Compare recursively objects

[Nicolas Fleury]

Hi everyone,
Is there a way to compare recursively two objects (compare their
members recursively)? I'm only interested in equality or non-equality
(no need for lower-than...).

Thx and Regards,
Nicolas

[Paul McGuire]

Re: Compare recursively objects

"Nicolas Fleury" <nid_oizo@yahoo .com_remove_the _> wrote in message
news:LcZ5c.6697 $kc2.157952@nnr p1.uunet.ca...[color=blue]
> Hi everyone,
> Is there a way to compare recursively two objects (compare their
> members recursively)? I'm only interested in equality or non-equality
> (no need for lower-than...).
>
> Thx and Regards,
> Nicolas[/color]
If you're comparing lists, here's an example (although list1==list2 also
works, see the verification in the test() function). If you're comparing
something else, the logic is similar.

This example also does short-circuiting, in that it stops testing for
equality once the first mismatch is found. Also, there is a slight
short-cut in isEqualElement, in comparison of two objects for equal identity
before testing for equal value.

-- Paul


def compareLists(ls t1, lst2):

def isEqualElement( e1,e2):
print "Comparing",e1, "and",e2
if e1 is e2:
return True

if type(e1) is type(e2):
if isinstance(e1,l ist):
return compareLists(e1 ,e2)
else:
return e1 == e2
else:
return False

# or the same thing as a lambda, just remove '0' from name to
# use the lambda instead
isEqualElement0 = (lambda e1,e2:
(e1 is e2) or
( ( type(e1) is type(e2) ) and
( ( isinstance(e1,l ist) and compareLists(e1 ,e2) ) or
(e1 == e2) )
)
)

if len(lst1) != len(lst2):
return False

for e1,e2 in zip(lst1,lst2):
if not isEqualElement( e1,e2):
return False

return True


def test(lst1, lst2):
print "Test:", lst1,"<->",lst2
print compareLists(ls t1,lst2)
print lst1 == lst2
print


test([1,2,3], [1,2,3])
test([1,2,3], [1,2])
test([1,'2',3], [1,2,3])
test([1,2,[3,4,5],6], [1,2,[3,4,5],6])
test([1,2,[3,4],6], [1,2,[3,4,5],6])
test([1,2,[7,4,5],6], [1,2,[3,4,5],6])

[Nicolas Fleury]

Re: Compare recursively objects

Do you know if the use of pickle would work? If I dump two objects and
compare the files, do I have the assurance that the files are the same
if the recursive members are the same?

Regards,
Nicolas

[Paul McGuire]

Re: Compare recursively objects

"Nicolas Fleury" <nid_oizo@yahoo .com_remove_the _> wrote in message
news:gF_5c.6730 $kc2.158843@nnr p1.uunet.ca...[color=blue]
> Do you know if the use of pickle would work? If I dump two objects and
> compare the files, do I have the assurance that the files are the same
> if the recursive members are the same?
>
> Regards,
> Nicolas[/color]

I am not very familiar with pickle, other than its purpose for persisting
object representations for later reconstitution. Given that, I suspect your
pickle-to-file-and-compare-the-files scheme would work.

However, I would be wary of a scheme that routinely used files for
temporary/scratch storage in this manner. There are just too many ordinary
ways for this to go wrong:
- disk has insufficient room to create files
- insufficient user privs to create files
- trying to create files in non-existent or protected directory
- program exits before scratch files get cleaned up, leaving behind growing
debris
- concurrent users end up using each others' files by mistake; accidentally
creating duplicate scratch file names
Plus, file i/o is just plain agonizingly slow.

In sum, I suspect you will get diverted to dealing with a whole bunch of
issues that are artifacts of using files, when what you really wanted to do
is compare some objects. Perhaps you could pickle to a string and compare
the strings? At least this avoids most of the file system complications.

-- Paul

[Nicolas Fleury]

Re: Compare recursively objects

Paul McGuire wrote:[color=blue]
> In sum, I suspect you will get diverted to dealing with a whole bunch of
> issues that are artifacts of using files, when what you really wanted to do
> is compare some objects. Perhaps you could pickle to a string and compare
> the strings? At least this avoids most of the file system complications.[/color]

Sounds like a good idea. I am only in need of that functionality for
unit testing, so I guess pickle to StringIO is a nice idea.

Thx
Nicolas