Introduction to Python defaultdict types in the collections module

Here we introduce the usage of the defaultdict type in Python's collections module, the biggest difference from the built-in dictionary class is the initialization, and look at it together:

The defaultdict is primarily used in situations where value initialization is required. For a dictionary, key must be hashable,immutable,unique data, and value can be any data type. If value is a data type such as list,dict, it must be initialized to null before use, and some cases require that value be initialized to a special value, such as 0 or '.

From collections Import Defaultdictperson_by_age = Defaultdict (list) for person in persons:  d[person.age].append ( Person.name)

Defaultdict and Dict Use the same method, only when the initialization must pass in a callable object X, when accessing a key that does not exist, the value is automatically set to X (). For example, in the example above, when a person who first accesses a certain age d[person.age] will become a list (), that is, [].

Of course, you can also use your own defined callable objects, such as:

D = defaultdict (lambda:0) d["Hello"] + = 1     # 1d["a"]         # 0

Defaultdict is more efficient than dict.set_default and more intuitive and convenient to use.

The standard dictionary includes a method, SetDefault (), to get a value that establishes a default value if the value does not exist. Defaultdict initializing a container causes the caller to specify the default value in advance.

Import Collectionsdef default_factory ():  return ' default value ' d = collections.defaultdict (default_factory, foo = ' Bar ') print ' d: ', dprint ' foo = ', d[' foo ']print ' var = ', d[' bar]

You can use this method as long as all keys have the same default value.
The above results are:

D:defaultdict (<function default_factory at 0x0201fab0>, {' foo ': ' Bar '}) foo = Barvar + = default value

Other instances

1#Dic1 = {}2#dic1["K1"].append ("KKKKK") #正常情况会报错3#Print (DIC1)45ImportCollections6 dic = collections.defaultdict (list)#Specifies the value of the dictionary as a list7 dic["k1 "].append (" kkkkk ")  8 print (DIC)  9 10 # Execution Result: 11 defaultdict (<class  "list", {k1": [ " kkkkk "})         

1#Has the following value collection,do not use the default dictionary code as follows2#[11,22,33,44,55,66,77,88,99,90]3#Save all values greater than 66 in the first key of the dictionary,4#Save a value of less than or equal to 66 in the value (list) of the second key.5#That is: {"K1": Greater Than, "K2": less than or equal to 66}67 Li = [11,22,33,44,55,66,77,88,99,90]8 dic ={}910For IInchLi11If i>66:12If"K1"InchDic.keys ():dic["K1"].append (i)14Else:dic["K1"] =[I,]16Else:17If"K2"InchDic.keys ():dic["K2 "].append (i) 19 else:20 dic[" k2 "] = [I,]21 print (DIC) 22 23 # execution result: Span style= "COLOR: #008080" >24 { "k2 ' k1": [All-in-a-.]}             

1#refine your code with the default dictionary2From collectionsImportDefaultdict #这样写, you don't have to write collections.defaultdict down here.3 values = [11, 22, 33,44,55,66,77,88,99,90]4 My_dict =Defaultdict (list)56For valueInchValues7If value>66:8 my_dict[‘K1‘].append (value)9Else:10 my_dict[ "k2 ' ].append (value) 11 print (my_dict) 12 13 # execution result: 14 defaultdict (<class " list", {k2 ": [11, 22, 33, 44, 55, 66],  ' k1 ": [77, 88, 99, 90]})  

Introduction to Python defaultdict types in the collections module