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Andrew, what does this do on Unix?  I don't *want* copyfile() 
to clobber read-only files, so I view this report as a small lack 
in the docs (which unconditionally say dst will be replaced).  
If the user lacks sufficient write permissions on Unix (a decent 
approximation to Windows "read only" state), does copyfile() 
succeed anyway?  I sure hope not.  On Windows,

    fdst = open(dst, 'wb')

dies with a (thoroughly appropriate, according to me)

    IOError: [Errno 13] Permission denied: <dst_value>

when dst is read-only.