Add intro to variable file

Author: sydasif

python_notes/03_variable.md

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+## Introduction to Variables
+
+**Variables** are a fundamental concept in programming, serving as containers
+to store data that can be used and manipulated throughout a program. They are
+used in every programming language and are essential for handling data in
+applications.
+
+### Key Characteristics of Variables:
+
+- **Storage**: Variables store data temporarily in memory, allowing the program
+  to access and manipulate it as needed.
+- **Dynamic Typing in Python**: Python variables are *dynamically typed*,
+  meaning that you don't need to declare the type of a variable when you create
+  it. The type is inferred based on the value assigned to it.
+
+### Example of Variables in Python:
+
+```python
+name = "John"
+# Output: 'John'
+
+number = 10
+# Output: 10
+
+# Checking the memory address of variables
+id(name)
+# Output: 1759753155568
+
+id(number)
+# Output: 140720682367704
+```
+
+In this example:
+
+- `name` stores a string value `"John"`.
+- `number` stores an integer value `10`.
+- `id()` function returns the memory address where the variable is stored.
+
+### Updating Variables:
+
+Variables in Python can be reassigned new values, and each time this happens,
+the variable points to a new memory address:
+
+```python
+name = "Ali"
+id(name)
+# Output: 1759753175616
+```
+
+Here, the variable `name` is updated from `"John"` to `"Ali"`, resulting in a
+change in its memory address, as Python creates a new object for the updated
+value.
+
+Variables are crucial for storing and manipulating data in programs. Python’s
+dynamic typing and ease of use make handling variables straightforward,
+allowing for quick development and flexibility.