamster']
owners = {'Safe': 'Cat', 'George': 'Dog'}

# Good
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!')

What is truth

3. Use in where possible

• 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)

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

# Not So Good
if name.find('H') != -1:
    print('This name has an H in it!')

# iteration
pets = ['Dog', 'Cat', 'Hamster']
# Good 
for pet in pets:
    print('A', pet, 'can be very cute!')

# Not So Good
i = 0
while i < len(pets):
    print('A', pets[i], 'can be very cute!')
    i += 1

4. Swap values without temp variable

• 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)

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

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

# Not So Good
temp = a
a = b
b = temp
print(a, b) # 6, 5

5. Build strings using sequence

• 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)

chars = ['S', 'a', 'f', 'e']

# Good
name = ''.join(chars)

# Not So Good
name = ''
for char in chars:
    name += char

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。

• 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(尽量不要依赖数据的类型)

# 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

7. Enumerate

• Available since Python 2.3!
• Use the start parameter available since Python 2.6 to start at a number other than 0. ```python names = ['Safe', 'George', 'Mildred']

Good

for i, name in enumerate(names): print(i, name) # 0 Safe, 1 Georg

Not So Good

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

### 8. Build lists using list comprehensions

* 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)

```python
data = [7, 20, 3, 15, 11]

# Good
result = [i * 3 for i in data if i > 10]

# Not So Good
result = []
for i in data:
    if i > 10:
    result.append(i * 3)

9. Create dict from keys and values using zip

• There are several ways of constructing dicts!(其实有很多方法来创建一个字典)

keys = ['Safe', 'Bob', 'Thomas']
values = ['Hammad', 'Builder', 'Engine']

# Good
d = dict(zip(keys, values))

# Not So Good
d = {}
for i, key in enumerate(keys):
    d[keys] = values[i]

10. And the rest ... !

• while True: break # This will spark discussion!!!
• Generators and generator expressions.
• Avoid from module import *
• Use _ for “throwaway” variables e.g.:

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

• dict.get() and dict.setdefault()
• collections.defaultdict
• Sort lists using l.sort(key=key_func)

原作者pdf