User’s Guide, Chapter 26: Stream Iteration and Filtering

We learned enough about streams in Chapter 6 to be able to get started, but you’ve preservered and hopefully are ready to learn more about how to get the most out of getting through a score. So this chapter will delve deeper into the concept of iteration, that is, going through an object one step at a time, and filtering out elements so only those in classes or areas you want are found. Let’s review and describe the concept of iteration in Python (or most programming languages) for a second.

Suppose you had a list like this:

letterList = ['a', 'b', 'c']

Now you could get your ABCs out of it in this way:

alphabet = ''
alphabet += letterList[0]
alphabet += letterList[1]
alphabet += letterList[2]

But it’s far easier, especially for a big list, to iterate over it using a for loop:

alphabet = ''
for letter in letterList:
    alphabet += letter


We can iterate over a list because lists are iterable (or, conversely, for the tautology department, because we can iterate over a list, we call it iterable) and there are some functions and methods that do great things on iterable objects, such as join them:


Or give the minimum value from a numeric list:

min([10, 20, 30, -3423, 40])

Or give the length of an iterable:


In Python, there’s a special type of iterable object called a generator which gives out objects as they are needed. One generator that we have seen already is the range() function:

zeroToFifty = range(51)
 range(0, 51)

We can find the first number in that range that is divisible by 5:

for n in zeroToFifty:
    if n != 0 and n % 5 == 0:

At this point we’ve stopped going through the range object and no more numbers are ever made or stored in memory – this point doesn’t matter to much for a set of numbers up to 50, but for numbers up to millions, or, as we will see, a repertory of scores of hundreds of thousands of notes, saving a few seconds here and there really adds up.

Streams, as we have seen, are iterable:

from music21 import *

s = stream.Part(id='restyStream')
s.append(note.Note('D', quarterLength=1.5))

for thing in s:
    print(thing, thing.quarterLength)
 <music21.note.Note C#> 1.0
 <music21.note.Rest half> 2.0
 <music21.note.Note D> 1.5
 <music21.note.Rest quarter> 1.0

When you iterate over a Stream, it is actually creating a lightweight object called a StreamIterator to help make things easier. We can create one directly by calling .iter() on any stream:

sIter = s.iter()
 < for Part:restyStream @:0>


Prior to v.7, a StreamIterator was a property .iter.

This information tells us that sIter is an iterator going over the Part object with id restyStream and it is currently ready to give out the first object, number 0. We can get the next thing in the Stream by calling next() on the Stream.

 <music21.note.Note C#>
 <music21.note.Rest half>
 < for Part:restyStream @:2>

But for the most part, you’ll want to use the built in way of going through an iterable, that is, with a for loop:

for el in sIter:
    print(el, el.quarterLength)
 <music21.note.Note C#> 1.0
 <music21.note.Rest half> 2.0
 <music21.note.Note D> 1.5
 <music21.note.Rest quarter> 1.0

Filtering elements in iteration

So this does exactly what iterating directly on the Stream does – but it’s good to know that a StreamIterator is silently being generated so that you can see what else these Iterators do. Most importantly, a StreamIterator can add filters to it. Let’s add a ClassFilter from the module:

restFilter = stream.filters.ClassFilter('Rest')
restIterator = sIter.addFilter(restFilter)
for el in restIterator:
    print(el, el.quarterLength)
 <music21.note.Rest half> 2.0
 <music21.note.Rest quarter> 1.0

Now when we go through sIter, we are only getting those objects that match all of the filters on it. We can also filter by offset. Let’s create a new iterator and add an OffsetFilter to it.

sIter2 = s.iter()
offsetFilter = stream.filters.OffsetFilter(offsetStart=0.5, offsetEnd=4.0)
offsetIterator = sIter2.addFilter(offsetFilter)
for el in offsetIterator:
    print(el, el.offset)
 <music21.note.Rest half> 1.0
 <music21.note.Note D> 3.0


prior to Music21 v.6, sIter.addFilter() would modify sIter in place and not return a new iterator. Thus in v.5, you would have written the last three lines of the code as:

>>> sIter2.addFilter(offsetFilter)
>>> for el in sIter2:
...     print(el, el.offset)

The changed behavior in v.6 did not affect most users, but it was one of the biggest backward incompatible changes – it was worth breaking code to finally get this right.

