Detailed explanation of Python dictionary operations

Python dictionary is another mutable container model and can store any type of object, such as strings, numbers, tuples and other container models.
1. Create a dictionary
A dictionary consists of pairs of keys and corresponding values. Dictionaries are also called associative arrays or hash tables. The basic syntax is as follows:

dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}

You can also create a dictionary like this:

dict1 = { 'abc': 456 };
dict2 = { 'abc': 123, 98.6: 37 };

Note:
Each key and value are separated by a colon (:), each pair is separated by a comma, and each pair is separated by a comma. The whole is enclosed in curly braces ({}).
Keys must be unique, but values ​​do not.
The value can be of any data type, but it must be immutable, such as string, number or tuple.
2. Access the values ​​in the dictionary

Put the corresponding keys in familiar square brackets, as shown in the following example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];
#以上实例输出结果：
#dict['Name']:  Zara
#dict['Age']:  7

If you access data with keys that are not in the dictionary, an error will be output. As follows:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

print "dict['Alice']: ", dict['Alice'];

#The output result of the above example:

#dict['Zara']:
#Traceback (most recent call last):
# File "test.py", line 4, in 4225fa317875f3e92281a7b1a5733569
# print "dict['Alice']: ", dict['Alice'];
#KeyError: 'Alice'[/code]
3. Modify the dictionary
The method to add new content to the dictionary is to add new key/value pairs, modify or delete existing key/value pairs as follows:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

dict['Age'] = 8; # update existing entry
dict['School'] = "DPS School"; # Add new entry

 
print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];
#以上实例输出结果：
#dict['Age']:  8
#dict['School']:  DPS School

4. Delete dictionary elements
You can delete a single element or clear it Dictionary, clearing only requires one operation.
Use the del command to display and delete a dictionary, as shown in the following example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};

del dict['Name']; # 删除键是'Name'的条目
dict.clear();     # 清空词典所有条目
del dict ;        # 删除词典

print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];
#但这会引发一个异常，因为用del后字典不再存在：
dict['Age']:
#Traceback (most recent call last):
#  File "test.py", line 8, in <module>
#    print "dict[&#39;Age&#39;]: ", dict[&#39;Age&#39;];
#TypeError: &#39;type&#39; object is unsubscriptable

5. Characteristics of dictionary keys
The dictionary value can be any python object without restrictions, either a standard object or It's user-defined, but not the keys.
Two important points to remember:
1) The same key is not allowed to appear twice. If the same key is assigned twice during creation, the latter value will be remembered, as in the following example:

#!/usr/bin/python

dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};

print "dict['Name']: ", dict['Name'];
#以上实例输出结果：
#dict['Name']:  Manni

2) The key must be immutable, so it can be used as a number, string or tuple, so use Lists will not work, as shown in the following example:

#!/usr/bin/python

dict = {['Name']: 'Zara', 'Age': 7};

print "dict['Name']: ", dict['Name'];
#以上实例输出结果：
#Traceback (most recent call last):
#  File "test.py", line 3, in <module>
#    dict = {[&#39;Name&#39;]: &#39;Zara&#39;, &#39;Age&#39;: 7};
#TypeError: list objects are unhashable

6. Dictionary built-in functions & methods
Python dictionary includes the following built-in functions:
1. cmp(dict1, dict2): Compare two dictionary elements.
2. len(dict): Calculate the number of dictionary elements, that is, the total number of keys.
3. str(dict): Output the printable string representation of the dictionary.
4. type(variable): Returns the input variable type. If the variable is a dictionary, return the dictionary type.

Python dictionary contains the following built-in methods:
1. radiansdict.clear(): Delete all elements in the dictionary
2. radiansdict.copy(): Return a shallow copy of the dictionary
3. radiansdict.fromkeys(): Create a new dictionary, using the elements in the sequence seq as the keys of the dictionary, and val is the initial value corresponding to all keys in the dictionary
4. radiansdict.get( key, default=None): Returns the value of the specified key. If the value is not in the dictionary, it returns the default value
5. radiansdict.has_key(key): If the key is in the dictionary dict, it returns true, otherwise it returns false
6. radiansdict.items(): Returns a traversable (key, value) tuple array as a list
7, radiansdict.keys(): Returns all the keys of a dictionary as a list
8, radiansdict.setdefault(key, default=None): Similar to get(), but if the key does not already exist in the dictionary, the key will be added and the value will be set to default
9. radiansdict.update(dict2): Change the key/value of dictionary dict2 Update to dict
10, radiansdict.values(): Return all values ​​in the dictionary as a list

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