Question

I have a function that performs a monthly task on one or more specified days (e.g., the first and the 15th.) For enhanced usability, I want to let users just pass in a single int when they only want it to happen one day each month, or a list of ints for multiple events per month.

monthly_event(days_of_month=1, event="paycheck") 
monthly_event(days_of_month=[1,15], event="bills", starting=date(2013,1,1))

Internally the function will iterate over the list and do the same thing it would do for a single int.

Since “int” is not an iterable, I need to do something to avoid a TypeError when the user just passes a single int. I was surprised to find that using an “or” expression and relying on short-circuiting does not work – the TypeError still happens. Here’s an example:

from datetime import date as date
dt = date.today()
days = 1
#days = [1,2]
if dt.day == days or dt.day in days:
    print "GOOD"
else:
    print "BAD"

My first question is: have I misunderstood Python, or does the internal typecheck really happen on the entire line of code before the short-circuiting of the boolean expression? That seems very strange.

My second question is: what is the Pythonic way to do this? I’d like to avoid doing an explicit type check on the “days” variable. Using a try/catch instead just bloats the code:

try:
    if dt.day == days:
        print "GOOD"
    else:
        print "BAD"
except TypeError:
    if dt.day in days:
        print "GOOD"
    else:
        print "BAD"

Is there something obvious I’ve overlooked?

Answer 1

I believe the problem occurs when dt.day != days (and the short circuit is not taken), Python will then try the dt.day in days expression, and get a TypeError.