Help understanding the decisions *behind* python?

[Phillip B Oldham]

My colleagues and I have been working with python for around 6 months
now, and while we love a lot of what python has done for us and what
it enables us to do some of the decisions behind such certain
data-types and their related methods baffle us slightly (when compared
to the decisions made in other, similarly powerful languages).

Specifically the "differences" between lists and tuples have us
confused and have caused many "discussions" in the office. We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.

Consider the following:

x = [2,1,3]
x.sort()
print x

[1, 2, 3]

Now, if the sort operations were unable to affect the original
structure of the list (as in JavaScript) you'd effectively have a
tuple which you could add/remove from, and the example above would
look more like:

x = [2,1,3]
print x.sort() [1, 2, 3]
print x

[2,1,3]

This make a lot more sense to us, and follows the convention from
other languages. It would also mean chaining methods to manipulate
lists would be easier:

x = [2,1,3]
print x.sort()[0] 3
print x

[2,1,3]

We often find we need to do manipulations like the above without
changing the order of the original list, and languages like JS allow
this. We can't work out how to do this in python though, other than
duplicating the list, sorting, reversing, then discarding.

We're not looking to start any arguments or religious wars and we're
not asking that python be changed into something its not. We'd simply
like to understand the decision behind the lists and tuple structures.
We feel that in not "getting" the difference between the two types we
may be missing out on using these data structures to their full
potential.

 

[Anthony Tolle]

...
Specifically the "differences" between lists and tuples have us
confused and have caused many "discussions" in the office. We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.
...

There's no hard-set rules, but tuples are typically used to hold
collections of heterogeneous data, e.g. (10, "spam", 3.21). As has
been mentioned, such a tuple can be used as a dictionary key, whereas
a list cannot be used as a dictionary key, because it is mutable.

Lists, on the other hand, typically hold collections of homogeneous
data, e.g. [1, 2, 5] or ["spam", "eggs", "sausage"].

Of course, you'll also see plenty of examples of tuples containing
homogeneous data and lists containing heterogeneous data

 

[Phillip B Oldham]

The main reason why you need both lists and tuples is that because a tuple
of immutable objects is itself immutable you can use it as a dictionary
key.

Really? That sounds interesting, although I can't think of any real-
world cases where you'd use something like that.

 

[J. Cliff Dyer]

Really? That sounds interesting, although I can't think of any real-
world cases where you'd use something like that.

Well, if you wanted to index a dictionary by coordinates, you might do
something like this:


fleet = {}
fleet[9,4] = 'destroyer'
fleet[8,4] = 'destroyer'
fleet[3,5] = 'aircraftcarrier'
fleet[4,5] = 'aircraftcarrier'
fleet[5,5] = 'aircraftcarrier'
fleet[6,5] = 'aircraftcarrier'
fleet[8,0] = 'battleship'
fleet[8,1] = 'battleship'
fleet[8,2] = 'battleship'


def checkattack(x, y, fleet):
if x,y in fleet:
return "You hit my %s' % fleet[x,y]

Maybe not the best implementation of Battleship, but you get the idea.

 

[Piet van Oostrum]

This make a lot more sense to us, and follows the convention from
other languages. It would also mean chaining methods to manipulate
lists would be easier:

x = [2,1,3]
print x.sort()[0]
3
print x
[2,1,3]

DB> You already have a way to do what you want:

x = [2,1,3]
print sorted(x)[0]

DB> 3

What kind of Python produces that?

 

[Chris Rebert]

This make a lot more sense to us, and follows the convention from
other languages. It would also mean chaining methods to manipulate
lists would be easier:

x = [2,1,3]
print x.sort()[0]
3
print x
[2,1,3]

DB> You already have a way to do what you want:

x = [2,1,3]
print sorted(x)[0]

DB> 3

What kind of Python produces that?

Assuming you're referring to the latter example, it was added in version 2.4
If you meant the former example, I think that's purely pseudo-Python.

Cheers,
Chris

 

[Paul Moore]

x = [2,1,3]
print sorted(x)[0]
DB> 3

What kind of Python produces that?

Assuming you're referring to the latter example, it was added in version 2.4
If you meant the former example, I think that's purely pseudo-Python.

I think he was referring to the fact that the result should be 1, not 3.

Paul.

 

[Hrvoje Niksic]

Really? That sounds interesting, although I can't think of any real-
world cases where you'd use something like that.

An application visiting files on a filesystem recursively needs a
dictionary or set keyed by (st_dev, st_ino) to make sure it doesn't
visit the same file twice. Memoization implementation (of a cache for
results of function application) might need to use a dictionary to map
function arguments, a tuple, to the function result.

 

[Hrvoje Niksic]

x = [2,1,3]
print sorted(x)[0]
DB> 3

What kind of Python produces that?

Assuming you're referring to the latter example, it was added in version 2.4
If you meant the former example, I think that's purely pseudo-Python.

sorted([2, 1, 3])[0] evaluates to 1, not 3.

