How do I convert a string to a datetime without many nested try…excepts?

December 12, 2021

I’m trying to check user input of a date/time in several allowable formats. (I know about the dateutil library. It’s not what I’m looking for in this case.)

If some user input was accepted, the function must return a datetime object.

If ALL "try…except" fail — the function must return NONE. But I have 30-50 different date/time formats that I need to check.

I’m confused by the huge indentation in my code! How do I organize this format checking in a good style with GOOD performance?

# Test format check program
import datetime

def datetime_format_check(str):
    try:
        dt = datetime.datetime.strptime(str, "%y-%m-%d %H:%M")
        return dt
    except:
        try:
            dt = datetime.datetime.strptime(str, "%Y-%m-%d %H:%M")
            return dt
        except:
            try:
                dt = datetime.datetime.strptime(str, "%y-%m-%d")
                return dt
            except:
                try:
                    dt = datetime.datetime.strptime(str, "%Y-%m-%d")
                    return dt
                except:
                    try:
                        dt = datetime.datetime.strptime(str, "%H:%M")
                        return dt
                    except:
                        try:
                               # . . .
                               # many many try...except blocks )))
                               # . . .
                                   return None  # last except far far away from a screen border. ))))

while True:
    str = input("Input date: ")
    print("Result: ", datetime_format_check(str))

>Solution :

Repetitive code? Well, that just begs to be replaced with a loop.

Put all of the formats in a list and iterate over it, checking each format:

def datetime_format_check(s):
    formats = ["%y-%m-%d %H:%M", "%Y-%m-%d %H:%M", "%y-%m-%d"] # etc
    for format in formats:
        try:
            dt = datetime.datetime.strptime(s, format)
            return dt
        except ValueError:
            pass
    return None

Some minor corrections I made to your code: