Python用户输入数据类型验证问题：判断字符串或数字

Hey there! Let's break down why your current code isn't behaving as expected, then fix it to meet your requirements.

Why your code isn't working #

The core issue here is that input() always returns a string in Python—so wrapping it in str() doesn't do anything useful, and it will never throw a TypeError. That means your except block never runs, even when someone enters a number (since the input is still a string like "123").

Fixed Code #

Here's a revised version that properly checks if the input is a number (integer or decimal) and prompts the user accordingly:

while True:
    member = input('Are you a child or adult member? ')
    # Check if the input is a number
    try:
        # Try converting the input to a float—if it works, it's a number
        float(member)
        print('Please input letters')
    except ValueError:
        # Conversion failed: input is non-numeric
        # Optional: Add validation for "child" or "adult" here
        if member.lower() in ['child', 'adult']:
            print(f"Got it, you're a {member} member!")
            break  # Exit the loop once valid input is received
        else:
            print("Oops, please enter either 'child' or 'adult' (letters only).")

How this works #

• Skip unnecessary str() conversion: Since input() already gives a string, we don't need to wrap it.
• Number check via type conversion: We try converting the input to a float—this works for both integers (like "10") and decimals (like "12.5"). If it succeeds, we know the input is numeric and show the "Please input letters" message.
• Non-numeric input handling: If the conversion fails (throws a ValueError), we then check if the input is one of the expected values (child/adult). You can adjust this part to fit your exact needs.

Alternative: Check for integers only #

If you only need to block whole numbers (not decimals), you can use the isdigit() method instead (note: this won't catch negative numbers like "-5"):

while True:
    member = input('Are you a child or adult member? ')
    if member.isdigit():
        print('Please input letters')
    elif member.lower() in ['child', 'adult']:
        print(f"Got it, you're a {member} member!")
        break
    else:
        print("Please enter either 'child' or 'adult'.")

内容的提问来源于stack exchange，提问作者R.Edwin