<measure number='30' implicit='yes'>
    <print new-page='yes'/>
    <barline location='left'>
        <bar-style>heavy-light</bar-style>
        <repeat direction='forward'/>
    </barline>
    <attributes>
        <key>
            <fifths>-1</fifths>
            <mode>major</mode>
        </key>
    </attributes>
    <direction>
        <direction-type>
            <dynamics  default-y='-82'>
                <p/>
            </dynamics>
        </direction-type>
        <staff>1</staff>
    </direction>
    <direction>
        <direction-type>
            <words default-y='15' relative-x='4'>
        </direction-type>
        <staff>1</staff>
    </direction>
    <note>
        <pitch>
            <step>F</step>
            <octave>5</octave>
        </pitch>
        <duration>768</duration>
        <voice>1</voice>
        <type>quarter</type>
        <stem>down</stem>
        <staff>1</staff>
        <notations>
            <ornaments>
                <trill-mark default-y='20'/>
                <wavy-line type='start' number='1'/>
                <wavy-line type='stop' number='1'/>
            </ornaments>
        </notations>
    </note>
</measure>

from music21 import *
s = converter.parse(filepath)
# assuming key changes are the same in all parts, just get the first part
p = s.parts[0]
pFlat = p.flat
keySigs = pFlat.getElementsByClass('KeySignature')
for k in keySigs:
    print k.measureNumber

//measure[attributes/key]/@number