Python Interview Questions

Roy Smith roy at
Tue Nov 20 04:14:39 CET 2012

In article <50aac66c$0$29983$c3e8da3$5496439d at>,
 Steven D'Aprano <steve+comp.lang.python at> wrote:

> I'm asking about the case where one might want the key to remain mutable 
> even after it is used as a key, but can't because Python won't let you.

Ah.  Now I see what you're getting at.  Thank you.

Well, I will admit that it probably doesn't make sense to mutate an 
object after it's put into a dict (or at least mutate it in a way which 
changes it's hash value and/or whether it compares equal to the original 
object).  If you did (assuming lists were allowed as keys):

l = [1, 2, 3]
d = {l: "spam"}
print d[l]

I'm not sure what I would expect to print.  It's not too hard to 
experiment, though.  All you need do is:

class HashableList(list):
    def __hash__(self):
        return hash(tuple(self))

and python is then happy to let you use a list as a key.  I just played 
around with this a bit off-line.  I think I got the results I was 
expecting, but since I'm not sure what I was expecting, that's hard to 

However, you didn't ask if it made sense to mutate an object after using 
it as a key.  You asked if it made sense to let the object remain 
mutable after using it as a key.  That's a harder question.

Let's say I had lots of of lists I wanted to use a dictionary keys.  As 
it stands now, I have to convert them to tuples, which means copying all 
the data.  For a lot of data, that's inefficient.

Wouldn't it be nice (or at least, more efficient) if I could just use 
the original lists as keys directly, without the extra copy?  I would 
have to understand that even though they are mutable, interesting (and 
perhaps, unwanted) things if I actually mutated them.  But, we're all 
consenting adults here.  If I'm willing to accept responsibility for the 
consequences of my actions in return for the efficiency gain, why 
shouldn't I be allowed to?

I guess the answer is, that I am allowed to.  I just need to do the 
HashableList deal, shown above (no broken URL required to read the code).

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