decision tree Python with exceptions 2.7 - if-statement

Question

I'm writing a script which calculates the date of Easter for years 1900 - 2099.
The thing is that for 4 certain years (1954, 1981, 2049, and 2076) the formula differs a little bet (namely, the date is off 7 days).
def main():
print "Computes the date of Easter for years 1900-2099.\n"
year = input("The year: ")
if year >= 1900 and year <= 2099:
if year != 2049 != 2076 !=1981 != 1954:
a = year%19
b = year%4
c = year%7
d = (19*a+24)%30
e = (2*b+4*c+6*d+5)%7
date = 22 + d + e # March 22 is the starting date
if date <= 31:
print "The date of Easter is March", date
else:
print "The date of Easter is April", date - 31
else:
if date <= 31:
print "The date of Easter is March", date - 7
else:
print "The date of Easter is April", date - 31 - 7
else:
print "The year is out of range."
main()
Exerything is working well but the 4 years computation.
I'm getting the:
if date <= 31:
UnboundLocalError: local variable 'date' referenced before assignment whenever I'm entering any of the 4 years as input.

Answer 1

You cannot chain a expression like that; chain the tests using and operators or use a not in expression instead:
# and operators
if year != 2049 and year != 2076 and year != 1981 and year != 1954:
# not in expression
if year not in (2049, 2076, 1981, 1954):
The expression year != 2049 != 2076 !=1981 != 1954 means something different, it is interpreted as (((year != 2049) != 2076) !=1981) != 1954 instead; the first test is either True or False, and neither of those two values will ever be equal to any of the other numbers and that branch will always evaluate to False.
You will still get the UnboundLocalError for date though, since your else branch refers to date but it is never set in that branch. When the else branch executes, all Python sees is:
def main():
print "Computes the date of Easter for years 1900-2099.\n"
year = input("The year: ")
if year >= 1900 and year <= 2099:
if False:
# skipped
else:
if date <= 31:
print "The date of Easter is March", date - 7
else:
print "The date of Easter is April", date - 31 - 7
and date is never assigned a value in that case. You need to calculate date separately in that branch still, or move the calculation of the date value out of the if statement altogether; I am not familiar with the calculation of Easter so I don't know what you need to do in this case.