python 列表表达式和生成器表达式

参考链接： Python | 生成器表达式

列表表达式：

 >>> a=[1,2,2,3,50]

 >>> b=[i+1 for i in a]

 >>> print(b)

 [2, 3, 3, 4, 51]

 >>> b=[i+1 for i in a if i>=5]

 >>> print(b)

 [51]

 列表表达式返回一个完整的列表

 生成器表达式：

 >>> c=(i+2 for i in a )

 >>> print(c)

 <generator object <genexpr> at 0x01612030>

 >>> print(list(c))

 [3, 4, 4, 5, 52]

 生成器表达式和列表表达式的形式上并无多大差别但是执行方式上却有很大的差别，生成器表达式返回的是一个iterator 每次只返回处理单个元素，而列表表达式返回的是整个的列表，如果列表的长度很大大到占满内存的程度，那么改用生成器是有很明显的好处的

 在生成器中嵌套使用：

 >>> c=(i*2 for i in( j+1 for j in a))

 >>> print(list(c))

 [4, 6, 6, 8, 102]

 嵌套使用for字句，注意和上面的区别，这里并行写for

 >>> c=((i,j) for i in a for j in a)

 >>> print(list(c))

 [(1, 1), (1, 2), (1, 2), (1, 3), (1, 50), (2, 1), (2, 2), (2, 2), (2, 3), (2, 50), (2, 1), (2, 2), (2, 2), (2, 3), (2, 50), (3, 1), (3, 2), (3, 2), (3, 3), (3, 50), (50, 1), (50, 2), (50, 2), (50, 3), (50, 50)]

>>>   

>>> c=([i,j+1,k+2] for i in a for j in a for k in a) >>> print(list(c)) [[1, 2, 3], [1, 2, 4], [1, 2, 4], [1, 2, 5], [1, 2, 52], [1, 3, 3], [1, 3, 4], [1, 3, 4], [1, 3, 5], [1, 3, 52], [1, 3, 3], [1, 3, 4], [1, 3, 4], [1, 3, 5], [1, 3, 52], [1, 4, 3], [1, 4, 4], [1, 4, 4], [1, 4, 5], [1, 4, 52], [1, 51, 3], [1, 51, 4], [1, 51, 4], [1, 51, 5], [1, 51, 52], [2, 2, 3], [2, 2, 4], [2, 2, 4], [2, 2, 5], [2, 2, 52], [2, 3, 3], [2, 3, 4], [2, 3, 4], [2, 3, 5], [2, 3, 52], [2, 3, 3], [2, 3, 4], [2, 3, 4], [2, 3, 5], [2, 3, 52], [2, 4, 3], [2, 4, 4], [2, 4, 4], [2, 4, 5], [2, 4, 52], [2, 51, 3], [2, 51, 4], [2, 51, 4], [2, 51, 5], [2, 51, 52], [2, 2, 3], [2, 2, 4], [2, 2, 4], [2, 2, 5], [2, 2, 52], [2, 3, 3], [2, 3, 4], [2, 3, 4], [2, 3, 5], [2, 3, 52], [2, 3, 3], [2, 3, 4], [2, 3, 4], [2, 3, 5], [2, 3, 52], [2, 4, 3], [2, 4, 4], [2, 4, 4], [2, 4, 5], [2, 4, 52], [2, 51, 3], [2, 51, 4], [2, 51, 4], [2, 51, 5], [2, 51, 52], [3, 2, 3], [3, 2, 4], [3, 2, 4], [3, 2, 5], [3, 2, 52], [3, 3, 3], [3, 3, 4], [3, 3, 4], [3, 3, 5], [3, 3, 52], [3, 3, 3], [3, 3, 4], [3, 3, 4], [3, 3, 5], [3, 3, 52], [3, 4, 3], [3, 4, 4], [3, 4, 4], [3, 4, 5], [3, 4, 52], [3, 51, 3], [3, 51, 4], [3, 51, 4], [3, 51, 5], [3, 51, 52], [50, 2, 3], [50, 2, 4], [50, 2, 4], [50, 2, 5], [50, 2, 52], [50, 3, 3], [50, 3, 4], [50, 3, 4], [50, 3, 5], [50, 3, 52], [50, 3, 3], [50, 3, 4], [50, 3, 4], [50, 3, 5], [50, 3, 52], [50, 4, 3], [50, 4, 4], [50, 4, 4], [50, 4, 5], [50, 4, 52], [50, 51, 3], [50, 51, 4], [50, 51, 4], [50, 51, 5], [50, 51, 52]] >>>