How to use try-except in Python function return boolean?

Question

I need to use try-except to validate the input string of a date and time.The input format should be MM/DD/YYYY HH:MM:SS. If the validate function returns false, then it's not valid.I tried a invalid input, but the try-except didn't work as expected. The error message wasn't printed. What's wrong with my code? How to fix it? Thanks.

def validate_user_input(input_string):
    # check the input validation

    validation = True
    if input_string[2] != "/" or input_string[5] != "/":
        validation = False
    elif int(date) > 31 or int(date) < 1:
        validation = False
    elif int(month) > 12 or int(month) < 1:
        validation = False
    elif int(hour) < 0 or int(hour) > 24:
        validation = False
    elif int(minute) < 0 or int(minute) > 59:
        validation = False
    elif int(second) < 0 or int(second) > 59:
        validation = False
    return validation


input = input("Please input a date and time within the format of MM/DD/YYYY HH:MM:SS")
# get the information from the input

year = input[6:10]
month = input[0:2]
date = input[3:5]
hour = input[11:13]
minute = input[14:16]
second = input[17:19]

# try-except
try:
    validate_user_input(input) == False
    print("Your input is valid!")
except:
    print("Error: the input date and time is invalid!")

Answer 1

The except will only catch an exception if one is thrown and not caught by an earlier try-except . You are not raise ing anything in your code, so the except block will never run. You should instead throw an error, perhaps a ValueError , when the validate_user_input function returns False :

try:
    if not validate_user_input(input):
        raise ValueError("Bad input")
except:
    ...

Answer 2

Use an if statement instead. This should be simpler than using try and except.

if validate_user_input(input) == False:
    print("Your input is valid!")
else:
    print("Error: the input date and time is invalid!")

Answer 3

# try-except
try: 
    validate_user_input(input) == False
    print("Your input is valid!")
except:
    print("Error: the input date and time is invalid!")

Seems like you're going to validate the entered date & time from the input. But this code lines doesn't do anything. It check the condition without doing anything, @dogman288 answer make it happen. I would like to suggest a library call python-dateutil . Within few lines you can do your validations very easily, I hope this will save your time. Just comment the relevant outputs.

from dateutil.parser import parse
from dateutil.parser._parser import ParserError


def validate_user_input(input_string: str):
    try:
        parse(input_string)
        print("Your input is valid!")
    except ParserError as error:
        print(error)


validate_user_input("12/31/2005 23:59:6")  # Your input is valid!
validate_user_input("13/31/2005 23:59:6")  # month must be in 1..12: 13/31/2005 23:59:6
validate_user_input("12/32/2005 23:59:6")  # day is out of range for month: 12/32/2005 23:59:6
validate_user_input("12/31/200511111 23:59:6")  # year 200511111 is out of range: 12/31/200511111 23:59:6
validate_user_input("12/31/2005 24:59:6")  # hour must be in 0..23: 12/31/2005 24:59:6
validate_user_input("12/31/2005 23:61:6")  # minute must be in 0..59: 12/31/2005 23:61:6
validate_user_input("12/31/2005 23:59:61")  # second must be in 0..59: 12/31/2005 23:59:61
validate_user_input("")  # String does not contain a date: 
validate_user_input("some_string")  # Unknown string format: some_string