What is a Variable?
A variable in Python is a container for storing data values. It acts as a container for data, allowing you to reference and manipulate it throughout your code.In other words it acts like a labeled box where you can store different types of information and retrieve them later.
Example:
Imagine a box labeled "favorite_color". If you put "blue" inside, that box now holds "blue".
Declaring Variables
A variable is created the moment you first assign a value to it. Python variables are dynamically typed, meaning you don’t need to specify their data type. The type is assigned automatically based on the value.
<span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span><span>age</span> <span>=</span> <span>12</span><span>score</span> <span>=</span> <span>95.5</span><span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>age</span> <span>=</span> <span>12</span> <span>score</span> <span>=</span> <span>95.5</span>name = "Vayola" age = 12 score = 95.5
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What happens here?
name holds the text “Vayola“.
age holds the number 12.
score holds a decimal number 95.5.
Variable Naming Rules
Must start with a letter (a-z, A-Z) or an underscore (_).
- Valid: `name`, `_age`, `Name1`- Invalid: `1name`, `@age`- Valid: `name`, `_age`, `Name1` - Invalid: `1name`, `@age`- Valid: `name`, `_age`, `Name1` - Invalid: `1name`, `@age`
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Cannot start with a number.
- Valid: `age1`- Invalid: `1age`- Valid: `age1` - Invalid: `1age`- Valid: `age1` - Invalid: `1age`
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Can only contain alphanumeric characters and underscores (A-Z, a-z, 0-9,_).
- Valid: `user_name`, `age_1`- Invalid: `user-name`, `age#1`- Valid: `user_name`, `age_1` - Invalid: `user-name`, `age#1`- Valid: `user_name`, `age_1` - Invalid: `user-name`, `age#1`
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Case-sensitive: myVar and myvar are different variables.
Cannot be a reserved keyword (e.g., if, else, for, while, etc.).
Examples of Variable Assignments
<span>#</span> <span>Integer</span><span>x</span> <span>=</span> <span>10</span><span>#</span> <span>Float</span><span>y</span> <span>=</span> <span>20.5</span><span>#</span> <span>String</span><span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span><span>#</span> <span>Boolean</span><span>is_valid</span> <span>=</span> <span>True</span><span>#</span> <span>List</span><span>my_list</span> <span>=</span> <span>[</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>,</span> <span>4</span><span>]</span><span>#</span> <span>Tuple</span><span>my_tuple</span> <span>=</span> <span>(</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>)</span><span>#</span> <span>Dictionary</span><span>my_dict</span> <span>=</span> <span>{</span><span>"</span><span>name</span><span>"</span><span>:</span> <span>"</span><span>Vayola</span><span>"</span><span>,</span> <span>"</span><span>age</span><span>"</span><span>:</span> <span>25</span><span>}</span><span>#</span> <span>Set</span><span>my_set</span> <span>=</span> <span>{</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>}</span><span>#</span> <span>Integer</span> <span>x</span> <span>=</span> <span>10</span> <span>#</span> <span>Float</span> <span>y</span> <span>=</span> <span>20.5</span> <span>#</span> <span>String</span> <span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>#</span> <span>Boolean</span> <span>is_valid</span> <span>=</span> <span>True</span> <span>#</span> <span>List</span> <span>my_list</span> <span>=</span> <span>[</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>,</span> <span>4</span><span>]</span> <span>#</span> <span>Tuple</span> <span>my_tuple</span> <span>=</span> <span>(</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>)</span> <span>#</span> <span>Dictionary</span> <span>my_dict</span> <span>=</span> <span>{</span><span>"</span><span>name</span><span>"</span><span>:</span> <span>"</span><span>Vayola</span><span>"</span><span>,</span> <span>"</span><span>age</span><span>"</span><span>:</span> <span>25</span><span>}</span> <span>#</span> <span>Set</span> <span>my_set</span> <span>=</span> <span>{</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>}</span># Integer x = 10 # Float y = 20.5 # String name = "Vayola" # Boolean is_valid = True # List my_list = [1, 2, 3, 4] # Tuple my_tuple = (1, 2, 3) # Dictionary my_dict = {"name": "Vayola", "age": 25} # Set my_set = {1, 2, 3}
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Variable Types
