Detailed explanation of Python dictionary operations_python

This article mainly introduces the detailed operation method of Python dictionary (Dictionary). Friends who need it can refer to it

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

Copy code The code is as follows:

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

You can also create a dictionary like this:

Copy code The code is as follows:

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

Note:
Each key and value are separated by a colon (: ), use commas for each pair, separate each pair with commas, and place the whole pair 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 into familiar square brackets, as shown in the following example:

Copy code The code is as follows:

#!/usr/bin/python

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

print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];
#Above examples Output result:

#dict['Name']: Zara
#dict['Age']: 7

If you access the data using keys that are not in the dictionary, it will The output error is as follows:

Copy code The code is as follows:

#!/usr/bin/python

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

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

#Output result of the above example:

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

Copy code The code is 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'];
#Output result of the above example:
#dict[ 'Age']: 8
#dict['School']: DPS School

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

Copy code The code is as follows:

#!/usr/bin/python

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

del dict['Name']; # The delete key is 'Name ''s entries
dict.clear(); # Clear all entries in the dictionary
del dict ; # Delete the dictionary

print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];
#But this will throw an exception because the dictionary no longer exists after using del:

dict[' Age']:
#Traceback (most recent call last):
# File "test.py", line 8, in
# print "dict['Age']: ", dict['Age'];
#TypeError: 'type' object is unsubscriptable

##5. Characteristics of dictionary keysDictionary values ​​can be taken without restrictions Any Python object can be either a standard object or user-defined, but keys cannot.
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 shown in the following example:

Copy code The code is as follows:

#!/usr/bin/python

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

print "dict['Name']: ", dict['Name'];
#The above example output result:
#dict['Name']: Manni

2) key It must be immutable, so it can be used as a number, string or tuple, so a list will not work. The following example:

Copy code The code is as follows:

#!/usr/bin/python

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

print "dict['Name']: ", dict['Name'];
#Output result of the above example:

#Traceback (most recent call last):
# File "test.py", line 3, in
# dict = {['Name']: 'Zara', 'Age': 7};
#TypeError: list objects are unhashable

6. Dictionary built-in functions & methods
Python dictionary contains 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): Return the specified The value of the key. If the value is not in the dictionary, return the default value
5. radiansdict.has_key(key): Return true if the key is in the dictionary dict, otherwise return false
6. radiansdict.items(): Return as a list Traversable (key, value) tuple array
7, radiansdict.keys(): Returns all the keys of a dictionary as a list
8, radiansdict.setdefault(key, default=None): and get() Similar, 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): Update the key/value pair of dictionary dict2 into dict
10 , radiansdict.values(): Returns all the values ​​in the dictionary as a list

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