Are these Python expressions equivalent to each other?

Question

Given the following Python function (ignoring its shortcomings):

def adjust_year(year):
  return year > 2000 and year - 2000 or year - 1900

If I change it to instead be:

def adjust_year(year):
  return year - 2000 if year > 2000 else year - 1900

Will the behavior be equivalent or have I changed it in some subtle way?

Answer 1

They are indeed equivalent, but the conditional expression variation is preferred.

Your expression narrowly avoids the typical and ... or pitfall where the middle expression evaluates to a falsy value (year >= 2000 and year - 2000 or year - 1900 and year = 2000 will result in 100).

Answer 2

There are several ways to tackle this. One is to logically analyse both versions; the other is to use brute force and compare the results for every valid input:

def adjust_year_1(year):
  return year > 2000 and year - 2000 or year - 1900

def adjust_year_2(year):
  return year - 2000 if year > 2000 else year - 1900

for y in range(-4000, 4000):
  if adjust_year_1(y) != adjust_year_2(y):
    print y, adjust_year_1(y), adjust_year_2(y)

This doesn't print anything, demonstrating that the functions are indeed equivalent for years between -4000 and 4000 (it's easy to see that they are also equivalent for inputs outside this range).