import re
print( re.sub(r'("[^"]*")|,',
lambda x: x.group(1) if x.group(1) else x.group().replace(",", ""),
'1,2,"test,3,7","4, 5,6, ... "') )
# => 12"test,3,7""4, 5,6, ... "
print( re.sub(r'(?s)("[^"\\]*(?:\\.[^"\\]*)*")|,',
lambda x: x.group(1) if x.group(1) else x.group().replace(",", ""),
r'1,2,"test, \"a,b,c\" ,03","4, 5,6, ... "') )
# => 12"test, \"a,b,c\" ,03""4, 5,6, ... "
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