推导式是Python中特有的一种表达方式,主要从一种序列推导出另外一种序列。
支持推导式的序列:
(1)链表(list)
(2)字典(dict)
(3)集合(set)
# (1)链表推导式 # [表达式 for item in seq] # [表达式 for item in seq if 条件表达式 ] l = [x*x for x in range(0, 3)] print l # [0, 1, 4] # 求[1, 2, 3]和[5, 6, 7]排列组合 l = [(x, y) for x in range(1, 3) for y in range(5, 7)] print l # [(1, 5), (1, 6), (2, 5), (2, 6)] # 求0 ~ 10之间的偶数 l = [x for x in range(0, 10) if x % 2 == 0] print l # [0, 2, 4, 6, 8] # 生成函数列表 l = [lambda y: y*x for x in range(0, 3)] # 此时x = 2, l包含三个表达式 lambda y: y*2 print l[0](1) # 2 print l[1](2) # 4 print l[2](3) # 6 # (2)字典推导式 # {key: value, for ... in ... if} d = {k: None for k in range(0, 3)} print d # {0: None, 1: None, 2: None} d = {k: v for k in range(0, 3) for v in range(2, 5) if k < v} print d # {0: 4, 1: 4, 2: 4} # (3)集合推导式 # {表达式 for item in seq} # {表达式 for item in seq if 条件表达式 } d = {k for k in range(0, 3)} print d