Two decimal places
Input
format_number(3.14159, 2)
Output
3.14
Rounded to two decimal places.
Full lesson preview
Format Numbers with Precision
Return a string representation of a number rounded to a specified number of decimal places, preserving trailing zeros.
Python practice16 minVariables & Data TypesIntermediateLast updated January 3, 2026
Problem statement
Write format_number(num, decimals) that takes a numeric value num (int or float) and an integer decimals (>= 0). Return a string representing num rounded to exactly decimals decimal places, including trailing zeros if necessary. If decimals is not a non-negative integer, raise a ValueError. Use standard Python rounding behavior.
Task
Implement a function that formats a numeric value to a fixed number of decimal places and returns it as a string.
Examples
Input format
A number num and an integer decimals passed as arguments to format_number(num, decimals).
Output format
A string representing num rounded to exactly decimals decimal places.
Constraints
- decimals is an integer and must be >= 0; otherwise raise ValueError.
- Do not use external libraries.
- The returned value must be a string (e.g., '2.000' for decimals=3).
Samples
Sample 1
Input
format_number(2, 3)
Output
2.000
Integer input should be formatted with trailing zeros to match decimals.