Python Tips

1. Is vs ==, mutable and immutable

• == is to compare value, is is compare address
• == is to compare by calling object.__eq__()
• list1 = list2, address is assigned, not value. need list1=list2[:]

>>> a = [1,2,3]
>>> id(a)
4463753048  <<<< address is 048
>>> a[0] = 11
>>> id(a)
4463753048
>>> a
[11, 2, 3]
>>> a.append(44)
>>> a
[11, 2, 3, 44]
>>> id(a)   <<<< can change item and append, address is same
4463753048
>>> b = a
>>> id(b)   <<<< list= is point to same address
4463753048
>>> b[0] = 111
>>> a
[111, 2, 3, 44]  <<<< change b = change a
>>> c = a[:]
>>> id(c)
4463919328
>>> c[0] = 1234
>>> a
[111, 2, 3, 44]
>>> b
[111, 2, 3, 44]
>>> c
[1234, 2, 3, 44]
>>>

2. keyword argument

>>> def ff(*args, **kwargs):
...     print args
...     print kwargs
...

>>> ff(1, '22', k1=333, k2='4444')
(1, '22')
{'k2': '4444', 'k1': 333}

>>> d = {'ka':'aaa', 'kb':123}
>>> ff(**d)
()
{'kb': 123, 'ka': 'aaa'}

3. Regexp \符号

https://docs.python.org/3/howto/regex.html

Remove全部的None-Alphanumeric char from string. 应该是

s = re.sub('\W', '', s)

还有记住以下的

\d, \D = [0-9]
\s, \S = [ \t\n\r\v\f]

\w, \W = alphanumeric,[0-9a-bA-Z_]

4. arstCli Script模版

from arstCliLib import *
import sys

dut = sys.argv[1]
openSshOnDut( dut )
setAccessMethod(dut, 'ssh')
cmd = ['show ip int brief | grep " 10\." | grep Vlan']
output = sendCmd(dut, cmd, prompt='enable', raw=True)

print '\n'.join(output)

[solomonyang@syscon] ~ $ python testCli.py
Traceback (most recent call last):
  File "testCli.py", line 4, in <module>
    from arstCliLib import *
ImportError: No module named arstCliLib

PATH=$PATH:$HOME/bin:$HOME/py
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$HOME/git/systest-infra/lib/
PYTHONPATH=$PYTHONPATH:$HOME/git/systest-infra/lib/

export PATH
export LD_LIBRARY_PATH
export PYTHONPATH

5. List vs Tuple, based on stackoverflow post

1. Literal/语法

>>> t = (1,2); l = [1,2]; t[1]; l[0]
2
1

2. Size/内存量，区别大概list +12%

>>> t = tuple(range(100000)); l =  list(range(100000)); t.__sizeof__(); l.__sizeof__()
800024
900088

3. Mutable vs Inmutable/可变 vs 不可变
>>> t = (1,2); l = [1,2]
>>> l[0] = 11; print l
[11, 2]
>>> t[0] = 11; print t
TypeError: 'tuple' object does not support item assignment

4. 都可以增加，但是不同，tuple += (3)是返回一个新的obj，而list.add()是改原来的
>>> t = (1,2); l = [1,2]; id(t); id(l); t+=(3,); l+=[3]; id(t); id(l); l.append(3); id(l)
4306859000 <<<<< t
4307014720 <<<<< 1, 都一个地址
4306913952 <<<<< t +=(3,)，新地址
4307014720

4307014720

5. 因为不可变，所以tuple (1,2)可以是Dict[key]
>>> d = {}; d[t] = '1,2'
>>> d = {}; d[l] = '1,2'
TypeError: unhashable type: 'list'

6. 用途
比方说，(10,11)是个bookmark，第10页的第11行，一般没有必要改；而list of bookmark = [ (1,10), (10,11), (22, 1)] 

7. Tuple != constant list
http://news.e-scribe.com/397
但是这么了解哪，好像也撮合:-) 好像更多的是字面的了解，tuple = lightweight record，比方说：DB API's fetchmany() 返回的是List of tuple. 每个tuple是一个record，不能改其中一项，改了就没有意义了。

List vs Set

1. Literal/语法/内存量
>>> s = set(range(100000)); s.__sizeof__(); l = list(range(100000)); l.__sizeof__()
4194504

900088

2. Set没有Index, 没有重复，可以数学操作 &, -, ^
>>> s1=set(range(1,20,2)); s2=set(range(10,30)); print 's1->', s1; print 's2->', s2; print '-:', s1 - s2; print '&:', s1&s2; s1^s2
s1-> set([1, 3, 5, 7, 9, 11, 13, 15, 17, 19])
s2-> set([10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29])
-: set([1, 3, 9, 5, 7])
&: set([19, 17, 11, 13, 15])

set([1, 3, 5, 7, 9, 10, 12, 14, 16, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29])