[Python Tutorial] Python Dictionary (Dictionary)

Python Dictionary

Dictionary is another mutable container model and can store any type of object, such as other container models.

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 };

Each key and value are separated by a colon (:), each pair is separated by a comma, and each pair is separated by a comma. 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.

Access the values ​​in the dictionary

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

#!/usr/bin/python
 
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
 
print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];

The output of the above example:

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

If you use a key that is not in the dictionary to access data, the 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 <module>
    print "dict[&#39;Alice&#39;]: ", dict[&#39;Alice&#39;];
KeyError: &#39;Alice&#39;

Modify the dictionary

The method of adding new content to the dictionary is 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'];

Output results of the above examples:

dict['Age']:  8
dict['School']:  DPS School

Delete dictionary elements

can delete a single element or clear the dictionary. Clearing only requires one operation.

Display the del command to 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'];

But this will cause an exception, because the dictionary no longer exists after using del:

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

Note: del The () method will also be discussed later.

Delete dictionary elements

Characteristics of dictionary keys

Dictionary values ​​can take any python object without restrictions, either standard objects or 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. The following example

#!/usr/bin/python
 
dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};
 
print "dict['Name']: ", dict['Name'];

The output result of the above example:

dict['Name']:  Manni

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

#!/usr/bin/python
 
dict = {['Name']: 'Zara', 'Age': 7};
 
print "dict['Name']: ", dict['Name'];

The output result of the above example:

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

Dictionary built-in functions & methods

Python dictionary Contains the following built-in functions:

Serial number

Function and description

1 cmp(dict1, dict2)
Compares 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, it returns the dictionary type.

Python dictionary contains the following built-in functions:

Serial number

Function and description

1 radiansdict .clear()
Delete all elements in the dictionary

2 radiansdict.copy()
Return a shallow copy of the dictionary

3 radiansdict.fromkeys()
Create one The new dictionary uses 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, returns the default value

5 radiansdict.has_key(key)
If the key is Returns true in the dictionary dict, otherwise returns false

6 radiansdict.items()
Returns a traversable (key, value) tuple array as a list

7 radiansdict.keys()
Returns all 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, it will be added key and set the value to default

9 radiansdict.update(dict2)
Update the key/value pair of dictionary dict2 into dict

10 radiansdict.values()
Return all values ​​in the dictionary as a list

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