> What if just use io.BufferedReader?
> 
>     if not isinstance(file, io.BufferedReader):
>         file = io.BufferedReader(file)
> 
> (at start of _Unpickler.__init__)

Two problems:

1. semantically, it is wrong; the BufferedReader will read bytes beyond
the pickle end, so the underlying stream will be desynchronized

2. performance-wise, it doesn't solve the issue either: read() method
calls are costly, even on an optimized C object