1、解析式是什么？为什么要用解析式？
对一个list里所有的数值求平方
In [123]: lst = list(range(6))
In [124]: ret = []
In [125]: for i in lst:
...: ret.append(i ** 2)
...:
In [126]: ret
Out[126]: [0, 1, 4, 9, 16, 25]解析式：
In [127]: lst = list(range(6))
In [128]: [ x ** 2 for x in lst ] # 列表解析
Out[128]: [0, 1, 4, 9, 16, 25]求list中偶数的平方
In [137]: lst
Out[137]: [0, 1, 2, 3, 4, 5]
In [138]: for i in lst:
...: if i % 2 == 0:
...: print(i ** 2)
...:
...:
0
4
16
In [140]: [ i ** 2 for i in lst if i % 2 == 0 ]
Out[140]: [0, 4, 16]列表解析的语法格式：
[ expr for i in iterable ]
[ expr for i in iterable if子句 ]
解析式代码简洁，可读性高；效率比普通迭代稍高
if子句可以出现多次：
In [143]: [ i ** 2 for i in lst if i % 2 == 0 if i > 0]
Out[143]: [4, 16]
In [151]: [ i ** 2 for i in lst if i % 2 == 0 and i > 0 and i < 4 ]
Out[151]: [4]
In [149]: [ i ** 2 for i in lst if i % 2 == 0 if i > 0 or i < 4 ]
Out[149]: [0, 4, 16]带多个if语句的都可以转化为条件的逻辑运算，所以一般不会带多个if语句
for语句也可以有多个： # 就是嵌套迭代
In [153]: [ x, y for x in range(5) for y in range(5, 10) ]
File "<ipython-input-153-8aadadd13ebf>", line 1
[ x, y for x in range(5) for y in range(5, 10) ]
^
SyntaxError: invalid syntax
In [154]: [ (x, y) for x in range(5) for y in range(5, 10) ] # 表达式只能有一个
Out[154]:
[(0, 5),
(0, 6),
(0, 7),
(0, 8),
(0, 9),
(1, 5),
(1, 6),
(1, 7),
(1, 8),
(1, 9),
(2, 5),
(2, 6),
(2, 7),
(2, 8),
(2, 9),
(3, 5),
(3, 6),
(3, 7),
(3, 8),
(3, 9),
(4, 5),
(4, 6),
(4, 7),
(4, 8),
(4, 9)]
In [155]:
In [161]: [ (x, y) for x in range(5) if x > 2 for y in range(5, 10) ]
Out[161]:
[(3, 5),
(3, 6),
(3, 7),
(3, 8),
(3, 9),
(4, 5),
(4, 6),
(4, 7),
(4, 8),
(4, 9)]
In [159]: [ (x, y) for x in range(5) for y in range(5, 10) if x > 2 ]
Out[159]:
[(3, 5),
(3, 6),
(3, 7),
(3, 8),
(3, 9),
(4, 5),
(4, 6),
(4, 7),
(4, 8),
(4, 9)]解析式也可以嵌套：
In [163]: [ x ** 2 for x in [ x for x in [1, 2, 3] ] ]
Out[163]: [1, 4, 9]一眼看不出解析式的结果是什么时候，就不要用解析式
2、三元表达式
expr1 if 条件子句 else expr2
当条件蛮族时，返回expr1 或则返回expr2
In [165]: x = 3
In [166]: x ** 2 if x % 2 == 0 else x ** 3
Out[166]: 27
In [172]: name = "xxj" if 3 > 0 else "xj"
In [173]: name
Out[173]: 'xxj'
In [174]: name = "xxj" if 3 <0 else "xj"
In [175]: name
Out[175]: 'xj'
In [176]: name = "xxj" if 3 > 0 else name = "xj"
File "<ipython-input-176-36d6075bedd6>", line 1
name = "xxj" if 3 > 0 else name = "xj"
^
SyntaxError: can't assign to conditional expression列表解析和三元表达式结合：
In [180]: [ x ** 2 if x % 2 == 0 else x ** 3 for x in [1, 2, 3, 4 ] ]
Out[180]: [1, 4, 27, 16]
In [178]: [ x ** 2 for x in [ 1, 2, 3, 4 ] if x % 2 == 0 else x ** 3 ]
File "<ipython-input-178-63a49a9bdf59>", line 1
[ x ** 2 for x in [ 1, 2, 3, 4 ] if x % 2 == 0 else x ** 3 ]
^
SyntaxError: invalid syntax3、生成器解析
列表解析返回的是list
In [8]: ( x ** 2 for x in range(1000) )
Out[8]: <generator object <genexpr> at 0x7f770e8eeb48>
In [9]: g = ( x ** 2 for x in range(1000) )
In [10]: type(g)
Out[10]: generator
In [11]: help(g)
In [12]: g.__next__
Out[12]: <method-wrapper '__next__' of generator object at 0x7f770de47b48>
In [13]: g.__next__()
Out[13]: 0
In [14]: g.__next__()
Out[14]: 1
In [15]: g.__next__()
Out[15]: 4
In [16]: next(g)
Out[16]: 9
In [17]: next(g)
Out[17]: 16 生成器解析返回一个生成器
生成器在取值的时候才计算
生成器解析式和列表解析式除了中括号改成小括号其它的完全一样
4、集合解析式
集合解析返回一个集合
集合解析式和列表解析式除了中括号改成小括号其它的完全一样
In [24]: [ x ** 2 for x in range(5) ]
Out[24]: [0, 1, 4, 9, 16]
In [25]: ( x ** 2 for x in range(5) )
Out[25]: <generator object <genexpr> at 0x7f770dd5c7d8>
In [27]: { x ** 2 for x in range(5) }
Out[27]: {0, 1, 4, 9, 16}5、字典解析式
字典解析返回一个集合
In [28]: { str(x):x ** 2 for x in range(5) }
Out[28]: {'0': 0, '1': 1, '2': 4, '3': 9, '4': 16}
In [32]: { str(x + 1):x ** 2 for x in range(5) }
Out[32]: {'1': 0, '2': 1, '3': 4, '4': 9, '5': 16}
In [46]: { k:v for k, v in [('a', 1), ('b', 2)] }
Out[46]: {'a': 1, 'b': 2}
