Equal sets with unequal print and str() representations

[Ganesh Gopalakrishnan]

This probably is known, but a potential pitfall (was, for me) 
nevertheless. I suspect it is due to hash collisions between 's3' and 
's13' in this case? It happens only rarely, depending on the contents of 
the set.


>>> S1 = {'s8', 's3', 's2', 's0', 's7', 's6', 's4', 's13', 's14'}
S1 = {'s8', 's3', 's2', 's0', 's7', 's6', 's4', 's13', 's14'}
>>> S2 = {'s8', 's13', 's2', 's0', 's7', 's6', 's4', 's3', 's14'}
S2 = {'s8', 's13', 's2', 's0', 's7', 's6', 's4', 's3', 's14'}
>>> S1
S1
{'s8', 's13', 's2', 's0', 's7', 's6', 's4', 's3', 's14'}
>>> S2
S2
{'s8', 's3', 's2', 's0', 's7', 's6', 's4', 's13', 's14'}
>>> S1==S2
S1==S2
True
>>> str(S1)
str(S1)
"{'s8', 's13', 's2', 's0', 's7', 's6', 's4', 's3', 's14'}"
>>> str(S2)
str(S2)
"{'s8', 's3', 's2', 's0', 's7', 's6', 's4', 's13', 's14'}"
>>> str(S1) == str(S2)
False

--
Ganesh

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Re: Equal sets with unequal print and str() representations

[Ganesh Gopalakrishnan]

Thanks to all who replied - also to Ben. I had foolishly assumed that 
the same set exhibits the same rep on at least one platform. Like any 
bug, the falsity of my assumption took months to expose - till then, 
things had worked fine. Needless to say I'm new to Python.  (The double 
printing is because I tend to work within an Emacs inferior shell.)


Cheers,

Ganesh

On 10/16/11 8:23 PM, Ben Finney wrote:

Ganesh Gopalakrishnan  writes:


This probably is known, but a potential pitfall (was, for me) nevertheless.
I suspect it is due to hash collisions between 's3' and 's13' in this
case?

What is the actual problem? What behaviour is occurring that doesn't
match your expectation?


S1==S2

S1==S2
True

str(S1)

str(S1)
"{'s8', 's13', 's2', 's0', 's7', 's6', 's4', 's3', 's14'}"

str(S2)

str(S2)
"{'s8', 's3', 's2', 's0', 's7', 's6', 's4', 's13', 's14'}"

str(S1) == str(S2)

False

Right, that's all correct (though I don't know why some of your
expressions are being shown twice). A deliberate property of a set is
that its items are unordered.
 http://docs.python.org/library/stdtypes.html#set>

Since sets are unordered, the string representation may show the items
in an arbitrary and unpredictable sequence. Don't write any code that
depends on a predictable sequence of retrieval from an unordered
collection.

So what did you expect instead, and what supports that expectation?


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