[issue38789] difflib lacks a way to check if results are empty

[Tim Peters]

Tim Peters  added the comment:

I'm taking Raymond's advice to close this for now.  The issue tracker isn't the 
right place to work out ideas - python-ideas is far better for that 
(StackOverflow isn't a good place for that either - StackOverflow is best for 
when you have a very specific use case and get stuck).

While the issues with generators are common to all generators, in the context 
of difflib something else Raymond said should be taken to heart:  using any 
difflib facility is an extremely expensive way to find out that two things are 
equal.  That's a value of use cases!  That is, if you had asked about this on 
StackOverflow and asked for help instead of proposing "a (vague) solution", 
they would have told you at once to check whether `a == b` before dragging 
difflib into it.

Indeed, that's probably why what you're asking about never came up before.  
People generally use difflib only when they know in advance (via a cheap 
equality test) that there _are_ differences to be found.

In any case, if a "specific & actionable" suggestion comes out of pursuing 
this, feel encouraged to open this report again!

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue38789] difflib lacks a way to check if results are empty

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Simon, I think the conversation is starting to drift and would best be 
continued on python-ideas or StackOverflow.  Ideas like peekable generators 
have been discussed before but there was almost no uptake.

Several thoughts:

* For equal inputs, ndiff() is supposed to generate non-empty output.  It does 
not just give differences.

* To the extent that you care about empty results from some other iterator, the 
easiest thing to do is follow Tim's advice and just list() the iterator.

* The special case of equal inputs is easily handled before running the diff:

 if a == b:
 do_something_for_the_equal_case(a, b)
 else:
 d = ndiff(a, b)
 do_something_for_the_non_equal_case(a, b, d)

I recommend closing this issue because it hasn't elicted anything that is both 
actionable and desireable.

--
nosy: +rhettinger

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[issue38789] difflib lacks a way to check if results are empty

[Simon Friedberger]

Simon Friedberger  added the comment:

And, just to state this explicitly, I think you are right that there are 
general idioms for checking if a generator can produce an item but I think it 
would be nicer if iterators which could do this is in a cheap way (like in this 
case) would allow it explicitly.

I don't know, maybe I'm wrong. Just seems nice. :)

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[issue38789] difflib lacks a way to check if results are empty

[Simon Friedberger]

Simon Friedberger  added the comment:

Hi Tim!

Sorry, if my explanation wasn't clear.

For some of the iterators - like the one produced by ndiff - the iterator will 
always return data, even if there is no difference in the files.

My current solution is to run difflib.unified_diff and check if the iterator is 
non-empty and then run difflib.ndiff again to get the output that I want.

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[issue38789] difflib lacks a way to check if results are empty

[Tim Peters]

Tim Peters  added the comment:

Please be explicit:  exactly which functions are you talking about, and exactly 
what do you want them to do instead.  Since, best I can tell, this is the first 
complaint of its kind, it's a pretty safe bet people can't guess what you want 
;-)

Note that, e.g., Differ(...).compare(...) returns a generator-iterator.  There 
is no general way in Python to know whether a generator will yield a non-empty 
sequence of results without running the generator.  This is common to all 
generators, not unique to those difflib returns.

So, of course, the same kinds of idioms can be used as for any other generator. 
 For example:

foundone = False
for line in difflib.Differ(...).compare(...):
if not foundone:
# there is at least one result, and this is the first
# maybe print a header line here, or whatever
foundone = True
process(line)
if not foundone:
# the generator produced nothing

Simpler to code is to force the results into a list instead, but then you lose 
the possible memory-saving advantages of iterating over a generator:

result = list(difflib.Differ(...).compare(...))
if result:
# there are results to deal with
else:
# the generator produced nothing

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[issue38789] difflib lacks a way to check if results are empty

[Raymond Hettinger]

Change by Raymond Hettinger :


--
assignee:  -> tim.peters
nosy: +tim.peters
versions: +Python 3.9 -Python 3.7

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[issue38789] difflib lacks a way to check if results are empty

[Simon Friedberger]

New submission from Simon Friedberger :

It seems there is no easy way to use difflib to show a diff but only when there 
actually are differences. The SequenceMatcher has ratio() but that isn't really 
available through Differ or any of the convenience functions. Vice versa, when 
using SequenceMatcher the pretty display is not available.

--
components: Library (Lib)
messages: 356536
nosy: simon_
priority: normal
severity: normal
status: open
title: difflib lacks a way to check if results are empty
versions: Python 3.7

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[issue38789] difflib lacks a way to check if results are empty

[Simon Friedberger]

Change by Simon Friedberger :


--
type:  -> enhancement

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