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Date: Thu, 07 Mar 2013 19:23:20 -0500
From: Dave Angel <davea@davea.name>
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Subject: Re: Unhelpful traceback
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On 03/07/2013 04:23 PM, John Nagle wrote:
>
>>   <snip>
>
>      raise RuntimeError, 'open() requires mode "r", "U", or "rU"'
> RuntimeError: open() requires mode "r", "U", or "rU"
>
>     "b" for files is about end of line handling (CR LF -> LF), anyway.
>

Only for Python 2.  Since originally you didn't specify, I took my best 
shot. If you omit the 'b' opening a binary file in Python 3, you'd get 
problems similar to yours.  Text files will be converted to Unicode.

That's one of the reasons that specifying the full environment is important.

-- 
DaveA