False == 0 in Python!?

I came across a very strange error when tweaking the skime compiler. I decided to use the more specific push_0 and push_1 instruction instead of the general push_literal when the literal is 0 and 1 respectively. However, after added this code, several test cases was broken immediately.

After examining the execution of the failing test cases, I found that I got a 0 when expecting a False. And I finally found the problem is that I use if literal == 0 to test whether I got a literal 0. But unfortunately, False == 0 (as well as True == 1) evaluates to True in Python. So there's the bug.

I'm feeling rather surprised when I found False == 0. Yes, I know we use 0 to represent a false value in C, but I really don't expect it to be in Python -- such a high-level and duck-typing language. However, since it is already there, I have to remember.

Comments (2)

Mithro July 21, 2008

If you want to check that something is "exactly" false (or exactly True, or exactly None), use the "x is False" rather then the "x == False".

Any non-zero value is "true" while any zero value is "false".

pluskid July 21, 2008

@Mithro,
Yes, you are right. But here I'm in fact willing to check something that is exactly 0, but I guess I can't use x is 0 here. I don't know how Python implement integers. If it is encoded as a immediate value in the pointer, every 0 should be the same. However, I found id(0) will be different when I start a different Python session.