When does using swapcase twice not return an identical answer?

Question

The Python docs for str.swapcase() say:

Note that it is not necessarily true that s.swapcase().swapcase() == s.

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Answer 1

While Volatility brought up the example of the uppercase mu and uppercase micro resolving to the same Unicode codepoint, here's another interesting situation where applying swapcase twice results in a different answer:

>>> 'ß'.swapcase().swapcase()
'ss'

Confused? The German consonant ß (pronounced [s]) becomes SS after one application of swapcase and then ss after the second.

Here's the whole list of them (→ represents one swapcase):

µ (0xb5) → Μ (0x39c) → μ (0x3bc) → Μ (0x39c)
ß (0xdf) → SS (0x5353) → ss (0x7373) → SS (0x5353)
İ (0x130) → i̇ (0x69307) → İ (0x49307) → i̇ (0x69307)
ı (0x131) → I (0x49) → i (0x69) → I (0x49)
ʼn (0x149) → ʼN (0x2bc4e) → ʼn (0x2bc6e) → ʼN (0x2bc4e)
ſ (0x17f) → S (0x53) → s (0x73) → S (0x53)
ǰ (0x1f0) → J̌ (0x4a30c) → ǰ (0x6a30c) → J̌ (0x4a30c)
ͅ (0x345) → Ι (0x399) → ι (0x3b9) → Ι (0x399)
ΐ (0x390) → Ϊ́ (0x399308301) → ΐ (0x3b9308301) → Ϊ́ (0x399308301)
ΰ (0x3b0) → Ϋ́ (0x3a5308301) → ΰ (0x3c5308301) → Ϋ́ (0x3a5308301)
ς (0x3c2) → Σ (0x3a3) → σ (0x3c3) → Σ (0x3a3)
ϐ (0x3d0) → Β (0x392) → β (0x3b2) → Β (0x392)
ϑ (0x3d1) → Θ (0x398) → θ (0x3b8) → Θ (0x398)
ϕ (0x3d5) → Φ (0x3a6) → φ (0x3c6) → Φ (0x3a6)
ϖ (0x3d6) → Π (0x3a0) → π (0x3c0) → Π (0x3a0)
ϰ (0x3f0) → Κ (0x39a) → κ (0x3ba) → Κ (0x39a)
ϱ (0x3f1) → Ρ (0x3a1) → ρ (0x3c1) → Ρ (0x3a1)
ϴ (0x3f4) → θ (0x3b8) → Θ (0x398) → θ (0x3b8)
ϵ (0x3f5) → Ε (0x395) → ε (0x3b5) → Ε (0x395)
և (0x587) → ԵՒ (0x535552) → եւ (0x565582) → ԵՒ (0x535552)
ẖ (0x1e96) → H̱ (0x48331) → ẖ (0x68331) → H̱ (0x48331)
ẗ (0x1e97) → T̈ (0x54308) → ẗ (0x74308) → T̈ (0x54308)
ẘ (0x1e98) → W̊ (0x5730a) → ẘ (0x7730a) → W̊ (0x5730a)
ẙ (0x1e99) → Y̊ (0x5930a) → ẙ (0x7930a) → Y̊ (0x5930a)
ẚ (0x1e9a) → Aʾ (0x412be) → aʾ (0x612be) → Aʾ (0x412be)
ẛ (0x1e9b) → Ṡ (0x1e60) → ṡ (0x1e61) → Ṡ (0x1e60)
ẞ (0x1e9e) → ß (0xdf) → SS (0x5353) → ss (0x7373) → SS (0x5353)
