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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: PEP 8 : Maximum line Length :
Date: Thu, 15 May 2014 18:21:27 -0400
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On 5/15/2014 9:58 AM, Rustom Mody wrote:

> As far as I can see the votaries of the mystical 79 have yet to explain
> how/where it appeared from

As has been explained before, and is implied in the PEP, 79 = 80 - 1.
80 chars - 1 character width cursor leaves 79 non-cursor characters. 
When <enter> is hit, the cursor replaced with \n. So you can think of 79 
as 79 visible + \n = 80.

PEP 8 says "The limits are chosen to avoid wrapping in editors with the 
window width set to 80," and goes on about dynamic wrapping or not. To 
put is another way, all 80 char terminals work with 79 visible chars. 
Some but not all work with 80 visible, and some work with 80 on all 
lines except the last.

I admit that this would have been clearer when most everyone reading it 
would have had some experience with 80 char sceens (including DOS). 
Still, did you really not notice that 79 = 80 - 1?

-- 
Terry Jan Reedy