Is "length and length" any different than just "length" in Python?
Question:
I’ve found the following code snippet:
length = length and length or len(string)
To me it appears that this should be equivalent to:
length = length or len(string)
Can I collapse the expression length and length to just be length or will I get a different result in some cases?
Answers:
length and length or len(string) is exactly equivalent to length or len(string). I think that that whoever wrote this code expected it to avoid the len(string) call when length is falsy, but that is not the case. For example:
>>> None and None or len('foo')
3
Since and had higher precedence than or, we can rewrite the expression as follows:
(length and length) or len('foo')
length and length will always evaluate to length, so the two expressions are equivalent.
The line is nonsense. Let’s take a look at the options we have:
-
length is falsy (so 0, an empty list, an empty string, etc.). Python will return the value of length because there is no point in looking at the rest of the expression as the and operator evaluates to False whatever the right-hand side evaluates to. It then tests length or len(string) and returns len(string).
-
length is not falsy. Python evaluates length and length, resolves that to the value of length, then tests length or len(string), and because length is truthy, returns length. After all, or will now always resolve to True and there is no point in looking at len(string).
This basically comes down to length = length or len(string), the length and length is a no-op.
I’ve found the following code snippet:
length = length and length or len(string)
To me it appears that this should be equivalent to:
length = length or len(string)
Can I collapse the expression length and length to just be length or will I get a different result in some cases?
length and length or len(string) is exactly equivalent to length or len(string). I think that that whoever wrote this code expected it to avoid the len(string) call when length is falsy, but that is not the case. For example:
>>> None and None or len('foo')
3
Since and had higher precedence than or, we can rewrite the expression as follows:
(length and length) or len('foo')
length and length will always evaluate to length, so the two expressions are equivalent.
The line is nonsense. Let’s take a look at the options we have:
-
lengthis falsy (so0, an empty list, an empty string, etc.). Python will return the value oflengthbecause there is no point in looking at the rest of the expression as theandoperator evaluates toFalsewhatever the right-hand side evaluates to. It then testslength or len(string)and returnslen(string). -
lengthis not falsy. Python evaluateslength and length, resolves that to the value oflength, then testslength or len(string), and becauselengthis truthy, returnslength. After all,orwill now always resolve toTrueand there is no point in looking atlen(string).
This basically comes down to length = length or len(string), the length and length is a no-op.