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Python's dictionaries are kind of hash table type. They work like associative arrays or hashes found in Perl and consist of key-value pairs. A dictionary key can be almost any Python type, but are usually numbers or strings. Values, on the other hand, can be any arbitrary Python object.

Dictionaries are enclosed by curly braces ({ }) and values can be assigned and accessed using square braces ([]).

For example − 
dict = {}
dict['one'] = "This is one"
dict[2] = "This is two"
tinydict = {'name': 'john','code':6734, 'dept': 'sales'}
print dict['one'] # Prints value for 'one' key
print dict[2] # Prints value for 2 key
print tinydict # Prints complete dictionary
print tinydict.keys() # Prints all the keys
print tinydict.values() # Prints all the values
This produce the following result −
This is one
This is two
{'dept': 'sales', 'code': 6734, 'name': 'john'}
['dept', 'code', 'name']
['sales', 6734, 'john']
Dictionaries have no concept of order among elements. It is incorrect to say that the elements are "out of order"; they are simply unordered.


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

When the above code is executed, it produces the following result −
Traceback (most recent call last):
File "", 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 −
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 −
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 −
Traceback (most recent call last):
File "", line 8, in <module>
print "dict['Age']: ", dict['Age'];
TypeError: 'type' object is unsubscriptable



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 −
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:
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 "", 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 −
Function with Description

cmp(dict1, dict2)

Compares elements of both dict.


Gives the total length of the dictionary. This would be equal to the number of items in the dictionary.


Produces a printable string representation of a dictionary


Returns the type of the passed variable. If passed variable is dictionary, then it would return a dictionary type.


Methods with Description


Removes all elements of dictionary dict


Returns a shallow copy of dictionary dict


Create a new dictionary with keys from seq and values set to value.

dict.get(key, default=None)

For key key, returns value or default if key not in dictionary


Returns true if key in dictionary dict, false otherwise


Returns a list of dict's (key, value) tuple pairs


Returns list of dictionary dict's keys

dict.setdefault(key, default=None)

Similar to get(), but will set dict[key]=default if key is not already in dict


Adds dictionary dict2's key-values pairs to dict


Returns list of dictionary dict's values