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Re: A sets algorithm

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From Tim Chase <python.list@tim.thechases.com>
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Subject Re: A sets algorithm
Date Sun, 7 Feb 2016 18:20:50 -0600
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On 2016-02-08 00:05, Paulo da Silva wrote:
> Às 22:17 de 07-02-2016, Tim Chase escreveu:
>>   all_files = list(generate_MyFile_objects())
>>   interesting = [
>>     (my_file1, my_file2)
>>     for i, my_file1
>>     in enumerate(all_files, 1)
>>     for my_file2
>>     in all_files[i:]
>>     if my_file1 == my_file2
>>     ]
> 
> "my_file1 == my_file2" can be implemented into MyFile class taking
> advantage of caching sizes (if different files are different),
> hashes or even content (for small files) or file headers (first n
> bytes). However this seems to have a problem:
> all_files: a b c d e ...
> If a==b then comparing b with c,d,e is useless.

Depends on what the OP wants to have happen if more than one input
file is equal. I.e., a == b == c.  Does one just want "a has
duplicates" (and optionally "and here's one of them"), or does one
want "a == b", "a == c" and "b == c" in the output?

> Another solution I thought of, could be defining some methods (I
> still don't know which ones) in MyFile so that I could use sets
> intersection. Would this one be a faster solution?

Adding __hash__ would allow for the set operations, but would
require (as ChrisA points out) knowing how to create a hash function
that encompasses the information you want to compare.

-tkc

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Thread

A sets algorithm Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2016-02-07 21:46 +0000
  Re: A sets algorithm Chris Angelico <rosuav@gmail.com> - 2016-02-08 08:58 +1100
  Re: A sets algorithm Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-02-07 22:03 +0000
  Re: A sets algorithm Tim Chase <python.list@tim.thechases.com> - 2016-02-07 16:17 -0600
    Re: A sets algorithm Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2016-02-08 00:05 +0000
      Re: A sets algorithm Tim Chase <python.list@tim.thechases.com> - 2016-02-07 18:20 -0600
  Re: A sets algorithm Cem Karan <cfkaran2@gmail.com> - 2016-02-07 20:07 -0500
  Re: A sets algorithm Paulo da Silva <p_s_d_a_s_i_l_v_a_ns@netcabo.pt> - 2016-02-08 02:22 +0000
  Re: A sets algorithm Random832 <random832@fastmail.com> - 2016-02-08 09:49 -0500
  Re: A sets algorithm Chris Angelico <rosuav@gmail.com> - 2016-02-09 02:11 +1100
    Re: A sets algorithm Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-02-09 15:13 +1100
      Re: A sets algorithm Chris Angelico <rosuav@gmail.com> - 2016-02-09 15:27 +1100
        Re: A sets algorithm Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-02-09 17:48 +1300

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