Python 3 Dictionary Explanation

Posted by big-dog1965 on Sun, 21 Jul 2019 11:51:17 +0200

Create and use dictionaries

Dictionaries can be created in the following ways:

phoneBook = {"Bill":"1234", "Mike":"4321"}

In a dictionary, keys are unique. If the key is not unique, then the program will not throw an exception, but the same key value will be overwritten by the last one:

phoneBook = {"Bill":"1234", "Mike":"4321", "Bill":"5678"}
# If positioned by Bill, the value found is "5678"

dict function

Dictionaries can be built using dick functions through other mappings or sequences of key-value pairs.

items = [["Bill","1234"], ("Mike","4321")]
d1 = dict(items)
d2 = dict(name = 'Bill', number = '5678', age = 45)
print(d1) # {'Bill': '1234', 'Mike': '4321'}
print(d2) # {'name': 'Bill', 'number': '5678', 'age': 45}

Basic operation of dictionary

Many operations of a dictionary are similar to lists:
- len(dict): Returns the number of elements in dictionary Dict
- dict[key]: Returns the value associated with the key
- dict[key] = value: assign a value to key
- del dict[key]: Delete items with keys
- key in dict: Check whether the dict contains items with keys

Dictionary format string (format_map() method)

dic = {'title':'MyTitle', 'url':'http://www.aaa.com', 'company':'Zerobit'}

str = """
<html>
	<head>
		<title>{title}</title>
	<head>
	<body>
		<h1>{title}</h1>
		<a href="{url}">{company}</a>
	</body>
</html>
"""
print(str.format_map(dic))

Iteration of Dictionary

1. Get a list of key s in the dictionary

dic = {'x':1, 'y':2, 'z':3}
# Output x y z
for key in dic:
	print(key, end=' ')

2. Get a list of key s and value s in the dictionary at the same time

dic = {'x':1, 'y':2, 'z':3}
for key,value in dic.items():
	print(key, value)

Dictionary method

clear method

clear method is used to empty all elements in a dictionary

dic = {'x':1, 'y':2, 'z':3}
dic.clear()

copy method and deepcopy function

The copy method is used to copy a dictionary, which returns the copied new dictionary.

dic = {'x':1, 'y':2, 'z':3}
newDic = dic.copy()

The copy method is only a shallow copy, only the dictionary data of the first layer is copied. As for all layers below the second level, both the original dictionary and the new dictionary point to the same value. Whether the elements in the original dictionary are modified or those in the new dictionary, the elements in the original dictionary and the new dictionary will change at the same time.

If you want to change the above situation, you need to use the deepcopy function in the copy module for deep replication.

from copy import deepcopy
dic = {'x':1, 'y':2, 'z':3}
newDic = deepcopy(dic)

fromkeys method

The fromkeys method is used to create new dictionaries based on keys. In the new dictionary, all keys have the same default value.

# Call the fromkeys method on an empty dictionary to create a new dictionary, specifying the key through a list
newDic1 = {}.fromkeys(['name', 'company', 'salary'])
print(newDic1) # {'name': None, 'company': None, 'salary': None}
newDic2 = {}.fromkeys(['name', 'company', 'salary'], 100)
print(newDic2) # {'name': 100, 'company': 100, 'salary': 100}

get method

If the key does not exist, the KeyError exception is thrown when the value is obtained from the dictionary in the form of dict[key].

If you want to avoid key s without throwing exceptions, you need to use the get() method

dic = {'x':1, 'y':2, 'z':3}
print(dic.get('a')) # None

items method and keys method

The items method is used to return all key-value pairs in the dictionary and can be used for iteration.
The keys method is used to return all keys in the dictionary, as well as to iterate.

pop method and popitem method

The pop method is used to get the value of the specified key and pop up the key-value pair from the dictionary.
The popitem method is used to return and pop up the last key-value pair in the dictionary.

For dictionaries, the elements are not in order, so the last key-value pair here refers to the last added key-value pair.

dic = {'c':10, 'a':40, 'b':12, 'x':44}
dic['z'] = 3
print(dic.pop('b')) # 12
print(dic.popitem()) # ('z', 3)
print(dic) # {'c': 10, 'a': 40, 'x': 44}

setdefault method

The setdefault method is used to set the default value of the key. This method receives two parameters, the first is key and the second is default. If the key does not exist, the key and default values are added to the dictionary

dic = {}
dic.setdefault("name","alex")
print(dic) # {'name': 'alex'}

update method

The update method can update another dictionary with elements in one dictionary.

dic1 = {'c':10, 'a':40, 'b':12, 'x':44}
dic2 = {'name': 'Bill', 'number': '5678', 'x': 150}
dic1.update(dic2)
print(dic1) # {'c': 10, 'a': 40, 'b': 12, 'x': 150, 'name': 'Bill', 'number': '5678'}

If the key-value pair in dict2 does not exist in dict1, a new key-value pair will be added to dict1. If the key already exists in dict1, the value of key in dict1 will be updated with the corresponding value of key in dict2.

values method

Values method is used to return a list of dictionary median values in iterator form

dic = {'c':10, 'a':40, 'b':12, 'x':44}
for val in dic.values():
	print(val, end=' ') # 10 40 12 44

Topics: Programming

Programmer Think