Python supports various data types, and the type of a variable is determined by the value it holds. Common types include:
<span>Integer</span><span>:</span> <span>x</span> <span>=</span> <span>10</span> <span>//Integer</span><span>Float</span><span>:</span> <span>y</span> <span>=</span> <span>20.5</span> <span>//Float</span><span>String</span><span>:</span> <span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>//String</span><span>Boolean</span><span>:</span> <span>is_valid</span> <span>=</span> <span>True</span> <span>//Boolean</span><span>List</span><span>:</span> <span>my_list</span> <span>=</span> <span>[</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>]</span> <span>//List</span><span>Tuple</span><span>:</span> <span>my_tuple</span> <span>=</span> <span>(</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>)</span> <span>//Tuple</span><span>Dictionary</span><span>:</span> <span>my_dict</span> <span>=</span> <span>{</span><span>"</span><span>name</span><span>"</span><span>:</span> <span>"</span><span>Alice</span><span>"</span><span>,</span> <span>"</span><span>age</span><span>"</span><span>:</span> <span>25</span><span>}</span> <span>//Dictionary</span><span>Set</span><span>:</span> <span>my_set</span> <span>=</span> <span>{</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>}</span> <span>//Set</span><span>Integer</span><span>:</span> <span>x</span> <span>=</span> <span>10</span> <span>//Integer</span> <span>Float</span><span>:</span> <span>y</span> <span>=</span> <span>20.5</span> <span>//Float</span> <span>String</span><span>:</span> <span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>//String</span> <span>Boolean</span><span>:</span> <span>is_valid</span> <span>=</span> <span>True</span> <span>//Boolean</span> <span>List</span><span>:</span> <span>my_list</span> <span>=</span> <span>[</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>]</span> <span>//List</span> <span>Tuple</span><span>:</span> <span>my_tuple</span> <span>=</span> <span>(</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>)</span> <span>//Tuple</span> <span>Dictionary</span><span>:</span> <span>my_dict</span> <span>=</span> <span>{</span><span>"</span><span>name</span><span>"</span><span>:</span> <span>"</span><span>Alice</span><span>"</span><span>,</span> <span>"</span><span>age</span><span>"</span><span>:</span> <span>25</span><span>}</span> <span>//Dictionary</span> <span>Set</span><span>:</span> <span>my_set</span> <span>=</span> <span>{</span><span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span><span>}</span> <span>//Set</span>Integer: x = 10 //Integer Float: y = 20.5 //Float String: name = "Vayola" //String Boolean: is_valid = True //Boolean List: my_list = [1, 2, 3] //List Tuple: my_tuple = (1, 2, 3) //Tuple Dictionary: my_dict = {"name": "Alice", "age": 25} //Dictionary Set: my_set = {1, 2, 3} //Set
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You can check the type of a variable using the type() function:
<span>x</span> <span>=</span> <span>10</span><span>print</span><span>(</span><span>type</span><span>(</span><span>x</span><span>))</span> <span>#</span> <span>Output</span><span>:</span> <span><</span><span>class</span> <span>'</span><span>int</span><span>'</span><span>> </span><span>x</span> <span>=</span> <span>10</span> <span>print</span><span>(</span><span>type</span><span>(</span><span>x</span><span>))</span> <span>#</span> <span>Output</span><span>:</span> <span><</span><span>class</span> <span>'</span><span>int</span><span>'</span><span>> </span>x = 10 print(type(x)) # Output: <class 'int'>
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Changing a Variable’s Value
Variables are not permanent! You can change them anytime.
<span>color</span> <span>=</span> <span>"</span><span>red</span><span>"</span><span>print</span><span>(</span><span>color</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>red</span><span>color</span> <span>=</span> <span>"</span><span>blue</span><span>"</span><span>print</span><span>(</span><span>color</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>blue</span><span>color</span> <span>=</span> <span>"</span><span>red</span><span>"</span> <span>print</span><span>(</span><span>color</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>red</span> <span>color</span> <span>=</span> <span>"</span><span>blue</span><span>"</span> <span>print</span><span>(</span><span>color</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>blue</span>color = "red" print(color) # Output: red color = "blue" print(color) # Output: blue
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First, color is “red“. Then, we change it to “blue“.
