Slight discrepancy with filecmp.cmp

[Ivan Van Laningham]

Hi All--

John Machin wrote:
> 
> On Sun, 17 Apr 2005 22:06:04 -0600, Ivan Van Laningham
> <ivanlan at pauahtun.org> wrote:
> [snip]
> > So I wrote a set of
> >programs to both index the disk versions with the cd versions, and to
> >compare, using filecmp.cmp(), the cd and disk version.  Works fine.
> >Turned up several dozen files that had been inadvertantly rotated or
> >saved with the wrong quality, various fat-fingered mistakes like that.
> >
> >However, it didn't flag the files that I know have bitrot.  I seem to
> >remember that diff uses a checksum algorithm on binary files, not a
> >byte-by-byte comparison.  Am I wrong?
> 
> According to the docs:
> 
> """
> cmp( f1, f2[, shallow[, use_statcache]])
> 
> Compare the files named f1 and f2, returning True if they seem equal,
> False otherwise.
> Unless shallow is given and is false, files with identical os.stat()
> signatures are taken to be equal
> """
> 
> and what is an os.stat() signature, you ask? So did I.
> 
> According to the code itself:
> 
> def _sig(st):
>     return (stat.S_IFMT(st.st_mode),
>             st.st_size,
>             st.st_mtime)
> 
> Looks like it assumes two files are the same if they are of the same
> type, same size, and same time-last-modified. Normally I guess that's
> good enough, but maybe the phantom bit-toggler is bypassing the file
> system somehow. What OS are you running?
> 

WinXP, SP2

> You might like to do two things: (1) run your comparison again with
> shallow=False (2) submit a patch to the docs.
> 

You know, I read that doc, tried it, and it made absolutely no
difference.  Then I read your message, read the docs again, and finally
realized I had flipped the sense of shallow in my head.  Sheesh.  So
then I tried it with shallow=False, not True, and it runs about ten
times slower, but it works.  Beautifully.

Now I have to go back and redo the first five thousand, but it's worth
it.  Thanks.  Shows how much you need another set of eyeballs to debug
your brain;-)

> (-:
> You have of course attempted to eliminate other variables by checking
> that the bit-rot effect is apparent using different display software,
> a different computer, an observer who's not on the same medication as
> you, ... haven't you?
> :-)
> 

;-)  Absolutely.  Several different viewers and several different OSs. 
And my wife never sees anything the way I do;-)

Metta,
Ivan
----------------------------------------------
Ivan Van Laningham
God N Locomotive Works
http://www.andi-holmes.com/
http://www.foretec.com/python/workshops/1998-11/proceedings.html
Army Signal Corps:  Cu Chi, Class of '70
Author:  Teach Yourself Python in 24 Hours