For the lowercase German alphabet `ß`, `upper()` returns `SS` instead of the uppercase German alphabet `ẞ`

[hyperkai]

Bug report

Bug description:

For the uppercase German alphabet ẞ, lower() returns the lowercase German alphabet ß as shown below. *Additionally, upper() returns ẞ same as the input:

up = 'ẞ'

print(up.lower()) # ß

# print(up.upper()) # ẞ 
# ↑↑↑↑ Additional ↑↑↑↑

But for the lowercase German alphabet ß, upper() returns SS instead of the uppercase German alphabet ẞ as shown below. *Additionally, lower() returns ß same as the input:

lo = 'ß'

print(lo.upper()) # SS

# print(lo.lower()) # ß
# ↑↑↑ Additional ↑↑↑

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[ned-deily]

Thanks for the report. However, this is expected behavior. As noted in the documentation for str.upper():

The uppercasing algorithm used is described in section 3.13 ‘Default Case Folding’ of the Unicode Standard.

and the Unicode standard still specifies that the upper-case mapping for ß (U+00DF) is SS even though there is now an upper-case ẞ (U+1E9E); see the Unicode FAQ here.