Python Basic Teaching dictionary operation detailed _python

Dictionary
Dictionary

1. Set of key-value pairs (map)

2. The dictionary is a collection of data surrounded by braces "{}"

3. Dictionaries are unordered and access members by key in the dictionary. Variable, can be nested, can be used to modify the extension, etc., do not produce a new dictionary

4. The key of the dictionary can be a string (case sensitive), a numeric constant or a tuple (immutable type), and a key of the same dictionary can be mixed with a type. The key of the dictionary must be hash

The criteria for a tuple as a key is that the values within the tuple are immutable types

Copy Code code as follows:

A = (1,2) #可以作为键
b = (1,2,[3,4]) #不可以

5. The value of a dictionary can be any type, can be nested, can be modified freely

Statement
Several ways to create a dictionary:

1. Basic

Copy Code code as follows:

D = {} #空字典
D = {' name ': ' Tom ', ' Age ': 22}
#等价
D = {}
d[' name ' = ' Tom '
d[' age ' = 22

2.dict

Copy Code code as follows:

D = dict () #空
D = dict (name= ' Tom ', age=22)

D = dict ([(' Name ', ' Tom '), (' age ', 22)]
#等价
Keys = [' name ', ' age ']
values = [' Tom ', 22]
D = dict (Zip (keys,values))

3.fromkeys

Without specifying Default_value, default None

Copy Code code as follows:

>>> Dict.fromkeys ([' Name ', ' age '], ' default_value ')
{' Age ': ' Default_value ', ' name ': ' Default_value '}

Basic operations

0. Get help

Copy Code code as follows:

Help (Dict)

1. Whether the decision key exists in the dictionary

Copy Code code as follows:

If k in D: #k isn't in
DoSomething ()

2. Read

Copy Code code as follows:

D = {' A ': 1, ' B ': 2}
Print d[' A '] #得到1, but if the key does not exist, an exception keyerror is thrown. Caution, no use recommended

Print D.get (' C ', 3) #得到3, the Get method, if the key does not exist, returns the second parameter default_value. If no default_value is set to return none
Handling Missing-key errors in three ways, depending on the specific needs of

Copy Code code as follows:

If k in D:
Print D[k]

Try
Print D[k]
Except Keyerror:
DoSomething ()

Print D.get (k, default)
#等价 D[k] If k in D else default

3. Traverse

Mode 1:

Copy Code code as follows:

For key in D:
Print key, D[key]
#等价 for key in D.keys ()

Mode 2:

Copy Code code as follows:

For Key,value in D.items ():
Print key, value

4. Modify Mode 1: A key value pair

Copy Code code as follows:

d[' key ' = ' newvalue '

Mode 2: Batch Add or update

Copy Code code as follows:

#另一个字典
D.update ({' key ': ' NewValue '}) #这里支持一整组值

#元组列表
D.update ([(' A ', 1), (' B ', 2]]) #每个元组两个元素, (Key,value)

#**key
D.update (c=3, e=4)

5. Delete

Copy Code code as follows:

Del d[' key ']
Value = D.pop (' key ') #删除并返回值
D.clear () #清空
6. Other:

Len (d) #长度
D.keys () #key列表
D.values () #value列表
D.items () # (key, value) list
c = d.copy () #浅拷贝
#返回迭代器, save memory
D.iterkeys ()
D.itervalues ()
D.iteritems ()
D.setdefault (' name ', ' Ken ') #若原来没有, set, otherwise the original value does not change

Other
1. Dictionary Sort by key

Copy Code code as follows:

Keys = D.keys ()
Keys.sort ()
For key in keys:
Print D.get (key)

Sort by value

Copy Code code as follows:

Sorted (D.items (), Lambda x,y:cmp (x[1],y[1))

Other than that:

Copy Code code as follows:

#假设d为字典
Sorted (d) #返回同 sorted (D.keys ()), which returns the key sort

2. Custom object as key

Have to:

Copy Code code as follows:

def __hash__ (self):
Pass
def __eq__ (self, Other):
Pass

3. Dictionary Copy Light copy:

Copy Code code as follows:

c = d.copy () #

Deep copy must be in copy module

Copy Code code as follows:

Form copy Import Deepcopy
c = Deepcopy (d)

4. A use scenario assumes that there is a large list of L, assuming that the 10w record

There is a small list B, to determine if the element in B is in L

If:

Copy Code code as follows:

For I in B:
If I in L:
DoSomething ()

You'll find it very, very slow ... Because the second in statement will traverse the 10w bar ....

Improved:

Copy Code code as follows:

D = Dict.fromkeys (l)
For I in B:
If I in D:
DoSomething ()
#空间换时间, O (n)-> O (1)