> ## Documentation Index
> Fetch the complete documentation index at: https://rajanand.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Python Variables

# Python Variables

## 1. **What is a Variable?**

* A **variable** is a named location in memory used to store data.
* It acts as a container for storing values that can be used and manipulated throughout a program.

## 2. **Variable Naming Rules**

* **Valid Names**:
  * Must start with a letter (a-z, A-Z) or an underscore (`_`).
  * Can contain letters, numbers (0-9), and underscores.
  * Case-sensitive (`age`, `Age`, and `AGE` are different variables).
* **Invalid Names**:
  * Cannot start with a number.
  * Cannot contain spaces or special characters (e.g., `@`, `#`, `$`).
  * Cannot be a Python keyword (e.g., `if`, `else`, `for`, `while`).

**Examples**:

```python theme={"system"}
valid_variable = 10
_valid_name = "Python"
invalid-variable = 5  # Error: hyphens are not allowed
2invalid = 20         # Error: cannot start with a number
```

## 3. **Assigning Values to Variables**

* Use the assignment operator `=` to assign a value to a variable.
* Python is dynamically typed, so you don’t need to declare the type of a variable explicitly.

**Examples**:

```python theme={"system"}
x = 10          # Integer
name = "Alice"  # String
is_valid = True # Boolean
pi = 3.14       # Float
```

## 4. **Multiple Assignments**

* You can assign multiple variables in a single line.

**Examples**:

```python theme={"system"}
a, b, c = 1, 2, 3  # a=1, b=2, c=3
x = y = z = 10     # x=10, y=10, z=10
```

## 5. **Variable Types**

* Python variables can hold data of different types:
  * **Integer**: Whole numbers (e.g., `10`, `-5`)
  * **Float**: Decimal numbers (e.g., `3.14`, `-0.001`)
  * **String**: Text (e.g., `"Hello"`, `'Python'`)
  * **Boolean**: `True` or `False`
  * **List**: Ordered, mutable collection (e.g., `[1, 2, 3]`)
  * **Tuple**: Ordered, immutable collection (e.g., `(1, 2, 3)`)
  * **Dictionary**: Key-value pairs (e.g., `{"name": "Alice", "age": 25}`)
  * **Set**: Unordered, unique elements (e.g., `{1, 2, 3}`)

**Examples**:

```python theme={"system"}
age = 25            # Integer
price = 19.99       # Float
name = "John"       # String
is_student = True   # Boolean
numbers = [1, 2, 3] # List
```

## 6. **Dynamic Typing**

* Python is dynamically typed, meaning the type of a variable is determined at runtime and can change.

**Example**:

```python theme={"system"}
x = 10      # x is an integer
x = "Hello" # x is now a string
```

## 7. **Type Checking**

* Use the `type()` function to check the type of a variable.

**Example**:

```python theme={"system"}
x = 10
print(type(x))  # Output: <class 'int'>
```

## 8. **Variable Scope**

* **Local Scope**: Variables defined inside a function are local to that function.
* **Global Scope**: Variables defined outside a function are global and can be accessed anywhere.

**Example**:

```python theme={"system"}
x = 10  # Global variable

def my_function():
    y = 5  # Local variable
    print(x)  # Access global variable
    print(y)

my_function()
print(y)  # Error: y is not defined globally
```

## 9. **Deleting Variables**

* Use the `del` keyword to delete a variable.

**Example**:

```python theme={"system"}
x = 10
del x
print(x)  # Error: x is not defined
```

## 10. **Constants**

* Python doesn’t have built-in support for constants, but by convention, use uppercase names for constants.

**Example**:

```python theme={"system"}
PI = 3.14159  # Convention for constants
```

## 11. **Best Practices**

* Use descriptive variable names (e.g., `user_age` instead of `ua`).
* Follow naming conventions (e.g., `snake_case` for variable names).
* Avoid using single-letter variable names unless in loops or mathematical contexts.