- Python Basics
- Python - Home
- Python - Overview
- Python - History
- Python - Features
- Python vs C++
- Python - Hello World Program
- Python - Application Areas
- Python - Interpreter
- Python - Environment Setup
- Python - Virtual Environment
- Python - Basic Syntax
- Python - Variables
- Python - Data Types
- Python - Type Casting
- Python - Unicode System
- Python - Literals
- Python - Operators
- Python - Arithmetic Operators
- Python - Comparison Operators
- Python - Assignment Operators
- Python - Logical Operators
- Python - Bitwise Operators
- Python - Membership Operators
- Python - Identity Operators
- Python - Operator Precedence
- Python - Comments
- Python - User Input
- Python - Numbers
- Python - Booleans
- Python Control Statements
- Python - Control Flow
- Python - Decision Making
- Python - If Statement
- Python - If else
- Python - Nested If
- Python - Match-Case Statement
- Python - Loops
- Python - for Loops
- Python - for-else Loops
- Python - While Loops
- Python - break Statement
- Python - continue Statement
- Python - pass Statement
- Python - Nested Loops
- Python Functions & Modules
- Python - Functions
- Python - Default Arguments
- Python - Keyword Arguments
- Python - Keyword-Only Arguments
- Python - Positional Arguments
- Python - Positional-Only Arguments
- Python - Arbitrary Arguments
- Python - Variables Scope
- Python - Function Annotations
- Python - Modules
- Python - Built in Functions
- Python Strings
- Python - Strings
- Python - Slicing Strings
- Python - Modify Strings
- Python - String Concatenation
- Python - String Formatting
- Python - Escape Characters
- Python - String Methods
- Python - String Exercises
- Python Lists
- Python - Lists
- Python - Access List Items
- Python - Change List Items
- Python - Add List Items
- Python - Remove List Items
- Python - Loop Lists
- Python - List Comprehension
- Python - Sort Lists
- Python - Copy Lists
- Python - Join Lists
- Python - List Methods
- Python - List Exercises
- Python Tuples
- Python - Tuples
- Python - Access Tuple Items
- Python - Update Tuples
- Python - Unpack Tuples
- Python - Loop Tuples
- Python - Join Tuples
- Python - Tuple Methods
- Python - Tuple Exercises
- Python Sets
- Python - Sets
- Python - Access Set Items
- Python - Add Set Items
- Python - Remove Set Items
- Python - Loop Sets
- Python - Join Sets
- Python - Copy Sets
- Python - Set Operators
- Python - Set Methods
- Python - Set Exercises
- Python Dictionaries
- Python - Dictionaries
- Python - Access Dictionary Items
- Python - Change Dictionary Items
- Python - Add Dictionary Items
- Python - Remove Dictionary Items
- Python - Dictionary View Objects
- Python - Loop Dictionaries
- Python - Copy Dictionaries
- Python - Nested Dictionaries
- Python - Dictionary Methods
- Python - Dictionary Exercises
- Python Arrays
- Python - Arrays
- Python - Access Array Items
- Python - Add Array Items
- Python - Remove Array Items
- Python - Loop Arrays
- Python - Copy Arrays
- Python - Reverse Arrays
- Python - Sort Arrays
- Python - Join Arrays
- Python - Array Methods
- Python - Array Exercises
- Python File Handling
- Python - File Handling
- Python - Write to File
- Python - Read Files
- Python - Renaming and Deleting Files
- Python - Directories
- Python - File Methods
- Python - OS File/Directory Methods
- Object Oriented Programming
- Python - OOPs Concepts
- Python - Object & Classes
- Python - Class Attributes
- Python - Class Methods
- Python - Static Methods
- Python - Constructors
- Python - Access Modifiers
- Python - Inheritance
- Python - Polymorphism
- Python - Method Overriding
- Python - Method Overloading
- Python - Dynamic Binding
- Python - Dynamic Typing
- Python - Abstraction
- Python - Encapsulation
- Python - Interfaces
- Python - Packages
- Python - Inner Classes
- Python - Anonymous Class and Objects
- Python - Singleton Class
- Python - Wrapper Classes
- Python - Enums
- Python - Reflection
- Python Errors & Exceptions
- Python - Syntax Errors
- Python - Exceptions
- Python - try-except Block
- Python - try-finally Block
- Python - Raising Exceptions
- Python - Exception Chaining
- Python - Nested try Block
- Python - User-defined Exception
- Python - Logging
- Python - Assertions
- Python - Built-in Exceptions
- Python Multithreading
- Python - Multithreading
- Python - Thread Life Cycle
- Python - Creating a Thread
- Python - Starting a Thread
- Python - Joining Threads
- Python - Naming Thread
- Python - Thread Scheduling
- Python - Thread Pools
- Python - Main Thread
- Python - Thread Priority
- Python - Daemon Threads
- Python - Synchronizing Threads
- Python Synchronization
- Python - Inter-thread Communication
- Python - Thread Deadlock
- Python - Interrupting a Thread
- Python Networking
- Python - Networking
- Python - Socket Programming
- Python - URL Processing
- Python - Generics
- Python Libraries
- NumPy Tutorial
- Pandas Tutorial
- SciPy Tutorial
- Matplotlib Tutorial
- Django Tutorial
- OpenCV Tutorial
- Python Miscellenous
- Python - Date & Time
- Python - Maths
- Python - Iterators
- Python - Generators
- Python - Closures
- Python - Decorators
- Python - Recursion
- Python - Reg Expressions
- Python - PIP
- Python - Database Access
- Python - Weak References
- Python - Serialization
- Python - Templating
- Python - Output Formatting
- Python - Performance Measurement
- Python - Data Compression
- Python - CGI Programming
- Python - XML Processing
- Python - GUI Programming
- Python - Command-Line Arguments
- Python - Docstrings
- Python - JSON
- Python - Sending Email
- Python - Further Extensions
- Python - Tools/Utilities
- Python - GUIs
- Python Useful Resources
- Python Compiler
- NumPy Compiler
- Matplotlib Compiler
- SciPy Compiler
- Python - Programming Examples
- Python - Quick Guide
- Python - Useful Resources
- Python - Discussion
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['Alice']: 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: unhashable type: 'list'
Built-in Dictionary Functions & Methods
Python includes the following dictionary functions −
Sr.No. | 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 −
Sr.No. | 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 |