Python学习小记(2)---[list, iterator, and, or, zip, dict.keys]

1.List行为

可以用 alist[:] 相当于 alist.copy() ，可以创建一个 alist 的 shallo copy，但是直接对 alist[:] 操作却会直接操作 alist 对象

>>> alist = [1,2,3]
>>> blist = alist[:]               #assign alist[:] to blist
>>> alist
[1, 2, 3]
>>> blist
[1, 2, 3]
>>> blist[2:] = ['a', 'b', 'c']   #allter blist
>>> alist
[1, 2, 3]
>>> blist
[1, 2, 'a', 'b', 'c']
>>> alist[:] = ['a', 'b', 'c']       #alter alist[:]
>>> alist
['a', 'b', 'c']

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2.循环技巧

#list
>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.items():
...     print(k, v)
...
gallahad the pure
robin the brave

#zip函数
>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
...     print('What is your {0}?  It is {1}.'.format(q, a))
...
What is your name?  It is lancelot.
What is your quest?  It is the holy grail.
What is your favorite color?  It is blue.

#reversed & sorted
#Note:  这两个函数不修改参数本身，返回一个iterator
#reversed
>>> for i in reversed(range(1, 10, 2)):
...     print(i)
...
9
7
5
3
1

#sorted
>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
>>> for f in sorted(set(basket)):
...     print(f)
...
apple
banana
orangez
pear

---

3.

enumerate()函数可以把创建ist，str的可迭代对象，迭代对象每次返回一个(index, value)，形式的元组

>>> astr = 'abc'
>>> alist = [1,2,3]
>>> enumerate(astr)
<enumerate object at 0x0374D760>
>>> enumerate(alist)
<enumerate object at 0x0374D698>
>>> def print_iterator(iterator):
...     for ele in iterator:
...             print(ele)
...
>>> print_iterator(astr)
a
b
c
>>> print_iterator(enumerate(astr))
(0, 'a')
(1, 'b')
(2, 'c')
>>> print_iterator(enumerate(alist))
(0, 1)
(1, 2)
(2, 3)
>>>                                     

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4.zip()示例

>>> a = [1,2,3]
>>> b = ['a', 'b', 'c']
>>> c = ['one', 'two', 'three']
>>> a,b,c
([1, 2, 3], ['a', 'b', 'c'], ['one', 'two', 'three'])
>>>
>>> def print_iterator(iterator):
...     for ele in iterator:
...             print(ele)
...
>>>
>>> print_iterator(zip(a))
(1,)
(2,)
(3,)
>>> print_iterator(zip(a,b))
(1, 'a')
(2, 'b')
(3, 'c')
>>>
>>> print_iterator(zip(a,b,c))
(1, 'a', 'one')
(2, 'b', 'two')
(3, 'c', 'three')                                                                                       

---

5.

注意 adict.keys() 返回的只是 adict 的 keys 的视图

>>> adict = dict(a=1, b=2)
>>> adict
{'a': 1, 'b': 2}
>>> view = adict.keys()
>>> view
dict_keys(['a', 'b'])
>>> adict['c'] = 3
>>> view
dict_keys(['a', 'b', 'c'])

---

6.不一样的逻辑运算返回值

大概规则就是返回第一个可以判别表达式真假对象

>>> '' and 'a' and 'b'
''
>>> 'c' and '' and 'b'
''
>>> 'c' and 0 and 'b'
0
>>> '' or 'a' or 'b'
'a'
>>> 'c' or '' or 'b'
'c'
>>> '' or 0 or 'b'
'b'
>>> 1 and 3 and 4
4
>>> 0 or '' or []
[]

7.注意list的迭代方式，若要获得 (k, v) ，需要调用 adict.items() , 直接迭代只能获得 key, 和 adict.keys() 是完全等效的

>>> adict = {'one':'first', 'two':'second', 'three':'third'}
>>> adict
{'one': 'first', 'two': 'second', 'three': 'third'}
>>> it = iter(adict)
>>> it
<dict_keyiterator object at 0x010A8F60>
>>> next(it)
'one'
>>> keys = adict.keys()
>>> keys
dict_keys(['one', 'two', 'three'])
>>> items = adict.items()
>>> items
dict_items([('one', 'first'), ('two', 'second'), ('three', 'third')])

>>> iter(items)
<dict_itemiterator object at 0x010BAC30>
>>> iter(keys)
<dict_keyiterator object at 0x010BAC90>