Basic sanitization
Input
sanitize_username(' Bob Smith ')
Output
bob_smith
Trims spaces, lowercases, replaces internal spaces with underscore, removes invalid characters.
Full lesson preview
Sanitize a username input
Create a sanitizer for raw username strings that enforces safe characters, casing, and length limits. Return None for invalid results.
Python practice15 minError Handling & Edge CasesIntermediateLast updated March 20, 2026
Problem statement
User-entered usernames can contain whitespace, punctuation, mixed casing, or other unwanted characters. Implement sanitize_username(name) to:
- Accept a value and attempt to coerce it to a string.
- Trim leading/trailing whitespace and lowercase the name.
- Replace internal whitespace sequences with a single underscore.
- Remove characters other than lowercase letters, digits, underscore (_), and hyphen (-).
- Collapse multiple underscores into a single underscore.
- Enforce a minimum length of 3 characters. If the cleaned name is shorter than 3, return None.
- Enforce a maximum length of 20 characters by truncating the cleaned name to 20 characters.
- If the cleaned username does not start with a lowercase letter (a-z), return None.
Return the sanitized username string or None when it is invalid.
Task
Implement sanitize_username(name) that returns a cleaned username or None when the input can't be turned into a valid username.
Examples
Input format
Single value (typically a string) representing a username.
Output format
A sanitized username string or None (represented as empty output by the test harness) if invalid.
Constraints
Only use the Python standard library. Be defensive: handle non-string inputs by converting to str where sensible, but still apply validation rules.
Samples
Sample 1
Input
sanitize_username('User!@#')
Output
user
Punctuation removed, case lowered.