====== music21 ======
The Python library //music21// is comprehensively documented on the project page of MIT Boston: In addition to a [[https://web.mit.edu/music21/doc/usersGuide/index.html|User Guide]] with chapters on numerous topics, there is an extensive [[https://web.mit.edu/music21/doc/moduleReference/index.html|module documentation]] and an alphabetical [[https://web.mit.edu/music21/doc/genindex.html|index]] concering all topics and commands. \\
Especially in view of this very extensive documentation, it makes sense to list some important commands related to //computer-aided analysis procedures// separately and to describe them with short examples.


==== Overview of the most important music21 commands ====

In the following, the most important and useful commands in //music21// are listed. More complex command sequences can be found with examples in the tutorials notes. 

(1) Handling files

The following commands load the music21 library and then the Prelude in C major from the Well-Tempered Clavier as variable 'sBach'.


   from music21 import *
   # This initial command imports the Python library music21
   
   sBach = converter.parse('https://analyse.hfm-weimar.de/database/04/BaJoSe_BWV846_COM_COM_DasWohltem_004_00816.xml')
   
Now the piece will be displayed with MuseSore:


   sBach.show()			

Here's how to see the score information in musicxml format:

   sBach.show('text') 

And this is how the score is played as midi:

   sBach.show('midi') 			


Selection of bars / parts:
   a = sBach.parts[0] 		
   # selects the first voice (here: right hand) as variable 'a'. 
   b = sBach.measures(4,7)		
   # chooses measure 4 to 7 as variable 'b'
   c = sBach.parts[0].measures(2,4)	
   # chooses bar 2 to 4 of the first voice as 'c'
   Alternative: sBach.measures(2,4).show()
   # selects and displays bar 2 to 4.

Show number of elements:
   len(sBach)	
   # shows the number of elements on the next lower level
   len(sBach.flat)	
   # names the number of all contained elements, e.g. notes

Save the selection:
   sBach.write()
   a.write()	
   # saves the variables 'sBach' or 'a' (see above: the first voice) as xml-file (default)
   # Other formats can be chosen, e.g. 
   sBach.write('midi')
   # alternative command for saving the first voice:
   sBach.parts[0].write()



(2) View graphs: 

   sBach.plot('pianoroll')		
   sBach.parts[2].plot('pianoroll')		
   # only the second voice is displayed as pianoroll
   sBach.measure(2,5).plot('pianoroll')	
   # only measures 2-5 are displayed
   graph.plotStream(sBach) 			
   # = alternative method
	
   sBach.plot('histogram')			
   # histogram of pitch frequencies


(3) Writing notes 

   n1 = note.Note('g4', type='quarter')
   n2 = note.Note('a4', type='quarter')
   n3 = note.Note('g4', type='half')
   # notes are connected ("streamed") to measures, measures to parts and parts to scores:
   m1 = stream.Measure()
   m1.append([n1, n2, n3])
   p1 = stream.Part
   p1.append([m1])