一些Python知识点

0. 新与旧

两种reverse

>>> L = [1,2,3,4]
>>> R = L[::-1] # new object
>>> R
[4, 3, 2, 1]
>>> L.reverse() # in place
>>> L
[4, 3, 2, 1]
>>>

两种sort

>>> sorted(r) # new object
[’black’, ’blue’, ’green’, ’red’, ’white’]
>>> r
[’white’, ’black’, ’green’, ’blue’, ’red’]
>>> r. sort() # in-place
>>> r
[’black’, ’blue’, ’green’, ’red’, ’white’]

1. 函数参数

def f(a, b, c=0, *argc, **kw):　　print a, b, c, argc, kw

>>> f(1, 2, 3, ‘a‘, ‘b‘, ‘c‘, key=‘value‘)
1 2 3 (‘a‘, ‘b‘, ‘c‘) {‘key‘: ‘value‘}

a, b为必选参数，c默认参数，argc可变参数，kw为关键字参数。传入函数时，argc是一个tuple，kw为dict。

注意默认参数，一定要指向不可变变量。否者多个函数的同一参数可能指向同一个instance

>>> def foo(bar=[]):
...    bar.append("baz")
...    return bar
>>> foo()
["baz"]
>>> foo()
["baz", "baz"]
>>> foo()
["baz", "baz", "baz"]

>>> def foo(bar=None):
...    if bar is None:        # or if not bar:
...        bar = []
...    bar.append("baz")
...    return bar
...
>>> foo()
["baz"]
>>> foo()
["baz"]
>>> foo()
["baz"]

2. 迭代

• 判断是否可迭代，使用Iterable

>>> from collections import Iterable
>>> isinstance(‘string‘, Iterable)
True
>>> isinstance([1,2,3], Iterable)
True
>>> isinstance(123, Iterable)
False

• 引入下标，可用enumerate

>>> for i, v in enumerate([‘a‘, ‘b‘, ‘c‘]):
...     print i, v
...
0 a
1 b
2 c
>>>

• 同时迭代多个list，使用函数zip

>>> for i, j in zip([1,2,3], [‘a‘, ‘b‘, ‘c‘]):
...     print i, j
...
1 a
2 b
3 c

3. List Comprehension中的if-else

>>> L = [‘We‘, ‘are‘, 42, ‘ALONE‘, ‘!‘]
>>> [s.lower() if isinstance(s,str) else s for s in L]
[‘we‘, ‘are‘, 42, ‘alone‘, ‘!‘]

4. class

http://www.toptal.com/python/python-class-attributes-an-overly-thorough-guide

限定class的成员

__slots__

>>> class Student(object):
...     __slots__ = (‘name‘, ‘age‘)
...

5. exception

检查多个异常，需要放在tuple内

>>> try:
...     l = ["a", "b"]
...     int(l[2])
... except (ValueError, IndexError) as e:
...     pass
...
>>>