Home1
Python dictionary data type Fri Sep 24, 2010 2:57 am
Erebus
Achiever
I did not write most of this i took different parts from different sites 
But stil +rep for bring it all together for you guys
A dictionary is mutable and is another container type that can store any number of Python objects, including other container types.
Dictionaries consist of pairs (called items) of keys and their corresponding values.
Python dictionaries are also known as associative arrays or hash tables. The general syntax of a dictionary is as follows:
You can create dictionary in the following way as well:
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 use the familiar square brackets along with the key to obtain its value:
Example:
If we attempt to access a data item with a key which is not part of the dictionary, we get an error as follows:
You can update a dictionary by adding a new entry or item (i.e., a key-value pair), modifying an existing entry, or deleting an existing entry as shown below:
Example:
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:
Example:
This will produce following result. Note an exception raised, this is because after del dict dictionary does not exist any more:
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.
Example:
(b) Keys must be immutable. Which means you can use strings, numbers, or tuples as dictionary keys but something like ['key'] is not allowed.
Example:
This will produce following result. Note an exception raised:
But stil +rep for bring it all together for you guys
A dictionary is mutable and is another container type that can store any number of Python objects, including other container types.
Dictionaries consist of pairs (called items) of keys and their corresponding values.
Python dictionaries are also known as associative arrays or hash tables. The general syntax of a dictionary is as follows:
- Code:
dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}
You can create dictionary in the following way as well:
- Code:
dict1 = { 'abc': 456 };
dict2 = { 'abc': 123, 98.6: 37 };
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 use the familiar square brackets along with the key to obtain its value:
Example:
- Code:
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];
This will produce 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:
- Code:
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Alice']: ", dict['Alice'];
This will produce following result:
dict['Zara']:
Traceback (most recent call last):
File "test.py", line 4, in <module>
print "dict['Alice']: ", dict['Alice'];
KeyError: 'Alice'
You can update a dictionary by adding a new entry or item (i.e., a key-value pair), modifying an existing entry, or deleting an existing entry as shown below:
Example:
- Code:
#!/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'];
This will produce 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:
Example:
- Code:
#!/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 will produce following result. Note an exception raised, this is because after del dict dictionary does not exist any more:
- Code:
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.
Example:
- Code:
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};
print "dict['Name']: ", dict['Name'];
This will produce 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.
Example:
- Code:
#!/usr/bin/python
dict = {['Name']: 'Zara', 'Age': 7};
print "dict['Name']: ", dict['Name'];
This will produce following result. Note an exception raised:
- Code:
Traceback (most recent call last):
File "test.py", line 3, in <module>
dict = {['Name']: 'Zara', 'Age': 7};
TypeError: list objects are unhashable