ὐ (0x1f50) → Υ̓ (0x3a5313) → ὐ (0x3c5313) → Υ̓ (0x3a5313)
ὒ (0x1f52) → Υ̓̀ (0x3a5313300) → ὒ (0x3c5313300) → Υ̓̀ (0x3a5313300)
ὔ (0x1f54) → Υ̓́ (0x3a5313301) → ὔ (0x3c5313301) → Υ̓́ (0x3a5313301)
ὖ (0x1f56) → Υ̓͂ (0x3a5313342) → ὖ (0x3c5313342) → Υ̓͂ (0x3a5313342)
ᾀ (0x1f80) → ἈΙ (0x1f08399) → ἀι (0x1f003b9) → ἈΙ (0x1f08399)
ᾁ (0x1f81) → ἉΙ (0x1f09399) → ἁι (0x1f013b9) → ἉΙ (0x1f09399)
ᾂ (0x1f82) → ἊΙ (0x1f0a399) → ἂι (0x1f023b9) → ἊΙ (0x1f0a399)
ᾃ (0x1f83) → ἋΙ (0x1f0b399) → ἃι (0x1f033b9) → ἋΙ (0x1f0b399)
ᾄ (0x1f84) → ἌΙ (0x1f0c399) → ἄι (0x1f043b9) → ἌΙ (0x1f0c399)
ᾅ (0x1f85) → ἍΙ (0x1f0d399) → ἅι (0x1f053b9) → ἍΙ (0x1f0d399)
ᾆ (0x1f86) → ἎΙ (0x1f0e399) → ἆι (0x1f063b9) → ἎΙ (0x1f0e399)
ᾇ (0x1f87) → ἏΙ (0x1f0f399) → ἇι (0x1f073b9) → ἏΙ (0x1f0f399)
ᾐ (0x1f90) → ἨΙ (0x1f28399) → ἠι (0x1f203b9) → ἨΙ (0x1f28399)
ᾑ (0x1f91) → ἩΙ (0x1f29399) → ἡι (0x1f213b9) → ἩΙ (0x1f29399)
ᾒ (0x1f92) → ἪΙ (0x1f2a399) → ἢι (0x1f223b9) → ἪΙ (0x1f2a399)
ᾓ (0x1f93) → ἫΙ (0x1f2b399) → ἣι (0x1f233b9) → ἫΙ (0x1f2b399)
ᾔ (0x1f94) → ἬΙ (0x1f2c399) → ἤι (0x1f243b9) → ἬΙ (0x1f2c399)
ᾕ (0x1f95) → ἭΙ (0x1f2d399) → ἥι (0x1f253b9) → ἭΙ (0x1f2d399)
ᾖ (0x1f96) → ἮΙ (0x1f2e399) → ἦι (0x1f263b9) → ἮΙ (0x1f2e399)
ᾗ (0x1f97) → ἯΙ (0x1f2f399) → ἧι (0x1f273b9) → ἯΙ (0x1f2f399)
ᾠ (0x1fa0) → ὨΙ (0x1f68399) → ὠι (0x1f603b9) → ὨΙ (0x1f68399)
ᾡ (0x1fa1) → ὩΙ (0x1f69399) → ὡι (0x1f613b9) → ὩΙ (0x1f69399)
ᾢ (0x1fa2) → ὪΙ (0x1f6a399) → ὢι (0x1f623b9) → ὪΙ (0x1f6a399)
ᾣ (0x1fa3) → ὫΙ (0x1f6b399) → ὣι (0x1f633b9) → ὫΙ (0x1f6b399)
ᾤ (0x1fa4) → ὬΙ (0x1f6c399) → ὤι (0x1f643b9) → ὬΙ (0x1f6c399)
ᾥ (0x1fa5) → ὭΙ (0x1f6d399) → ὥι (0x1f653b9) → ὭΙ (0x1f6d399)
ᾦ (0x1fa6) → ὮΙ (0x1f6e399) → ὦι (0x1f663b9) → ὮΙ (0x1f6e399)
ᾧ (0x1fa7) → ὯΙ (0x1f6f399) → ὧι (0x1f673b9) → ὯΙ (0x1f6f399)
ᾲ (0x1fb2) → ᾺΙ (0x1fba399) → ὰι (0x1f703b9) → ᾺΙ (0x1fba399)
ᾳ (0x1fb3) → ΑΙ (0x391399) → αι (0x3b13b9) → ΑΙ (0x391399)
ᾴ (0x1fb4) → ΆΙ (0x386399) → άι (0x3ac3b9) → ΆΙ (0x386399)
ᾶ (0x1fb6) → Α͂ (0x391342) → ᾶ (0x3b1342) → Α͂ (0x391342)
ᾷ (0x1fb7) → Α͂Ι (0x391342399) → ᾶι (0x3b13423b9) → Α͂Ι (0x391342399)