Multiple Assignments
Python allows you to assign values to multiple variables in a single line:
<span>a</span><span>,</span> <span>b</span><span>,</span> <span>c</span> <span>=</span> <span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span> <span>#</span> <span>a</span> <span>=</span> <span>1</span><span>,</span> <span>b</span> <span>=</span> <span>2</span><span>,</span> <span>c</span> <span>=</span> <span>3</span><span>a</span><span>,</span> <span>b</span><span>,</span> <span>c</span> <span>=</span> <span>1</span><span>,</span> <span>2</span><span>,</span> <span>3</span> <span>#</span> <span>a</span> <span>=</span> <span>1</span><span>,</span> <span>b</span> <span>=</span> <span>2</span><span>,</span> <span>c</span> <span>=</span> <span>3</span>a, b, c = 1, 2, 3 # a = 1, b = 2, c = 3
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Can also assign like this too
<span>x</span> <span>=</span> <span>y</span> <span>=</span> <span>z</span> <span>=</span> <span>10</span> <span>#</span> <span>x</span><span>,</span> <span>y</span><span>,</span> <span>and</span> <span>z</span> <span>all</span> <span>have</span> <span>the</span> <span>value</span> <span>10</span><span>x</span> <span>=</span> <span>y</span> <span>=</span> <span>z</span> <span>=</span> <span>10</span> <span>#</span> <span>x</span><span>,</span> <span>y</span><span>,</span> <span>and</span> <span>z</span> <span>all</span> <span>have</span> <span>the</span> <span>value</span> <span>10</span>x = y = z = 10 # x, y, and z all have the value 10
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Swapping Variables
Python makes it easy to swap values between variables without needing a temporary variable
<span>x</span><span>,</span> <span>y</span> <span>=</span> <span>10</span><span>,</span> <span>20</span><span>x</span><span>,</span> <span>y</span> <span>=</span> <span>y</span><span>,</span> <span>x</span> <span>#</span> <span>Now</span> <span>x</span> <span>=</span> <span>20</span> <span>and</span> <span>y</span> <span>=</span> <span>10</span><span>x</span><span>,</span> <span>y</span> <span>=</span> <span>10</span><span>,</span> <span>20</span> <span>x</span><span>,</span> <span>y</span> <span>=</span> <span>y</span><span>,</span> <span>x</span> <span>#</span> <span>Now</span> <span>x</span> <span>=</span> <span>20</span> <span>and</span> <span>y</span> <span>=</span> <span>10</span>x, y = 10, 20 x, y = y, x # Now x = 20 and y = 10
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Deleting Variables
You can delete a variable using the del keyword. Once deleted, the variable no longer exists in memory.
<span>x</span> <span>=</span> <span>10</span><span>del</span> <span>x</span> <span>#</span> <span>x</span> <span>is</span> <span>deleted</span><span>#</span> <span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>This</span> <span>will</span> <span>raise</span> <span>a</span> <span>NameError</span><span>x</span> <span>=</span> <span>10</span> <span>del</span> <span>x</span> <span>#</span> <span>x</span> <span>is</span> <span>deleted</span> <span>#</span> <span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>This</span> <span>will</span> <span>raise</span> <span>a</span> <span>NameError</span>x = 10 del x # x is deleted # print(x) # This will raise a NameError
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Combining Variables
You can combine variables in Python!