Multiple filters can be chained together to get something more powerful:

for el in s.iter().addFilter(restFilter).addFilter(offsetFilter):
    print(el, el.offset)
 <music21.note.Rest half> 1.0

Other filters that music21 has in the include:

  • IsFilter which returns elements that are exactly the same as the objects passed in (useful for getting the context of an object in a stream)

  • IsNotFilter, even more useful, for getting everything but an object or list of objects

  • IdFilter for finding items by Id.

  • ClassNotFilter for finding items other than a list of classes.

  • and GroupFilter for finding elements which have a particular group name.

Filter Shortcuts

Filtering elements by offset or by class is so common, that music21 has some shortcuts for adding filters to it, like this:

sIter4 = s.iter()
restIterator = sIter4.getElementsByClass('Rest')
restOffsetIterator = restIterator.getElementsByOffset(0.5, 4.0)
for el in restOffsetIterator:
    print(el, el.offset)
 <music21.note.Rest half> 1.0

Easier still, since each of these methods returns a new filter object, you can chain them right in the for loop:

for el in s.iter().getElementsByClass('Rest').getElementsByOffset(0.5, 4.0):
    print(el, el.offset)
 <music21.note.Rest half> 1.0

And you can even skip the s.iter() step for getting an iterator for the most common of these filters, and music21 will recognize what you want to do and create the iterator for you:

for el in s.getElementsByClass('Rest').getElementsByOffset(0.5, 4.0):
    print(el, el.offset)
 <music21.note.Rest half> 1.0

The shortcut methods that music21 exposes on Iterators include:

And there are also properties (that is, written without parentheses) which add certain filters:

  • notes which filters out everything but Note and Chord objects

  • notesAndRests which filters out everything except GeneralNote objects

  • parts which returns all the Part objects

  • voices which returns all the Voice objects

  • spanners which returns all the Spanner objects

Custom Filters

Creating your own filter is pretty easy too. The easiest way is to create a function that takes in an element and returns True or False depending on whether the object matches the filter.

We will create a filter to see if the element has a .pitch attribute and then if that pitch attribute has a sharp on it:

def sharpFilter(el):
    if (hasattr(el, 'pitch')
            and el.pitch.accidental is not None
            and el.pitch.accidental.alter > 0):
        return True
        return False

sharpIterator = s.iter().addFilter(sharpFilter)

for el in sharpIterator:
 <music21.note.Note C#>

Recursive and Offset Iterators

Music21 comes with two other iterators that let you do powerful operations. The most commonly used is the RecursiveIterator which burrows down into nested Streams to get whatever you want. Let’s load in a nested stream:

bach = corpus.parse('bwv66.6')
for thing in bach:
 <music21.metadata.Metadata object at 0x10f4cb6d0>
 < Soprano>
 < Alto>
 < Tenor>
 < Bass>
 <music21.layout.StaffGroup < Soprano>< Alto>< Tenor>< Bass>>

Right, we remember that often the actual notes of a piece can be hidden inside Parts, Measures, and Voices. A recursive iterator gets to them, and they’re created by calling recurse() on a stream.

recurseIter = bach.recurse()
 < for Score:bach/bwv66.6.mxl @:0>

Let’s add a filter for only E#s to it, and look into it. Instead of checking to see if each element has a .name attribute we’ll put a try...except clause around it, and if it does not have the .name attribute (and thus raises and AttributeError we will return False.

