Review

Core idea

An error is evidence. It tells you which assumption the program could not keep.

Tracebacks

Read near the bottom first.

Syntax errors

A syntax error means Python could not parse the program.

Common causes:

• missing colon after if, for, while, or def;
• unclosed quote, parenthesis, bracket, or brace;
• indentation that does not match the expected block.

Fix syntax before debugging logic.

Name errors

A NameError means Python could not find a name.

Common causes:

• typo;
• name used before assignment;
• name assigned only in a branch that did not run;
• local name used outside its function.

Type errors

A TypeError means an operation received a value of an unsuitable type.

Good questions:

• What operation failed?
• What type did the operation expect?
• What type did the value actually have?
• Should I convert at the boundary?

Index errors

An IndexError means an index is outside a sequence.

For a sequence of length n, valid positive indexes are:

0 through n - 1

Prefer direct iteration when you do not need the index:

Assertions

Use assertions to state assumptions while developing:

assert len(values) > 0, "values must not be empty"

Assertions are for programmer assumptions. Public user input usually needs normal checks and helpful responses.

Reducing failures

A reduced failing example:

• still fails;
• removes unrelated code;
• keeps the operation or assumption being tested.

Change one thing at a time and run again.