This patch modifies the SimpleHTTPServer to serve Last-Modified headers 
with its responses, which allows browsers to cache the result, preventing 
annoying reloads of image files every time you view a page, and so on.

Fixes 1417554.

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If the server doesn't handle the If-Modified-Since header on 
requests, does this really help much?  (I suppose it does 
give browsers something to use as a heuristic.)

But adding that to Simple doesn't really make sense.  Adding 
it to BaseHTTPServer might.

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Yes, it does help. RFC 2616 (HTTP/1.1 spec) says "HTTP caches typically 
assign heuristic expiration times, employing algorithms that use other header 
values (such as the Last-Modified time) to estimate a plausible expiration 
time" and indeed major browsers and caches actually do this.

Why doesn't it make sense to add this to SimpleHTTPServer? 
SimpleHTTPServer serves files from disk; this patch grabs the last-modified 
time for those files. It certainly wouldn't fit in BaseHTTP -- how would it know 
what the Last-Modified time would be?

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Sorry,  I wasn't clear enough.  

Adding the Last-Modified to responses created by 
SimpleServer makes perfect sense.

I had been (wrongly) thinking it was only useful in 
conjunction with a possible "304 Not Modified" response.  
Adding the 304 logic to a "simple" server may not fit, but 
adding it to BaseServer (and calling it from SimpleServer, 
which has the file's timestamp) would be reasonable.

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Thanks, I committed a similar patch (your date handling was
suboptimal).