Tuple question

Benjamin Niemann pink at odahoda.de
Sun Sep 5 14:37:31 CEST 2004


Roy Smith wrote:
> "Donn Cave" <donn at drizzle.com> wrote:
> 
>>I expect it's used relatively infrequently, and for different
>>reasons.  "if len(info) == 5", for example - just from that
>>line from a relatively popular Python application, would you
>>guess info is a list, or a tuple?
> 
> 
> I'd guess it was something which had a __len__ method.
> 
> 
>>Maybe the problem is that tuples have too many features already.
>>It's sort of silly that they're indexed by number, and if that
>>weren't allowed, we would find fewer people trying to make lists
>>of them.
> 
> 
> If tuples weren't indexed, the only way you'd be able to access the 
> elements would be to unpack them, which would be rather inconvenient.  
> Unless of course you had an alternate way to name the elements, but if 
> you're going to allow named element access, and forbid indexed access, 
> then you might as well just create a normal class instance.
> 
> The more I look into this, the more I realize just how inconsistent the 
> whole thing is.
> 
> For example, tuples can be used as dictionary keys because they are 
> immutable.  Or so it's commonly said.  But, that's not true.  The real 
> reason they can be used as keys is because they're hashable.  If you try 
> to use a list as a key, it doesn't complain that it's immutable, it 
> complains that it's unhashable:
> 
> 
>>>>d = {}
>>>>d[[1, 2]] = 3
> 
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: list objects are unhashable
> 
> Furthermore, regular objects can be used as keys, even though they *are* 
> mutable.  You can do this:
> 
> class data:
>     pass
> 
> key = data ()
> key.x = 1
> key.y = 2
> 
> d = {}
> d[key] = None
> 
> dictKey = d.keys()[0]
> print dictKey.x, dictKey.y
> 
> key.x = 42
> dictKey = d.keys()[0]
> print dictKey.x, dictKey.y
> 
> If a mutable class instance object can be used as a dictionary key, then 
> I don't really see any reason a list shouldn't be usable as a key.  How 
> is a class instance's mutability any less of disqualifier for key-ness 
> than a list's mutability?
> 
> And, once you allow lists to be keys, then pretty much the whole raison 
> d'etre for tuples goes away.  And if we didn't have tuples, then we 
> wouldn't have to worry about silly syntax warts like t = (1,) to make a 
> 1-tuple :-)

A very handy feature of lists is:

a = [1, 2, 3]
b = [1, 2, 3]
if a == b:
	print "List equality is based on content"

while:

a = data()
a.x = 42
b = date()
b.x = 42
if a != b:
	print "Other objects have an identity that is independent of\
content"

This special behaviour of lists is implemented by implementing the 
__eq__ method. Objects with non-standard __eq__ usually don't have the 
expected behaviour when used as keys:

a = [1, 2, 3]
b = [1, 2, 4]
d = {}
d[a] = "first"
d[b] = "second"
a[2] = 4
b[2] = 3
print d[[1, 2, 3]]

Which result would you expect here?



More information about the Python-list mailing list