def eSharpFilter(el):
        if == 'E#':
            return True
            return False
    except AttributeError:
        return False

eSharpIterator = recurseIter.addFilter(eSharpFilter)

for el in eSharpIterator:
    print(el, el.measureNumber)
 <music21.note.Note E#> 9
 <music21.note.Note E#> 3
 <music21.note.Note E#> 7
 <music21.note.Note E#> 7
 <music21.note.Note E#> 2
 <music21.note.Note E#> 6

Note that the measure numbers don’t keep increasing. That’s because the recurse iterator finishes one part before returning to the next. We can use the fancy .getContextByClass to figure out what part it is in:

for el in eSharpIterator:
    pId = el.getContextByClass(stream.Part).id
    print(el, el.measureNumber, pId)
 <music21.note.Note E#> 9 Soprano
 <music21.note.Note E#> 3 Alto
 <music21.note.Note E#> 7 Alto
 <music21.note.Note E#> 7 Tenor
 <music21.note.Note E#> 2 Bass
 <music21.note.Note E#> 6 Bass

(as an aside, .measureNumber is just a shortcut for .getContextByClass(stream.Measure).number, so we are actually looking up two contexts)

If you want to recurse into a stream and get elements of a certain class, you can do s.recurse().getElementsByClass(chord.Chord) but there’s another simpler way of doing it: s[chord.Chord] (with square brackets). As this example shows:

chopin = corpus.parse('chopin/mazurka06-2')
chopinExcerpt = chopin.measures(1, 5)
for ch in chopinExcerpt[chord.Chord]:
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>
 <music21.chord.Chord G#2 D#3>

(when Chopin likes a chord, he really likes a chord!). Note that each of these is a chord in one voice in one hand of the piano. To see how to get chords between both hands, turn back to the chordify() chapter.

Another great iterator is the OffsetIterator, which returns lists of elements grouped by offset. Let’s add some more things to our Stream before we see how it works.

s.insert(0, clef.TrebleClef())
s.insert(0, key.KeySignature(3))
s.insert(1, instrument.Trumpet())

# normal iterator
for el in s:
    print(el, el.offset)
 <music21.clef.TrebleClef> 0.0
 <music21.key.KeySignature of 3 sharps> 0.0
 <music21.note.Note C#> 0.0
 Trumpet 1.0
 <music21.note.Rest half> 1.0
 <music21.note.Note D> 3.0
 <music21.note.Rest quarter> 4.5

Unlike with the normal StreamIterator or the RecursiveIterator, there is no method on Stream to create an offset iterator, so we will create one directly:

oIter = stream.iterator.OffsetIterator(s)
for elementGroup in oIter:
    print(elementGroup[0].offset, elementGroup)
 0.0 [<music21.clef.TrebleClef>, <music21.key.KeySignature of 3 sharps>, <music21.note.Note C#>]
 1.0 [<music21.instrument.Trumpet 'Trumpet'>, <music21.note.Rest half>]
 3.0 [<music21.note.Note D>]
 4.5 [<music21.note.Rest quarter>]

From Iterator to Stream

From either a StreamIterator or a RecursiveIterator a new Stream object can be generated by calling .stream() on it. On a RecursiveIterator, this does not put the elements into substreams.

onlyESharps = bach.recurse().addFilter(eSharpFilter)
esharpStream ='text')
 {8.0} <music21.note.Note E#>
 {10.0} <music21.note.Note E#>
 {23.0} <music21.note.Note E#>
 {25.5} <music21.note.Note E#>
 {27.0} <music21.note.Note E#>
 {34.5} <music21.note.Note E#>
 <Derivation of < bach/bwv66.6.mxl> from < bach/bwv66.6.mxl> via 'eSharpFilter'>

This can be useful if you’d like to do plots on the resulting stream, though this one is a bit too obvious…


But maybe this one could tell someone something:


Perhaps not. But iterators are not the main point – what you can do with them is more important, so we will return to working with actual musical objects in Chapter 27 when we talk about Grace Notes.