2.4 Python Variables (BT101CO)

In many older languages, a variable is like a "box" where you drop a value. In Python, it is more accurate to view a variable as a tag or pointer tied to an object in memory.

• When you write x = 10, an object 10 is created in memory, and the label x is pointed at it.
• If you then write y = x, you aren't creating a second "10"; you are simply attaching a second label (y) to the same memory location.

To ensure the interpreter recognizes a variable, it must follow these strict syntax rules:

• Start with a letter (A-Z, a-z) or an underscore (_).
• No Digits at the start: 1variable is invalid; variable1 is fine.
• Case-Sensitivity: Age, age, and AGE are three completely different variables.
• No Keywords: Reserved words like if, while, or class cannot be used as names.
• No Special Characters: Symbols like @, $, and % are prohibited (except for the underscore).

Python uses Dynamic Typing, meaning you don't have to tell the computer what kind of data a variable will hold (like int or string) beforehand. The type is automatically determined by the value assigned to it:

• score = 100 $\rightarrow$ The variable is an Integer.
• score = "High" $\rightarrow$ The same variable now points to a String.

Values are assigned using the = operator (the assignment operator).

• Single Assignment: price = 99.99
• Multiple Assignment (Same Value): a = b = c = 0 (All three point to zero).
• Multiple Assignment (Different Values): name, age = "Ashok", 25 (Assigns "Ashok" to name and 25 to age simultaneously).

A variable's "Scope" determines where in the code it can be accessed:

• Local Variables: Defined inside a function; they only exist while that function is running.
• Global Variables: Defined outside functions; they are accessible throughout the entire script.