如何用Python检查字符串指定长度？信用卡号码16位校验代码求助

Hey there! Let's work through the kinks in your credit card validation code—there are a couple of straightforward fixes needed to get it working right:

1. Mixed-Up Length Check Logic #

Your current code uses str(CreditCardNum) != len(16) which is backwards. len() needs to take the input string as its argument, and you want to compare the result to 16. Also, since input() returns a string by default, you don’t need to wrap CreditCardNum in str().

2. Incorrect First Digit Comparison #

str(CreditCardNum[0:]) == 4 is checking if the entire card number string equals the number 4, which isn’t your goal. You should grab just the first character (CreditCardNum[0]) and compare it to the string "4" (since input values are strings, not integers).

Here’s the corrected code with clear explanations:

# Get user input (already a string, no conversion needed)
CreditCardNum = input("Input a credit card number(no spaces/hyphens): ")

# Check if the input is exactly 16 characters long
if len(CreditCardNum) != 16:
    print("This is not a valid number, make sure the number is 16 characters.")
elif CreditCardNum[0] == "4":
    # Add your specific logic for this card type here
    print("Valid length and starts with 4—proceed with further checks!")
else:
    # Handle other valid lengths or invalid starting digits
    print("Valid length, but starting digit doesn't match expected value.")

A quick extra tip: If you want to ensure the input only contains numbers (no letters or symbols), you can add a check like if not CreditCardNum.isdigit(): before the length validation to catch non-numeric entries early.

内容的提问来源于stack exchange，提问作者Simpson