Brief Overview of Python

Python basics: identifiers and keywords, data types, operators and their precedence, type conversion, errors, if-elif-else, and for and while loops.

Key ideas

• An identifier starts with a letter or _, has only letters, digits and _, is case-sensitive and cannot be a keyword such as for, if, True or None.
• Python works out ** first (right to left), then *, /, // and %, then + and −, going left to right; brackets come before all of them.
• / always gives a float (10 / 5 is 2.0); // gives the quotient rounded down and % the remainder.
• type() reports int, float, str, bool, list, tuple or dict; lists and dicts are mutable, while int, float, str and tuple are immutable.
• input() always returns a str; int(), float() and str() convert explicitly, and int() simply drops the fraction part.
• In an if-elif-else ladder only the FIRST true condition runs; indentation shows which lines belong to each block.
• range(a, b, c) starts at a, steps by c and stops before b; a while loop repeats as long as its condition is True.
• Syntax errors (a missing colon, bad indentation) stop a program before it runs; runtime errors such as ZeroDivisionError or ValueError happen while it runs; logical errors give wrong answers silently.

Remember

Watch out

• Doing + before *, or working 2 ** 3 ** 2 from the left: it is 512, not 64.
• Writing 3 for 12 / 4: / always prints a float, 3.0.
• Counting range(a, b) up to b: the stop value is never included.
• Joining a str and an int with + (TypeError): convert with str() or int() first.

Worked example