On Sun, Feb 23, 2014 at 3:06 PM, Sam <lightai...@gmail.com> wrote:
> My understanding of Python tuples is that they are like immutable lists. If
> this is the cause, why can't we replace tuples with lists all the time (just
> don't reassign the lists)? Correct me if I am wrong.
>
One reason is performance/efficiency. If Python knows this is never
going to change, it can save some effort. But a more important reason
is hashability.
>>> mapping = {}
>>> key = (1,2)
>>> mapping[key] = "Hello"
>>> key = (1,3)
>>> mapping[key] = "World"
>>> key = (2,3)
>>> mapping[key] = "!"
>>> mapping
{(1, 2): 'Hello', (1, 3): 'World', (2, 3): '!'}
You can't do this with lists:
>>> key = [1,2]
>>> mapping[key]
Traceback (most recent call last):
File "<pyshell#12>", line 1, in <module>
mapping[key]
TypeError: unhashable type: 'list'
This is because any two tuples are either equal or not equal, they
can't be otherwise. I can create another tuple and look up something
in the above mapping:
>>> mapping[1,3]
'World'
But with lists, their equality can change.
>>> lst1 = [1,2]
>>> lst2 = [1,3]
>>> lst1 == lst2
False
>>> lst1[1] += 1
>>> lst1 == lst2
True
>>> lst1[1] += 1
>>> lst1 == lst2
False
So it would be very difficult and dangerous to try to use a list as a
dictionary key. The only safe way to do it is by identity, and that's
only useful in a very small set of situations. So Python happily
declares that a tuple can be a dict key and a list can't, and that's
now a key (if you'll excuse the pun) difference between them.
ChrisA
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