Passing a comparison operator in Python

October 28, 2013
tags: python, programming languages, stupid tricks, code-dump

So, earlier today I was refactoring some code to be more general purpose, and said code involved a comparison operator. I was already using the same code in two places, but copy-pasted with different comparison operators switched in. Eww.

I didn't quite know how to pull this off, given that I'm not that well versed on Python's introspection faculties... but I gave it a whack. My first thought was the niave approach, of passing in a string to specify mode...

	## approach 1

	def myfun(a, b, mode='<'):
	valid_modes = ['<', '<=', '>', '>=', '==', '!=']
	if mode not in valid_modes:
	print 'Error!'
	... # error handling code here
	return
	if mode == '<':
	if a < b:
	...
	elif mode == '<=':
	if a <= b:
	...
	elif mode == '>':
	if a > b:
	...
	elif mode == '>=':
	if a >= b:
	...
	elif mode == '==':
	if a == b:
	...
	elif mode == '!=':
	if a != b:
	...
	else:
	print 'Something went horribly wrong. Blame cosmic rays.'

view raw approach_1.py hosted with ❤ by GitHub

Yuck.

Then I realized that functions are pretty much first class in python. Additionally, lambdas are short and sweet. A few minutes later, I was rolling with this instead:

	## approach 2

	# comparators
	cmp_lt = lambda x,y: x < y
	cmp_gt = lambda x,y: x > y
	cmp_eq = lambda x,y: x == y
	cmp_lte = lambda x,y: x <= y
	cmp_gte = lambda x,y: x >= y
	cmp_neq = lambda x,y: x != y

	def myfun(a, b, comparator):
	if comparator(a,b):
	...

view raw approach_2.py hosted with ❤ by GitHub

So much nicer. Instead of passing a mode string, now you just pass one of those comparators, ala:

myfun(4,7,comparator=cmp_lt)

Update: A friend pointed out a few other ways of accomplishing this -- one way more pythonic, and one incredibly unsafe.

The pythonic solution relies on the 'operator' module:

	## python is batteries included!

	import operator

	def myfun(a, b, comparator):
	if comparator(a,b):
	...

view raw approach_3.py hosted with ❤ by GitHub

That's much like approach 3, but the call is now:

myfun(4,7,comparator=operator.lt)

The unsafe way is to evaluate it as a string! Here we go:

	## don't do this, dummy

	def myfun(a, b, mode='<'):
	if eval(str(a)+mode+str(b)):
	...

view raw approach_4.py hosted with ❤ by GitHub

Man, I love Python. We're not starved for options!

Hope this helps someone else!

a→ab

computation, game design, and experimentation

Passing a comparison operator in Python