Input Validation

Unit ID: M06-U03 Estimated active time: 25-35 minutes

Validation enforces a contract

Suppose accepted hours may be:

• an exact integer from 0 to 40; or
• text containing an integer from 0 to 40 after outside whitespace is removed.

Booleans, missing values, decimal text, words, and out-of-range values are rejected.

Validate by type before operation

def parse_hours(raw_hours):
    if type(raw_hours) is int:
        hours = raw_hours
    elif type(raw_hours) is str:
        hours = int(raw_hours.strip())
    else:
        raise TypeError("hours must be an integer or integer text")

    if not 0 <= hours <= 40:
        raise ValueError("hours must be from 0 to 40")

    return hours

The type branches prevent calling .strip() on an integer. Exact type rejects booleans.

Conversion is not validation by itself

int(3.8) produces 3, which silently changes the value. If decimal input is not allowed, reject floats instead of converting them.

int(True) produces 1, but a boolean is not an hour count under this contract.

Preserve raw input

Do not overwrite raw_hours. Store the validated result in hours. This keeps the transformation inspectable.

Validate at a clear boundary

Parse and validate when data enters the trusted part of the program. Downstream calculations can then rely on the documented result type and range.

Do not repeat inconsistent partial checks throughout the notebook.

Practice

Write expected results for 0, 40, -1, 41, " 8 ", "8.5", True, and None. Then run parse_hours() and record returned values or exception types.

Takeaway

Validation defines accepted representations, types, and ranges before unsafe operations. Conversion must not silently change disallowed input. Next, we will handle only the expected per-record exceptions while keeping useful details.