Python dictionary operations

1. Construct the dictionary

• Create directly

dict1={}

dict1['key']=value

• With the collections module

Import defaultdict from the collections module, defaultdict can implement a key corresponding to multiple values, to ensure that the order of inserted elements remains unchanged, you can use defaultdict (list), and then use the append function to add elements; add function to add an element

dict2=dd(list)
dict2['a'].append(1)
dict2['a'].append(2)
dict2['b'].append(2)
dict2['c'].append(44)
dict2>>>defaultdict(<type 'list'>, {'a': [1, 2], 'c': [44], 'b': [2]})

• ordered dictionary

Import OrderedDict to implement
dict3['aa']=1
dict3['bb']=11
dict3['cc']=5
dict3>>>OrderedDict([('aa', 1), ('bb', 11) , ('cc', 5)])

2. Built-in functions

dir(dict) to see supported methods:
'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', ' values'

• clear clears the elements in the dictionary
• copy Shallow copy
• fromkeys uses the sequence in fromkeys(seq, value) as the key of the new dictionary; value is the initial value, optional
• get 同dict[key]
• items returns an iterable key value
• values ​​return value
• keys return key
• popitem returns and deletes a key-value pair
• update adds a new key-value pair
• setdefault is added if the key does not exist, similar to fromkeys