Python Basic Tutorial Notes 14: Dictionary (Dictionary)

Python  dictionary (Dictionary)

Dictionaries are another mutable container model and can store objects of any type.

Each key-value  key=>value pair of the dictionary is separated by a colon  : , and each key-value pair is separated by a comma  , , and the entire dictionary is enclosed in curly braces  {}. The format is as follows:

d = {key1 : value1, key2 : value2 }

The keys are generally unique. If the last key-value pair is repeated, the previous one will be replaced, and the value does not need to be unique.

>>>dict = {'a': 1, 'b': 2, 'b': '3'};
>>> dict['b']
'3'
>>> dict
{'a': 1, 'b': '3'}

Values ​​can take any data type, but keys must be immutable like strings, numbers or tuples.

A simple dictionary instance:

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

Dictionaries can also be created like this:

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

access value in dictionary

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

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
 
print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];

result:

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

If you access the data with a key that is not in the dictionary, you will get the following error:

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
 
print "dict['Alice']: ", dict['Alice'];

result:

dict['Alice']: 
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    print "dict['Alice']: ", dict['Alice'];
KeyError: ' Alice '

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:

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'];

result:

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

remove dictionary element

You can delete a single element and also clear the dictionary, and clearing is only one operation.

Display delete a dictionary with the del command, the following example:

dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
 
del dict[ ' Name ' ]; #delete the entry whose key is 'Name' 
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 throws an exception because the dictionary no longer exists after del:

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

Characteristics of dictionary keys

Dictionary values ​​can take any python object without restriction, either standard objects or user-defined, but not keys.

Two important points to remember:

1) Do not allow the same key to appear twice. If the same key is assigned twice during creation , the latter value will be remembered, as in the following example:

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

result:

dict['Name']:  Manni

2) The key must be immutable, so it can be used as a number, string or tuple, so a list cannot be used, as in the following example:

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

result:

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