lesson 55 String Algorithms · intermediate

Validate text with character tests

isdigit(), isalpha() and isalnum() answer whether every character of a text is a digit, a letter, or either. Combined with length and position checks, they validate codes, usernames and numbers before the program trusts them.

You'll be able to

Use isdigit, isalpha and isalnum, and know they are False for an empty text
Combine length, position and character checks into one validation function
Collect every rule a text breaks, not just the first

The idea

Text typed into a form can't be trusted until it's checked: a ZIP code must be five digits before int sees it, and a username must contain only allowed characters before it's stored. Three methods test what a text is made of:

text.isdigit(): every character is a digit (0–9).
text.isalpha(): every character is a letter.
text.isalnum(): every character is a letter or a digit.

All three are False for an empty text, and they say nothing about length or position: combine them with len and indexing.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · What each test says

for text in ["77002", "7700a", "", "-5", "3.5", "Nora", "Nora Garcia", "HOM22847"]:
    print("[" + text + "]", text.isdigit(), text.isalpha(), text.isalnum())

Real validation is several rules. Check the cheap structural ones first (length, a separator at a fixed position), then the characters of each part. A function that returns a list of every problem lets a form show all its errors at once instead of one per attempt.

example-2.py · Password rules, all at once

def password_problems(password):
    """Return a list of every rule the password breaks."""
    problems = []
    if len(password) < 10:
        problems.append("at least 10 characters")
    has_digit = False
    has_letter = False
    has_symbol = False
    for ch in password:
        if ch.isdigit():
            has_digit = True
        elif ch.isalpha():
            has_letter = True
        elif ch != " ":
            has_symbol = True
    if not has_digit:
        problems.append("a digit")
    if not has_letter:
        problems.append("a letter")
    if not has_symbol:
        problems.append("a symbol")
    return problems

print(password_problems("harbor"))
print(password_problems("night-harbor-2024"))
print(password_problems("12345678901"))

When to use it: before converting or storing anything typed by a person or read from a file. Mistakes to avoid:

Expecting isdigit to accept negative or decimal numbers.
Forgetting that an empty text fails every test, which is usually what you want, but check it deliberately.
Validating after converting: int("7a") raises ValueError before your check runs.
Forgetting to strip first when spaces at the ends are allowed.

Review

isdigit, isalpha and isalnum test every character, and are False for empty text.
Validate structure first (length, fixed positions), then characters, then convert.
Return every problem so a form can show them all.
Next module: collection algorithms: filter, transform, group, join and de-duplicate records.

Practise this