Try and Except in Python

Question

My code is:

def less_than_equal(start_day, start_mon, start_year, \
 end_day, end_mon, end_year):
 try:
 start_day <= end_day and start_mon <= end_mon
 return True
 except:
 start_day > end_day or start_mon > end_mon or start_year >end_year
 print("Start date must be less than or equal end date.")

where

>>> less_than_equal(12, 4, 1111, 12, 5, 1111)
True
>>> less_than_equal(12, 7, 1111, 12, 5, 1111)
"Start date must be less than or equal end date." (but my code gives True)
>>> less_than_equal(17, 7, 1111, 12, 5, 1111)
"Start date must be less than or equal end date." (mine gives True here too)

I also am very confused on how to use raise and try and except. Which is placed within the body of the code, which is placed at the very end? For example, under the #TODO

def count_days(start_date, end_date):
 # date is represented as a string in format dd/mm/yyyy
 start_day, start_mon, start_year = get_day_month_year(start_date)
 end_day, end_mon, end_year = get_day_month_year(end_date)
# TODO: check for data validity here #
# if start date is not valid...
# if end date is not valid...
# if start date > end date...
 if is_valid( start_day, start_mon, start_year) == False:
 raise ValueError( "Not a valid date: " + start_date )
 elif is_valid(end_day, end_mon, end_year) == False:
 raise ValueError( "Not a valid date: " + end_date )
 if less_than_equal(start_day, start_mon, start_year, \
 end_day, end_mon, end_year) == "Start date must be less than or equal end date.":
 raise ValueError( "Not a valid date: " + start_date )
# lazy - let the computer count from start date to end date
 count = 0
 while less_than_equal(start_day, start_mon, start_year, end_day, end_mon, end_year):
 count = count + 1
 start_day, start_mon, start_year = next_date(start_day, start_mon, start_year)
# exclude end date
 return count - 1

Thanks a lot

• devnull: You don't have a condition to check in your first snippet.
• sshashank124: you should add a return False statement in your except block
• Jeff Mercado: You need to go back and read about what the try/except blocks are used for and if/how it applies here. The way you're using it is not how it's meant to be used. It would be pointless to explain how to make your code work using it. Just do it right.

Answer 1

You don't need to use try and except here.

def less_than_equal(start_day, start_mon, start_year, \
 end_day, end_mon, end_year):
 if start_day <= end_day and start_mon <= end_mon:
 return True
 elif start_day > end_day or start_mon > end_mon or start_year >end_year:
 print("Start date must be less than or equal end date.")
 return False

Try and except are used for catching exceptions.

For your second question, you can

count = 0
while True:
 try:
 if not less_than_equal(start_day, start_mon, start_year, end_day, end_mon, end_year):
 break
 except TypeError: # for example
 break
 count = count + 1
 start_day, start_mon, start_year = next_date(start_day, start_mon, start_year)

• user3251511: Thanks! If I want to implement try and except in my last code, should try and except be placed at the middle or at the end? Is the indentation the same line following "def"?
• Jeffrey Tang: Since both your validate functions return True and False, you just need to check their return values. Try and except are still unneeded in your example. Your third if clause in count_days should be checking for the return value of less_than_equal, not something it printed. Did I understand correctly?
• user3251511: But what about if the function meets with some errors and I want to continue the loop till the end?
• Jeffrey Tang: Ah, I see what you mean.

Answer 2

There are built-in library functions which would make this much simpler:

import datetime
import time
def parse_date(datestring):
 dt = time.strptime(datestring, "%Y/%m/%d")
 return datetime.date(dt.tm_year, dt.tm_mon, dt.tm_mday)
def count_days(start_date, end_date):
 s = parse_date(start_date)
 e = parse_date(end_date)
 if s > e:
 raise ValueError("Start date must be <= end date")
 else:
 return (e - s).days

then

count_days("2012/2/27", "2012/3/5") # => 7