String Concatenation
<span>first_name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span><span>last_name</span> <span>=</span> <span>"</span><span>Pradeep</span><span>"</span><span>full_name</span> <span>=</span> <span>first_name</span> <span>+</span> <span>"</span><span> </span><span>"</span> <span>+</span> <span>last_name</span> <span>#</span> <span>Combine</span> <span>with</span> <span>a</span> <span>space</span><span>print</span><span>(</span><span>full_name</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>Vayola</span> <span>Pradeep</span><span>first_name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>last_name</span> <span>=</span> <span>"</span><span>Pradeep</span><span>"</span> <span>full_name</span> <span>=</span> <span>first_name</span> <span>+</span> <span>"</span><span> </span><span>"</span> <span>+</span> <span>last_name</span> <span>#</span> <span>Combine</span> <span>with</span> <span>a</span> <span>space</span> <span>print</span><span>(</span><span>full_name</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>Vayola</span> <span>Pradeep</span>first_name = "Vayola" last_name = "Pradeep" full_name = first_name + " " + last_name # Combine with a space print(full_name) # Output: Vayola Pradeep
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*Using f-strings
<span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span><span>age</span> <span>=</span> <span>12</span><span>print</span><span>(</span><span>f</span><span>"</span><span>My name is {name} and I am {age} years old.</span><span>"</span><span>)</span><span>#</span> <span>Output</span><span>:</span> <span>My</span> <span>name</span> <span>is</span> <span>Vayola</span> <span>and</span> <span>I</span> <span>am</span> <span>12</span> <span>years</span> <span>old</span><span>.</span><span>name</span> <span>=</span> <span>"</span><span>Vayola</span><span>"</span> <span>age</span> <span>=</span> <span>12</span> <span>print</span><span>(</span><span>f</span><span>"</span><span>My name is {name} and I am {age} years old.</span><span>"</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>My</span> <span>name</span> <span>is</span> <span>Vayola</span> <span>and</span> <span>I</span> <span>am</span> <span>12</span> <span>years</span> <span>old</span><span>.</span>name = "Vayola" age = 12 print(f"My name is {name} and I am {age} years old.") # Output: My name is Vayola and I am 12 years old.
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Variables in Math Operations
Python allows you to perform calculations using variables.
<span>a</span> <span>=</span> <span>10</span><span>b</span> <span>=</span> <span>5</span><span>sum</span> <span>=</span> <span>a</span> <span>+</span> <span>b</span> <span>#</span> <span>Addition</span><span>difference</span> <span>=</span> <span>a</span> <span>-</span> <span>b</span> <span>#</span> <span>Subtraction</span><span>product</span> <span>=</span> <span>a</span> <span>*</span> <span>b</span> <span>#</span> <span>Multiplication</span><span>quotient</span> <span>=</span> <span>a</span> <span>/</span> <span>b</span> <span>#</span> <span>Division</span><span>print</span><span>(</span><span>sum</span><span>,</span> <span>difference</span><span>,</span> <span>product</span><span>,</span> <span>quotient</span><span>)</span><span>a</span> <span>=</span> <span>10</span> <span>b</span> <span>=</span> <span>5</span> <span>sum</span> <span>=</span> <span>a</span> <span>+</span> <span>b</span> <span>#</span> <span>Addition</span> <span>difference</span> <span>=</span> <span>a</span> <span>-</span> <span>b</span> <span>#</span> <span>Subtraction</span> <span>product</span> <span>=</span> <span>a</span> <span>*</span> <span>b</span> <span>#</span> <span>Multiplication</span> <span>quotient</span> <span>=</span> <span>a</span> <span>/</span> <span>b</span> <span>#</span> <span>Division</span> <span>print</span><span>(</span><span>sum</span><span>,</span> <span>difference</span><span>,</span> <span>product</span><span>,</span> <span>quotient</span><span>)</span>a = 10 b = 5 sum = a + b # Addition difference = a - b # Subtraction product = a * b # Multiplication quotient = a / b # Division print(sum, difference, product, quotient)
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Special Variable: None
None represents an empty or undefined value.
<span>result</span> <span>=</span> <span>None</span><span>print</span><span>(</span><span>result</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>None</span><span>result</span> <span>=</span> <span>100</span> <span>#</span> <span>Now</span> <span>it</span> <span>holds</span> <span>a</span> <span>value</span><span>print</span><span>(</span><span>result</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>100</span><span>result</span> <span>=</span> <span>None</span> <span>print</span><span>(</span><span>result</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>None</span> <span>result</span> <span>=</span> <span>100</span> <span>#</span> <span>Now</span> <span>it</span> <span>holds</span> <span>a</span> <span>value</span> <span>print</span><span>(</span><span>result</span><span>)</span> <span>#</span> <span>Output</span><span>:</span> <span>100</span>result = None print(result) # Output: None result = 100 # Now it holds a value print(result) # Output: 100
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Constants in Python
A constant is a variable that should not change.
Python doesn’t have built-in constants, but we use UPPERCASE names by convention.
<span>PI</span> <span>=</span> <span>3.14159</span><span>GRAVITY</span> <span>=</span> <span>9.8</span><span>PI</span> <span>=</span> <span>3.14159</span> <span>GRAVITY</span> <span>=</span> <span>9.8</span>PI = 3.14159 GRAVITY = 9.8
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Using Variables in a Simple Program
Let’s create a program that asks for user input and prints a message.
<span>name</span> <span>=</span> <span>input</span><span>(</span><span>"</span><span>Enter your name: </span><span>"</span><span>)</span><span>age</span> <span>=</span> <span>input</span><span>(</span><span>"</span><span>Enter your age: </span><span>"</span><span>)</span><span>print</span><span>(</span><span>f</span><span>"</span><span>Hello {name}! You are {age} years old.</span><span>"</span><span>)</span><span>name</span> <span>=</span> <span>input</span><span>(</span><span>"</span><span>Enter your name: </span><span>"</span><span>)</span> <span>age</span> <span>=</span> <span>input</span><span>(</span><span>"</span><span>Enter your age: </span><span>"</span><span>)</span> <span>print</span><span>(</span><span>f</span><span>"</span><span>Hello {name}! You are {age} years old.</span><span>"</span><span>)</span>name = input("Enter your name: ") age = input("Enter your age: ") print(f"Hello {name}! You are {age} years old.")
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What Happens?
-
input()takes user input. - The input is stored in variables.
- The program prints a message using
f-string.
Scope of Variables
The scope of a variable determines where it can be accessed in a program. There are two main types of variable scope in Python:
Local Variables: Defined inside a function and accessible only within that function.
Global Variables: Defined outside functions and accessible throughout the program.
<span>x</span> <span>=</span> <span>10</span> <span>#</span> <span>Global</span> <span>variable</span><span>def</span> <span>my_function</span><span>():</span><span>y</span> <span>=</span> <span>20</span> <span>#</span> <span>Local</span> <span>variable</span><span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>Access</span> <span>global</span> <span>variable</span><span>print</span><span>(</span><span>y</span><span>)</span><span>my_function</span><span>()</span><span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>Access</span> <span>global</span> <span>variable</span><span>#</span> <span>print</span><span>(</span><span>y</span><span>)</span> <span>#</span> <span>This</span> <span>will</span> <span>raise</span> <span>an</span> <span>error</span> <span>because</span> <span>y</span> <span>is</span> <span>local</span> <span>to</span> <span>my_function</span><span>``` </span><span>x</span> <span>=</span> <span>10</span> <span>#</span> <span>Global</span> <span>variable</span> <span>def</span> <span>my_function</span><span>():</span> <span>y</span> <span>=</span> <span>20</span> <span>#</span> <span>Local</span> <span>variable</span> <span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>Access</span> <span>global</span> <span>variable</span> <span>print</span><span>(</span><span>y</span><span>)</span> <span>my_function</span><span>()</span> <span>print</span><span>(</span><span>x</span><span>)</span> <span>#</span> <span>Access</span> <span>global</span> <span>variable</span> <span>#</span> <span>print</span><span>(</span><span>y</span><span>)</span> <span>#</span> <span>This</span> <span>will</span> <span>raise</span> <span>an</span> <span>error</span> <span>because</span> <span>y</span> <span>is</span> <span>local</span> <span>to</span> <span>my_function</span><span>``` </span>x = 10 # Global variable def my_function(): y = 20 # Local variable print(x) # Access global variable print(y) my_function() print(x) # Access global variable # print(y) # This will raise an error because y is local to my_function```
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原文链接：What Are Variables in Python? – The Ultimate Beginner’s Guide!
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