Type errors: when a number is secretly a string
Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.
What is type error?
A value is not the type the code assumes. Python usually raises immediately; JavaScript usually converts silently and gives a wrong answer, which is worse because nothing tells you anything went wrong.
Why it happens
Data crossing a boundary loses its type. Form fields, JSON, query strings and CSV files all arrive as text. JSON object keys are always strings even when they look like numbers. JavaScript's + means addition or concatenation depending on its operands, and its default sort compares elements as strings.
How to recognise it
- A total is a string of digits stuck together rather than a sum.
- Sorting puts 10 before 9 — string ordering, not numeric.
- KeyError on a key you can see in the printed dict (it is "1", not 1).
- In Python 3, / always returns a float, so it can never be an index.
Errors and symptoms this causes
- TypeError: unsupported operand type(s)
- TypeError: '>' not supported between instances of 'str' and 'int'
- TypeError: list indices must be integers or slices, not float
- sort puts 10 before 9
- numbers concatenating instead of adding
How to fix it
Convert once, at the boundary where data enters, rather than scattering int() and Number() through your logic. Use === over == in JavaScript so type mismatches fail loudly. When a value surprises you, print its type before you print its value.
Practise type errors
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