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From: Christopher Reimer <christopher_reimer@icloud.com>
Newsgroups: comp.lang.python
Subject: Re: Not x.islower() has different output than x.isupper() in list output...
Date: Fri, 29 Apr 2016 18:55:35 -0700
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On 4/29/2016 6:29 PM, Stephen Hansen wrote:
> If isupper/islower were perfect opposites of each-other, there'd be no 
> need for both. But since characters can be upper, lower, or *neither*, 
> you run into this situation.

Based upon the official documentation, I was expecting perfect opposites.

str.islower(): "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?highlight=islower#str.islower

str.isupper(): "Return true if all cased characters [4] in the string 
are uppercase and there is at least one cased character, false otherwise."

https://docs.python.org/3/library/stdtypes.html?highlight=isupper#str.isupper

Here's the footnote that may or not be relevant to this discussion: "[4] 
Cased characters are those with general category property being one of 
“Lu” (Letter, uppercase), “Ll” (Letter, lowercase), or “Lt” (Letter, 
titlecase)."

A bug in the docs?

Thank you,

Chris R.