Python迭代器生成器,私有变量及列表字典集合推导式(二)详解编程语言

a = [1,2,3] 
b = {'a':1,'b':2,'c':3} 
c = True 
print(type(a),type(b),type(c)) # <type 'list'> <type 'dict'> <type 'bool'> 
print(isinstance(a,list))  # True

(1)[exp for iter_var in iterable if_exp] 
#工作过程： 
1 迭代iterable中的每个元素，每次迭代都先判断if_exp表达式结果为真，如果为真则进行下一步，如果为假则进行下一次迭代； 
2 把迭代结果赋值给iter_var，然后通过exp得到一个新的计算值； 
3 最后把所有通过exp得到的计算值以一个新列表的形式返回。 
 
#相当于这样的过程： 
L = [] 
for iter_var in iterable: 
    if_exp: 
        L.append(exp) 
 
#也可以循环嵌套 
(2)[exp for iter_var_A in iterable_A for iter_var_B in iterable_B] 
工作过程： 
每迭代iterable_A中的一个元素，就把ierable_B中的所有元素都迭代一遍。 
 
#相当于这样的过程： 
L = [] 
for iter_var_A in iterable_A: 
    for iter_var_B in iterable_B: 
        L.append(exp)

d = {key: value for (key, value) in iterable} 
 
#快速更改字典key,value 
mcase = {'a': 10, 'b': 34} 
mcase_frequency = {v: k for k, v in mcase.items()} 
print(mcase_frequency) 
#  Output: {10: 'a', 34: 'b'}

squared = {x**2 for x in [1, 1, 2]} 
print(squared) 
# Output: set([1, 4])

>>> class MyClass(): 
...     def __init__(self): 
...             self.__superprivate = "Hello" 
...             self._semiprivate = ", world!" 
... 
>>> mc = MyClass() 
>>> print(mc.__superprivate)  #私有变量不能直接访问 
    #print(mc._Myclass__superprivate)  也可以访问,但是不建议这样访问 
Traceback (most recent call last): 
  File "<stdin>", line 1, in <module> 
AttributeError: myClass instance has no attribute '__superprivate' 
>>> print(mc._semiprivate) 
, world! 
>>> print(mc.__dict__) 
{'_MyClass__superprivate': 'Hello', '_semiprivate': ', world!'}

tu = (12,45,22222,103,6) 
print('{0} {2} {1} {2} {3} {2} {4} {2}'.format(*tu)) 
#结果 12 22222 45 22222 103 22222 6 22222

li = [12,45,78,784,2,69,1254,4785,984] 
print(map('the number is {}'.format,li))   
 
from datetime import datetime,timedelta 
once_upon_a_time = datetime(2010, 7, 1, 12, 0, 0) 
delta = timedelta(days=13, hours=8,  minutes=20) 
 
gen =(once_upon_a_time + x*delta for x in xrange(20)) 
 
print '/n'.join(map('{:%Y-%m-%d %H:%M:%S}'.format, gen)) 
 
2010-07-01 12:00:00 
2010-07-14 20:20:00 
2010-07-28 04:40:00 
2010-08-10 13:00:00 
2010-08-23 21:20:00 
2010-09-06 05:40:00 
2010-09-19 14:00:00 
2010-10-02 22:20:00 
2010-10-16 06:40:00 
2010-10-29 15:00:00 
2010-11-11 23:20:00 
2010-11-25 07:40:00 
2010-12-08 16:00:00 
2010-12-22 00:20:00 
2011-01-04 08:40:00 
2011-01-17 17:00:00 
2011-01-31 01:20:00 
2011-02-13 09:40:00 
2011-02-26 18:00:00 
2011-03-12 02:20:00

>>> L = [x*x for x in range(10)]   #迭代器 
>>> L 
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]#列表 
>>> g = (x*x for x in range(10))   #生成器 
>>> g 
<generator object <genexpr> at 0x0000028F8B774200>#生成器对象

>>> def createGenerator(): 
...    mylist = range(3) 
...    for i in mylist: 
...        yield i*i 
... 
>>> mygenerator = createGenerator() # 创建生成器 
>>> print(mygenerator) # mygenerator is an object! 
<generator object createGenerator at 0xb7555c34> 
>>> for i in mygenerator: 
...     print(i) 
0 
1 
4