Invalid token?

Hi, I wrote this in python and do not understand the error message:

Here is my code:

"cmd.load(\'/home/lockhartag/Wim/onlyXray_geoff/$filenamein\')\n";
"f=open(\'/home/lockhartag/Wim/HIS/dih2/$filenameout\',\'w\')\n";
"a=cmd.get_dihedral(\'$chain/$resn/ca\',\'$chain/$resn/cb\',\'$chain/$resn/cg\',\'$chain/$resn/cd1\')\n";
"f.write(\" $chain $resn %8.3f\\n\"\\%a)\n";
"f.close()\n";
"cmd.delete('$filenamein')\n";

and the error is:

PyMOL>a=cmd.get_dihedral('A/138/ca','A/138/cb','A/138/cg','A/138/cd1')
GetDihedral-Error: Selection 4 doesn't contain a single atom/vertex.
PyMOL>f.write(" A 138 %8.3f\n"\%a)
Traceback (most recent call last):
File "/usr/local/bin/pymol/modules/pymol/parser.py", line 455, in parse
exec(layer.com2+"\n",self.pymol_names,self.pymol_names)
File "<string>", line 1
f.write(" A 138 %8.3f\n"\%a)
^
SyntaxError: invalid token


I think the error relates to the bolded line. Any suggestions?

My guess is the complaint is about the %a ending up outside the string, or that it can't work out the resulting \% as a meaningful token.