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##Python 2, 118 111 bytes ( 113 - 2 )

a,b=0,1;print a
f=open(__file__,'r+')
s=f.read()
s=s.replace(s[4:s.find(';')],`b`+','+`a+b`)
f.seek(0)
f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

Thanks to FlipTack for reminding me, close() is not mandatory.

Python 2, 118 111 bytes ( 113 - 2 )

a,b=0,1;print a
f=open(__file__,'r+')
s=f.read()
s=s.replace(s[4:s.find(';')],`b`+','+`a+b`)
f.seek(0)
f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

Thanks to FlipTack for reminding me, close() is not mandatory.

##Python 2, 118 111 bytes ( 113 - 2 )

a,b=0,1;print a
f=open(__file__,'r+')
s=f.read()
s=s.replace(s[4:s.find(';')],`b`+','+`a+b`)
f.seek(0)
f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

Thanks to FlipTack for reminding me, close() is not mandatory.

##Python 2, 118 111 bytes ( 113 - 2 )

a,b=0,1;print a
f=open(__file__,'r+')
s=f.read()
s=s.replace(s[4:s.find(';')],`b`+','+`a+b`)
f.seek(0)
f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

Thanks to FlipTack for reminding me, close() is not mandatory.

##Python 2, 118 bytes (120 - 2)

a,b=0,1;print a
with open(__file__,'r+')as f:s=f.read();s=s.replace(s[4:s.find(';')],`b`+','+`a+b`);f.seek(0);f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

##Python 2, 118 111 bytes ( 113 - 2  )

a,b=0,1;print a
f=open(__file__,'r+') 
s=f.read()
s=s.replace(s[4:s.find(';')],`b`+','+`a+b`)
f.seek(0)
f.write(s)

It works with any valid filename. There is not much to explain here, the code itself is very verbose.

Thanks to FlipTack for reminding me, close() is not mandatory.