When is `string.swapcase().swapcase()` not equal to `string`?

Question

Documentation for str.swapcase() method says:

Return a copy of the string with uppercase characters converted to lowercase and vice versa. Note that it is not necessarily true that s.swapcase().swapcase() == s.

I can't think of an example where s.swapcase().swapcase() != s, can anyone think of one?

Answer 1

A simple example would be:

s = "ß"

print(s.swapcase().swapcase())

Ouput:

ss

ß is German lowercase double s (The "correct" uppercase version would be ẞ). The reason this happens is that the Unicode standard has defined the capitalization of ß to be SS:

The data in this file, combined with
# the simple case mappings in UnicodeData.txt, defines the full case mappings
# Lowercase_Mapping (lc), Titlecase_Mapping (tc), and Uppercase_Mapping (uc).

...

# The entries in this file are in the following machine-readable format:
#
# <code>; <lower>; <title>; <upper>; (<condition_list>;)? # <comment>

...

# The German es-zed is special--the normal mapping is to SS.
# Note: the titlecase should never occur in practice. It is equal to titlecase(uppercase(<es-zed>))

00DF; 00DF; 0053 0073; 0053 0053; # LATIN SMALL LETTER SHARP S

(00DF is ß, 0053 is S, and 0073 is s)