Data Types, Operators and Errors in Python

Python data types

• Numbers: int (whole: 25, −3), float (decimal: 3.14, 2e3), complex (3 + 4j).
• bool: True or False (a kind of int: True = 1, False = 0).
• None: means 'no value'; type NoneType.
• Sequences (ordered, indexed): str 'hello', list [1, 'a', 2.5], tuple (1, 2, 3).
• Mapping: dict {'name': 'Riya', 'age': 16} of key: value pairs.

Use type(x) to see a value's type.

Mutable and immutable types

Mutable: the value can be changed in place without making a new object: list, dict (and set).

Immutable: cannot be changed; 'changing' makes a new object with a new id: int, float, bool, complex, str, tuple.

L = [1, 2, 3]; L[0] = 5   # OK, same id
s = 'cat';  s[0] = 'b'   # TypeError

Operators in Python

• Arithmetic: + − * / (true division, gives float) // (floor division) % (remainder) ** (power). 7/2 = 3.5, 7//2 = 3, 7%2 = 1, 2**3 = 8, −7//2 = −4.
• Relational: < > <= >= == != give True/False.
• Logical: and, or, not. 5 > 3 and 2 > 4 → False.
• Assignment: = and augmented += −= *= /= //= %= **=. x += 2 means x = x + 2.
• Identity: is, is not — do two names refer to the same object (same id)?
• Membership: in, not in — is a value inside a sequence? 'a' in 'cat' → True.

Operator precedence

Highest to lowest:

1. () brackets
2. ** (right to left: 2**3**2 = 2**9 = 512)
3. unary +x −x
4. * / // %
5. + −
6. relational < > == != …, is, in
7. not
8. and
9. or

Same level: left to right (except **). Example: 2 + 3 * 4 ** 2 = 2 + 3 * 16 = 2 + 48 = 50.

Type conversion

Implicit (automatic): Python converts to avoid data loss, e.g. 5 + 2.0 = 7.0 (int → float).

Explicit (type casting): you convert with functions: int('25') → 25, float('3.5') → 3.5, str(10) → '10', int(7.9) → 7 (cuts the decimal), bool(0) → False. int('abc') gives a ValueError.

Input and output

input(prompt) shows the prompt and returns what the user types as a string. Convert for numbers: n = int(input('Enter n: ')).

print(values, sep=' ', end='\n') shows values separated by sep and ends with end. print(1, 2, 3, sep='-') → 1-2-3; print('Hi', end='!') → Hi! with no new line.

Errors: syntax, logical and runtime

• Syntax error: the code breaks Python's grammar, so it does not run at all: missing colon, unclosed bracket, prnt('hi') is a NameError at run time but print('hi' is a syntax error.
• Runtime error (exception): the code is valid but crashes while running: division by zero (ZeroDivisionError), int('abc') (ValueError), using an undefined name (NameError), wrong index (IndexError).
• Logical error: the program runs but gives the wrong answer, e.g. average = a + b / 2 instead of (a + b) / 2. Hardest to find; needs testing.

Key formulas and definitions

• / → float; // → floor; % → remainder; ** → power
• Precedence: () > ** > unary > * / // % > + − > relational > not > and > or
• Mutable: list, dict · Immutable: int, float, bool, str, tuple
• input() returns str → use int()/float()
• Errors: syntax, runtime (exception), logical

Worked examples

Common mistakes

• Expecting 7 / 2 to give 3. / always gives a float (3.5); use // for 3.
• Forgetting to convert input(): input() + 5 gives a TypeError.
• Using is to compare values. Use == for values; is checks the same object.
• Thinking -7 // 2 = -3. Floor division rounds down: -4.