romanNumeralFromChord() adds cautionary 'b' to VI and VII despite other system defaults

[napulen]

Here is the minimal reproducibility code:

In [1]: import music21

In [2]: music21.__version__
Out[2]: '7.1.0'

In [3]: from music21.roman import romanNumeralFromChord, RomanNumeral

In [4]: from music21.chord import Chord

In [5]: c = Chord("E-4 G4 B-4")

In [6]: rn = romanNumeralFromChord(c, "f")

In [7]: rn.pitches
Out[7]:
(<music21.pitch.Pitch E-4>,
 <music21.pitch.Pitch G4>,
 <music21.pitch.Pitch B-4>)

In [8]: rn2 = RomanNumeral("bVII", "f")

In [9]: rn2.pitches
Out[9]:
(<music21.pitch.Pitch E--5>,
 <music21.pitch.Pitch G-5>,
 <music21.pitch.Pitch B--5>)

The idea here is that two different paths to obtain the RomanNumeral object lead to the same figure but different pitches.

[jacobtylerwalls]

Yeah, the lack of round-trip equivalence feels surprising. Both directions are currently working as documented. The result of [8] can be changed by passing seventhMinor=roman.Minor67Default.CAUTIONARY, but since that's not the default, then I suppose [6] should put out a figure VII instead of bVII so that the round trip works by default.

This diff seems to help, but maybe the subsequent branches need to be looked at also?

diff --git a/music21/roman.py b/music21/roman.py
index 7286a7003..ae427215f 100644
--- a/music21/roman.py
+++ b/music21/roman.py
@@ -695,7 +695,7 @@ def correctRNAlterationForMinor(figureTuple, keyObj):
         rootAlterationString = ''
     elif alter == 0.0:
         alter = 0  # NB! does not change!
-        rootAlterationString = 'b'
+        rootAlterationString = ''
     # more exotic:
     elif alter > 1.0:
         alter = alter - 1

[mscuthbert]

There was so much discussion about ^6 and ^7 roman numerals just a few years back, so we're not going to change the defaults now. But it does seem that this is a bug. But the fix should take into account roman.Minor67Default in what it displays.