Python Basics: Modes, Variables, Data Types and Operators

Two ways to run Python: interactive and script mode

Interactive mode: you type one statement after the >>> prompt and Python answers at once. Good for testing small things. Nothing is saved.

Script mode: you write many lines in a file ending in .py, save it and run it (F5 in IDLE). Good for real programs; you can run it again and again.

Structure of a program and indentation

A Python program is a list of statements, run from top to bottom. A comment starts with #; Python ignores it, but it helps humans read the code.

A line that ends with a colon : (like if, for, while) starts a block. Every line of the block must be pushed right by the same amount, usually 4 spaces. This is indentation. Python uses it instead of brackets. Wrong indentation gives an IndentationError.

if marks >= 33:
    print("Pass")
else:
    print("Try again")

Identifiers, keywords, constants and variables

Keywords are reserved words with a fixed meaning: if, else, elif, for, while, True, False, None, and, or, not, in, is, def, import, etc. You cannot use them as names.

An identifier is a name you give to a variable or function. Rules: letters, digits and _ only; cannot start with a digit; no spaces or symbols; not a keyword; case-sensitive (Marks and marks are different). Valid: total_marks, _x, mark2. Invalid: 2mark, total marks, if.

A variable is a name that refers to a value in memory: age = 16. Python works out its type from the value; no declaration is needed. = means "tie this name to this value", not "equals".

A constant is a value that should not change. Python has no true constants; by habit we write the name in capitals: PI = 3.14. Fixed values written directly (10, "hi", 3.5) are called literals.

Data types

• Numbers: int (whole: 7, −3), float (decimal: 7.5), complex (3+4j).
• bool: True or False.
• str (string): text in quotes, "Ravi".
• list: ordered, changeable, [1, 2, 3].
• tuple: ordered, not changeable, (1, 2, 3).
• dict: key-value pairs, {"roll": 5}.
• None: means "no value".

Use type(x) to see a type.

Mutable and immutable types

Mutable: can be changed in place, same object. list, dict (and set).

Immutable: cannot be changed in place. int, float, bool, str, tuple. When you "change" them, Python makes a new value and moves the name tag. Example: s = "cat"; s[0] = "b" gives a TypeError, but L = [1,2]; L[0] = 9 works.

Operators and precedence

• Arithmetic: + - * / (always float), // (floor division), % (remainder), ** (power).
• Relational: == != < > <= >=, give True/False.
• Logical: not, and, or.
• Assignment: = += -= *= /= //= %= **=.
• Identity: is, is not (same object?).
• Membership: in, not in ("a" in "cat" → True).

An expression is a mix of values, variables and operators that gives one value, like 2 + 3 * 4.

Precedence (high to low)

1. ()
2. ** (right to left)
3. unary +x -x
4. * / // %
5. + -
6. relational == != < > <= >=, is, in
7. not
8. and
9. or

Same level: left to right (except **).

Input and output

input("prompt") shows a message, waits for the user, and always returns a string.

print(a, b, sep=" ", end="\n") shows values. sep is put between values, end at the end.

name = input("Name: ")
print("Hello", name, sep=", ", end="!\n")

Type conversion

Explicit (you ask for it): int("25") → 25, float("2.5") → 2.5, str(10) → "10", int(7.9) → 7 (cuts the decimal). int("abc") gives a ValueError.

Implicit (Python does it): 3 + 2.0 → 5.0 (int becomes float so no data is lost).

Since input() gives a string, write age = int(input("Age: ")) to do maths with it.

Debugging: three kinds of errors

• Syntax error: breaking the grammar rules. Python will not start. Example: print("hi" (missing bracket), missing colon, wrong indentation.
• Runtime error (exception): the grammar is fine but something fails while running. Example: 10 / 0 (ZeroDivisionError), int("abc") (ValueError), using a name not defined (NameError).
• Logical error: the program runs but gives a wrong answer. Example: avg = a + b / 2 instead of (a + b) / 2. Hardest to find, because there is no message.

Debugging means finding and fixing errors: read the error message and line number, print values in between, and test with small known inputs.

Try it: predict before you run

Open Python (IDLE or any online Python). Before pressing Enter, write down your guess for each: 7 // 2, 7 % 2, 2 ** 3 ** 2, "5" + "2", int("5") + 2, 10 / 5. Then run each. Every wrong guess: open step 6 in the 3D and watch that expression reduce.

Key formulas and definitions

• a // b = floor of a ÷ b; a % b = remainder; a == (a // b) * b + a % b
• / always returns float: 10 / 5 = 2.0
• Precedence: () > ** > unary > * / // % > + - > relational > not > and > or
• input() returns str; use int(), float() to convert

Worked examples

Common mistakes

• Forgetting that input() gives a string, so "4" + "5" = "45".
• Thinking 2 ** 3 ** 2 = 64. Power goes right to left: 512.
• Using = (assign) when you mean == (compare).
• Mixing tabs and spaces or forgetting to indent after a colon.