字典是Python中常用的内置数据类型之一。
字典是无序的对象集合，只能通过key-value的方式存取数据，字典是一种映射类型，其次key的必须是可hash的不可变类型。字典中的key必须唯一。
1.创建方法
dic = {} dic = dict() dic = dict(a=1,b=2) dic = dict([('a',1),('b',2),('c',3)]) dic = {x: x**2 for x in (2,4,6)} dic = dict([('a',1),('b',2),('c',3)]) dic = dict({('a',1),('b',2)}) dic = dict([['a',1],['b',2],['c',3]]) dic = dict((('a',1),('b',2),('c',3)))
2.常用的几个方法
2.1 setdefault(k,d) -> if k not in D ,return d,else return D[k]
#方法说明
#如果k存在则返回k在D字典中的v,如果k不存在，则返回d
setdefault(...)
D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D
#练习
>>> dic = {'a':1,'b':2,'c':3}
>>> dic.setdefault('a',4)
1
>>> dic
{'a': 1, 'b': 2, 'c': 3}
>>> dic.setdefault('d',5)
5
>>> dic
{'a': 1, 'b': 2, 'c': 3, 'd': 5}
2.2 update 更新字典
update(...)
D.update([E, ]**F) -> None. Update D from dict/iterable E and F.
If E is present and has a .keys() method, then does: for k in E: D[k] = E[k]
If E is present and lacks a .keys() method, then does: for k, v in E: D[k] = v
In either case, this is followed by: for k in F: D[k] = F[k]
#练习
>>> dic
{'a': 1, 'b': 2, 'c': 3, 'd': 5}
>>> dic.update({'e':6,'f':7})
>>> dic
{'a': 1, 'b': 2, 'c': 3, 'd': 5, 'e': 6, 'f': 7}