ι (0x1fbe) → Ι (0x399) → ι (0x3b9) → Ι (0x399)
ῂ (0x1fc2) → ῊΙ (0x1fca399) → ὴι (0x1f743b9) → ῊΙ (0x1fca399)
ῃ (0x1fc3) → ΗΙ (0x397399) → ηι (0x3b73b9) → ΗΙ (0x397399)
ῄ (0x1fc4) → ΉΙ (0x389399) → ήι (0x3ae3b9) → ΉΙ (0x389399)
ῆ (0x1fc6) → Η͂ (0x397342) → ῆ (0x3b7342) → Η͂ (0x397342)
ῇ (0x1fc7) → Η͂Ι (0x397342399) → ῆι (0x3b73423b9) → Η͂Ι (0x397342399)
ῒ (0x1fd2) → Ϊ̀ (0x399308300) → ῒ (0x3b9308300) → Ϊ̀ (0x399308300)
ΐ (0x1fd3) → Ϊ́ (0x399308301) → ΐ (0x3b9308301) → Ϊ́ (0x399308301)
ῖ (0x1fd6) → Ι͂ (0x399342) → ῖ (0x3b9342) → Ι͂ (0x399342)
ῗ (0x1fd7) → Ϊ͂ (0x399308342) → ῗ (0x3b9308342) → Ϊ͂ (0x399308342)
ῢ (0x1fe2) → Ϋ̀ (0x3a5308300) → ῢ (0x3c5308300) → Ϋ̀ (0x3a5308300)
ΰ (0x1fe3) → Ϋ́ (0x3a5308301) → ΰ (0x3c5308301) → Ϋ́ (0x3a5308301)
ῤ (0x1fe4) → Ρ̓ (0x3a1313) → ῤ (0x3c1313) → Ρ̓ (0x3a1313)
ῦ (0x1fe6) → Υ͂ (0x3a5342) → ῦ (0x3c5342) → Υ͂ (0x3a5342)
ῧ (0x1fe7) → Ϋ͂ (0x3a5308342) → ῧ (0x3c5308342) → Ϋ͂ (0x3a5308342)
ῲ (0x1ff2) → ῺΙ (0x1ffa399) → ὼι (0x1f7c3b9) → ῺΙ (0x1ffa399)
ῳ (0x1ff3) → ΩΙ (0x3a9399) → ωι (0x3c93b9) → ΩΙ (0x3a9399)
ῴ (0x1ff4) → ΏΙ (0x38f399) → ώι (0x3ce3b9) → ΏΙ (0x38f399)
ῶ (0x1ff6) → Ω͂ (0x3a9342) → ῶ (0x3c9342) → Ω͂ (0x3a9342)
ῷ (0x1ff7) → Ω͂Ι (0x3a9342399) → ῶι (0x3c93423b9) → Ω͂Ι (0x3a9342399)
Ω (0x2126) → ω (0x3c9) → Ω (0x3a9) → ω (0x3c9)
K (0x212a) → k (0x6b) → K (0x4b) → k (0x6b)
Å (0x212b) → å (0xe5) → Å (0xc5) → å (0xe5)
ff (0xfb00) → FF (0x4646) → ff (0x6666) → FF (0x4646)
fi (0xfb01) → FI (0x4649) → fi (0x6669) → FI (0x4649)
fl (0xfb02) → FL (0x464c) → fl (0x666c) → FL (0x464c)
ffi (0xfb03) → FFI (0x464649) → ffi (0x666669) → FFI (0x464649)
ffl (0xfb04) → FFL (0x46464c) → ffl (0x66666c) → FFL (0x46464c)
ſt (0xfb05) → ST (0x5354) → st (0x7374) → ST (0x5354)
st (0xfb06) → ST (0x5354) → st (0x7374) → ST (0x5354)
ﬓ (0xfb13) → ՄՆ (0x544546) → մն (0x574576) → ՄՆ (0x544546)
ﬔ (0xfb14) → ՄԵ (0x544535) → մե (0x574565) → ՄԵ (0x544535)
ﬕ (0xfb15) → ՄԻ (0x54453b) → մի (0x57456b) → ՄԻ (0x54453b)
ﬖ (0xfb16) → ՎՆ (0x54e546) → վն (0x57e576) → ՎՆ (0x54e546)
ﬗ (0xfb17) → ՄԽ (0x54453d) → մխ (0x57456d) → ՄԽ (0x54453d)