User Input Utilities#

Note

Source: Adapted from the C# edition (while/userinput.rst), which described the UI helper library. Python has no equivalent built-in library, so we build our own helper functions here. The try/except pattern is original Python-specific content.

The C# edition used a UI helper library to prompt for validated input. Python has no such built-in library, but building one is a good exercise in combining loops, functions, and error handling.

Simple Prompt Helpers#

The simplest helper re-prompts the user when input cannot be converted:

def prompt_int(message):
    """Keep prompting until the user enters a valid integer."""
    while True:
        try:
            return int(input(message))
        except ValueError:
            print("Please enter a whole number.")

def prompt_float(message):
    """Keep prompting until the user enters a valid float."""
    while True:
        try:
            return float(input(message))
        except ValueError:
            print("Please enter a number.")

A Brief Note on `try`/`except`#

int(input(...)) raises a ValueError if the user types something that cannot be converted to an integer (like "abc"). A try/except block catches that error so the program can respond gracefully instead of crashing:

try:
    risky_operation()
except SomeError:
    handle_the_error()

If risky_operation() raises SomeError, execution jumps to the except block. If no error occurs, the except block is skipped. Error handling is covered more fully in a later chapter; for now just use this pattern as written.

Prompting Within a Range#

A more useful helper also enforces a valid range:

def prompt_int_in_range(message, low, high):
    """Keep prompting until the user enters an integer in [low, high]."""
    while True:
        value = prompt_int(message)
        if low <= value <= high:
            return value
        print(f"{value} is out of range!  Enter a value from {low} to {high}.")

Sample interaction:

Enter a score (0-100): 233
233 is out of range!  Enter a value from 0 to 100.
Enter a score (0-100): -1
-1 is out of range!  Enter a value from 0 to 100.
Enter a score (0-100): 85

Yes/No Prompt#

Another common need is a yes/no confirmation:

def prompt_yes_no(message):
    """Return True if user answers yes, False if no."""
    while True:
        answer = input(message + " (yes/no): ").strip().lower()
        if answer in ("yes", "y"):
            return True
        if answer in ("no", "n"):
            return False
        print("Please answer yes or no.")

Using the Helpers#

Collect these functions in a module (e.g., ui.py) and import them:

from ui import prompt_int, prompt_int_in_range, prompt_yes_no

age  = prompt_int("Enter your age: ")
score = prompt_int_in_range("Enter score (0-100): ", 0, 100)
if prompt_yes_no("Save result?"):
    print(f"Saved: age={age}, score={score}")

These helpers keep your main program free of repetitive validation code — the same separation-of-concerns idea we applied to computation and output in the functions chapter.

User Input Utilities

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