Question

Picking up a tip from someone else on this site, I’ve been using logical operators to express conditional logic in list comprehension as a shortcut to using full for loops with if statements.

For example, if I had a list [1, 0, 0, 1, 1] that I wanted to turn into [“Yes”, “No”, “No”, “Yes”, “Yes”] I could do it like this:

original = [1, 0, 0, 1, 1]
new = [((x==1) and "Yes") or "No" for x in original]

The problem I have is that I get some very strange results when I use the logical operators with 0 in this way. The results occur regardless of whether I’m using list comprehension or not.

These:

print((True and 1.0) or "Wrong")
print((True and -1.0) or "Wrong")
print((True and "1.0") or "Wrong")
print((True and 1) or "Wrong")

Correctly (I think) output:

1.0
-1.0
1.0
1

However these:

print((True and 0) or "Wrong")
print((True and 0.0) or "Wrong")

Output:

Wrong
Wrong

I think that Python is implicitly converting both 0 and 0.0 to a boolean False, and any other value of any other type to True

Can someone confirm whether:
a. This is the case and/or
b. Is there a way to achieve what I’m after in list comprehension another way (with a lamda or something)

Many thanks
Chris

Answer 1

Yes 0 is returning a false in your short-circuit evaluation (you can test via bool(0) which returns a false ). I think this is a lot simpler:

new = ["yes" if x else "no" for x in list]