Python字典方法
关于Python的深复制和浅复制， 字典列表集合可变容器模型， a = b 都是潜复制，当将b[i]修改，则a也被修改，但是若对b重新赋值(及将b指向新的对象）， 则a 与b 将不同。而元组字符串不可变容器模型没有copy方法。
a = b.copy()是深拷贝，
copy模块里有copy和deepcopy，copy是浅拷贝，deepcopy是深拷贝，import copy a = copy.copy(b)
a = copy.deepcopy(b)
>>>
a
=
dict
(one
=
1
, two
=
2
, three
=
3
)
>>>
b
=
{
'one'
:
1
,
'two'
:
2
,
'three'
:
3
}
>>>
c
=
dict
(
zip
([
'one'
,
'two'
,
'three'
], [
1
,
2
,
3
]))
>>>
d
=
dict
([(
'two'
,
2
), (
'one'
,
1
), (
'three'
,
3
)])
>>>
e
=
dict
({
'three'
:
3
,
'one'
:
1
,
'two'
:
2
})
>>>
a
==
b
==
c
==
d
==
e
True
字典（dictionary）是Python中另一个非常有用的内置数据类型。
列表是有序的对象结合，字典是无序的对象集合。两者之间的区别在于：字典当中的元素是通过键来存取的，而不是通过偏移存取。
字典是一种映射类型，字典用"{ }"标识，它是一个无序的键(key) : 值(value)对集合。
键(key)必须使用不可变类型。
在同一个字典中，键(key)必须是唯一的。
注意：
- 1、字典是一种映射类型，它的元素是键值对。
- 2、字典的关键字必须为不可变类型，且不能重复。
- 3、创建空字典使用 { }。
（1）字典含的内置函数
- a = {'a':2, 'b':3}
- len(a)输出a 的长度
- str（a）输出字典可打印的字符串标识。"{'a':2, 'b':3} "
- type(variable)返回输入的变量类型，如果变量是字典就返回字典类型
-
- iter(d)
Return an iterator over the keys of the dictionary. This is a shortcut for iter(d.keys()). -
- d[key] = value
Set d[key] to value. -
- del d[key]
Remove d[key] from d. Raises a KeyError if key is not in the map. -
- key in d
-
Return True if d has a key key , else False .
key not in dEquivalent to not key in d . -
- clear ( )
Remove all items from the dictionary.
（2） Python字典包含的内置方法
- dict.clear()
- dict.copy()是深复制
- dict.fromkeys(,)fromkeys(iterable, value=None, /) method of builtins.type instance Returns a new dict with keys from iterable and values equal to value.
- dict.get(key, default = None)返回指定键的值，如果值不在字典中，返回default
-
- items ( )
Return a new view of the dictionary’s items ( (key, value) pairs). See the documentation of view objects. -
- pop ( key [ , default ] )
If key is in the dictionary, remove it and return its value, else return default. If default is not given and key is not in the dictionary, a KeyError is raised. -
- popitem ( )
Remove and return an arbitrary (key, value) pair from the dictionary.popitem() is useful to destructively iterate over a dictionary, as often used in set algorithms. If the dictionary is empty, calling popitem()raises a KeyError. -
- setdefault ( key [ , default ] )
If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None. -
- update ( [ other ] )
Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.update() accepts either another dictionary object or an iterable of key/value pairs (as tuples or other iterables of length two). If keyword arguments are specified, the dictionary is then updated with those key/value pairs: d.update(red=1, blue=2). - dict.keys()
-
- values ( )
Return a new view of the dictionary’s values. See the documentation of view objects.