Python Dictionary
Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.
Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.
Accessing Values in Dictionary:
To access dictionary elements, you can use the familiar square brackets along with the key to obtain its value. Following is a simple example −
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Name']: ", dict['Name']
print "dict['Age']: ", dict['Age']
When the above code is executed, it produces the following result −
dict['Name']: Zara
dict['Age']: 7
If we attempt to access a data item with a key, which is not part of the dictionary, we get an error as follows −
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Alice']: ", dict['Alice']
When the above code is executed, it produces the following result −
dict['Zara']:
Traceback (most recent call last):
File "test.py", line 4, in <module>
print "dict['Alice']: ", dict['Alice'];
KeyError: 'Alice'
Updating Dictionary
You can update a dictionary by adding a new entry or a key-value pair, modifying an existing entry, or deleting an existing entry as shown below in the simple example −
#!/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']
When the above code is executed, it produces the following result −
dict['Age']: 8
dict['School']: DPS School
Delete Dictionary Elements
You can either remove individual dictionary elements or clear the entire contents of a dictionary. You can also delete entire dictionary in a single operation.
To explicitly remove an entire dictionary, just use the del statement. Following is a simple example −
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
del dict['Name']; # remove entry with key 'Name'
dict.clear(); # remove all entries in dict
del dict ; # delete entire dictionary
print "dict['Age']: ", dict['Age']
print "dict['School']: ", dict['School']
This produces the following result. Note that an exception is raised because after del dict dictionary does not exist any more −
dict['Age']:
Traceback (most recent call last):
File "test.py", line 8, in <module>
print "dict['Age']: ", dict['Age'];
TypeError: 'type' object is unsubscriptable
Note: del() method is discussed in subsequent section.
Properties of Dictionary Keys
Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.
There are two important points to remember about dictionary keys −
(a) More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example −
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};
print "dict['Name']: ", dict['Name']
When the above code is executed, it produces the following result −
dict['Name']: Manni
(b) Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example:
#!/usr/bin/python
dict = {['Name']: 'Zara', 'Age': 7};
print "dict['Name']: ", dict['Name']
When the above code is executed, it produces the following result −
Traceback (most recent call last):
File "test.py", line 3, in <module>
dict = {['Name']: 'Zara', 'Age': 7};
TypeError: list objects are unhashable
Built-in Dictionary Functions & Methods −
Python includes the following dictionary functions −
SN |
Function with Description |
1 |
cmp(dict1, dict2) Compares elements of both dict. |
2 |
len(dict) Gives the total length of the dictionary. This would be equal to the number of items in the dictionary. |
3 |
str(dict) Produces a printable string representation of a dictionary |
4 |
type(variable) Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type. |
Python includes following dictionary methods −
SN |
Methods with Description |
1 |
dict.clear() Removes all elements of dictionary dict |
2 |
dict.copy() Returns a shallow copy of dictionary dict |
3 |
dict.fromkeys() Create a new dictionary with keys from seq and values set to value. |
4 |
dict.get(key, default=None) For key key, returns value or default if key not in dictionary |
5 |
dict.has_key(key) Returns true if key in dictionary dict, false otherwise |
6 |
dict.items() Returns a list of dict's (key, value) tuple pairs |
7 |
dict.keys() Returns list of dictionary dict's keys |
8 |
dict.setdefault(key, default=None) Similar to get(), but will set dict[key]=default if key is not already in dict |
9 |
dict.update(dict2) Adds dictionary dict2's key-values pairs to dict |
10 |
dict.values() Returns list of dictionary dict's values |