Inconsistent output for two string methods

[pmanas88]

Bug report

Bug description:

# Add a code block here, if required
s = ''
s.islower()
Output: False
s.isascii()
Output: True

Output of these two mthods such as islower() and isascii() are inconsistent.

s.isascii()
Output: True

Explanation for this output is "an empty string passes the isascii() check because it does not contain any non-ASCII characters". This same logic should be applied to islower() method as well or the vice versa.

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[hugovk]

Hello, this isn't a bug, str.islower() is documented as:

Return True if all cased characters [4] in the string are lowercase and there is at least one cased character, False otherwise.

https://docs.python.org/3/library/stdtypes.html#str.islower

So an empty string gives False.

If we changed it, it would likely break a lot of existing code, and we'd need a very good reason to do so, beyond consistency.

If you'd like to propose the change, please open a discussion at https://discuss.python.org/c/ideas/6, but I don't think it's likely to happen.

[terryjreedy]

s.isupper has an equivalent definition; both can only be true if there is at least 1 cased char. They are both false for all strings with no cased chars. Most of the other tests require at least 1 char, but of any type. The 2 oddballs are s.isascii "Return True if the string is empty ..." and s.isprintable "Return true if all characters in the string are printable". If the senses had been inverted (s.nonascii and s.nonprintable) they would 'conform'. I suspect they follow C or Unix definitions, which are likely based on which placement of empty strings is more useful. Change is extremely unlikely.

If a user wants empty strings in the other partition of non-empty, they can use s and s.isascii() or not s or s.isalnum(), etc.

Rephrased, this would have been a good question for the 'discuss' help channel. Note that similar issues arise for 0 and empty collections when the truthy objects are partitioned.

[pmanas88]

"Rephrased, this would have been a good question for the 'discuss' help channel. Note that similar issues arise for 0 and empty collections when the truthy objects are partitioned". @terryjreedy, I agree with you completely without a doubt. It was my bad that I created this post as a bug.