note of code in python

1.different of assignment variable and list
    for variable: just copy the content of the other
    like below:

>>> var1 = 'hello'
>>> var2 = var1
>>> var1 = 'world'
>>> var1
'world'
>>> var2
'hello'

    for list: copy the address of the other
    like below:

>>> list1 = []
>>> nested = [list1, list1, list1]
>>> nested
[[], [], []]
>>> nested[1].append('Hello ')
>>> nested
[['Hello '], ['Hello '], ['Hello ']]
>>> list1.append('world!!')
>>> nested
[['Hello ', 'world!!'], ['Hello ', 'world!!'], ['Hello ', 'world!!']]

2. contrast from string\list\pair
    the difference like the code below:

strings = 'I am a good student!!'
>>> lines = ['zhang', 'pei', 'hua', 'hello', 'world']
>>> pairs = (6, 'dog', 'cat')
>>> strings[2], lines[2], pairs[2]
('a', 'hua', 'cat')
>>> len(strings), len(lines), len(pairs)
(21, 5, 3)

3. the defence style function
    prevent unreasonable input and suggest reasonable solution. like below function the argument  must be string,if not return warn.
    

>>> def tag(word):
... assert isinstance(word, basestring), "argument \
...     to tag() must be a string"
... if word in ['a', 'the', 'all']:
... return 'det'
... else:
... return 'noun'
...
>>> tag('hello')
'noun'
>>> tag(['hello'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in tag
AssertionError: argument     to tag() must be a string

4. dictionary in python like hash pair in other programming language
    the character reflected in below code:

>>> pos = {'colorless':'ADJ', 'ideas':'N', 'sleep':'V'}
>>> pos
{'sleep': 'V', 'ideas': 'N', 'colorless': 'ADJ'}
>>> pos2 = dict(colorless='ADJ', ideas='N', sleep='V')
>>> pos2
{'sleep': 'V', 'ideas': 'N', 'colorless': 'ADJ'}
>>> pos['sleep']
'V'
>>> pos['sleep'] = ['N','V']
>>> pos
{'sleep': ['N', 'V'], 'ideas': 'N', 'colorless': 'ADJ'}