Tamil Nadu HSE First Year Computer Science: Introduction to Programming and Python Basics

Introduction to Programming and Python Basics tests how a Python program is written and executed, the token categories, variable naming, standard data types, input/output, type conversion and operator precedence.

Core rules

• Python is an interpreted, dynamically typed language: a variable is a name bound to an object, and its type can change on reassignment.
• Tokens are keywords, identifiers, literals, operators and punctuators. Keywords such as if, for and def cannot be used as identifiers.
• Core data types: int, float, str, bool and complex. Use type(x) to inspect one.
• input() always returns a string. Convert with int(), float() or str() before arithmetic.
• print() displays values; commas insert spaces, and sep and end control formatting.
• Precedence: **, then unary + -, then * / // %, then + -, then comparisons, then not, and, or. Use brackets when in doubt.
• / gives a float, // gives floor division, and % gives the remainder.

Worked example

Evaluate 2 + 3 * 4 ** 2 // 8. Exponentiation binds first: 4 ** 2 = 16. Multiplication and floor division are evaluated left to right: 3 * 16 = 48, then 48 // 8 = 6. Finally 2 + 6 = 8. If you evaluated left to right without precedence, you would wrongly get 10.

Traps that cost marks

• Writing input('Marks: ') + 5, which raises a TypeError because input returns a string.
• Confusing = (assignment) with == (comparison).
• Assuming // always rounds down toward zero; for negative numbers it rounds toward negative infinity.
• Starting an identifier with a digit or using a keyword.
• Expecting '5' + '3' to give 8; it gives the string '53'.