 

[Niels L. Ellegaard]

We often find we need to do manipulations like the above without
changing the order of the original list, and languages like JS allow
this. We can't work out how to do this in python though, other than
duplicating the list, sorting, reversing, then discarding.

If you just want a one-liner, and you don't care about speed you can
do the following (but I don't think this is considered best practice)

x = [2,1,3]
print list(sorted(x)) [1, 2, 3]
print x

[2, 1, 3]

Niels

 

[Piet van Oostrum]

x = [2,1,3]
print sorted(x)[0] DB> 3

What kind of Python produces that?

Assuming you're referring to the latter example, it was added in version 2.4
If you meant the former example, I think that's purely pseudo-Python.

PM> I think he was referring to the fact that the result should be 1, not 3.

The underlying thought was that you should copy and paste examples from
a real Python interpreter.

 

[Hendrik van Rooyen]

Really? That sounds interesting, although I can't think of any real-
world cases where you'd use something like that.

simplest is something like a point in the cartesian plane with
an associated attribute like colour.

- Hendrik

 

[Inky 788]

[snip] We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.

My guess is that it was probably for optimization reasons long ago.
I've never heard a *good* reason why Python needs both.

 

[Piet van Oostrum]

HvR> simplest is something like a point in the cartesian plane with
HvR> an associated attribute like colour.

There are numerous other examples. Anytime you need a key that is not a
single object but composed of more than one:
Name + birthday
Street + number + City
Student + Course
etc.

 

[David Smith]

There are numerous other examples. Anytime you need a key that is not a
single object but composed of more than one:
Name + birthday
Street + number + City
Student + Course
etc.

Compound keys (like what's listed above) can also be used for sorting
lists of dictionaries using DSU style sorting. Something I believe (and
I could be wrong) won't work with mutable types like lists.

--David

 

[Piet van Oostrum]

HvR> simplest is something like a point in the cartesian plane with
HvR> an associated attribute like colour.

DS> Compound keys (like what's listed above) can also be used for sorting
DS> lists of dictionaries using DSU style sorting. Something I believe (and
DS> I could be wrong) won't work with mutable types like lists.

For sorting there is no problem with mutable arrays as long as you don't
mutate them during the sorting process (for example in the comparison
routine). Doing that would be extremely perverse. And there is no
enforcement of that.

 

[Simon Forman]

My colleagues and I have been working with python for around 6 months
now, and while we love a lot of what python has done for us and what
it enables us to do some of the decisions behind such certain
data-types and their related methods baffle us slightly (when compared
to the decisions made in other, similarly powerful languages).

Specifically the "differences" between lists and tuples have us
confused and have caused many "discussions" in the office. We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.

Consider the following:

x = [2,1,3]
x.sort()
print x

[1, 2, 3]

Now, if the sort operations were unable to affect the original
structure of the list (as in JavaScript) you'd effectively have a
tuple which you could add/remove from, and the example above would
look more like:

x = [2,1,3]
print x.sort() [1, 2, 3]
print x

[2,1,3]

This make a lot more sense to us, and follows the convention from
other languages. It would also mean chaining methods to manipulate
lists would be easier:

x = [2,1,3]
print x.sort()[0]

3

(As others have already pointed out this would print 1, not 3.)

[2,1,3]

We often find we need to do manipulations like the above without
changing the order of the original list, and languages like JS allow
this. We can't work out how to do this in python though, other than
duplicating the list, sorting, reversing, then discarding.

In this case you can use:

x = [2,1,3]
print min(x)

1

(There's also a max() function.)

We're not looking to start any arguments or religious wars and we're
not asking that python be changed into something its not. We'd simply
like to understand the decision behind the lists and tuple structures.
We feel that in not "getting" the difference between the two types we
may be missing out on using these data structures to their full
potential.

One way to think about tuples (as distinct from lists) is as a short-
hand version of Pascal's 'record' type, or C's 'struct' (with the
caveats that the fields are not named, only indexed, and the types of
the fields are implied by use, not explicitly declared.)

(FWIW: http://en.wikipedia.org/wiki/Record_(computer_science) )

Broadly speaking, lists are useful for things like stacks and queues,
and sorting, while tuples are useful for aggregating heterogeneous
data into coherent units, and you can hash them (provided their
contents are also hashable.)

HTH,
~Simon

 

[Hendrik van Rooyen]

[snip] We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.

My guess is that it was probably for optimization reasons long ago.
I've never heard a *good* reason why Python needs both.

The good reason is the immutability, which lets you use
a tuple as a dict key.

- Hendrik

 

[Steven D'Aprano]

I think Phillip is confused if he thinks that other languages can sort a
list without modifying or duplicating the original. If you don't
duplicate the list, how can you sort it without modifying the original?
The only difference is whether the language makes the duplicate for you,
or expects you to do it yourself.

If you just want a one-liner, and you don't care about speed you can do
the following (but I don't think this is considered best practice)

x = [2,1,3]
print list(sorted(x))

The call to list is redundant, as sorted() already creates a list.


But that's the wrong solution to the problem. The OP wants the largest
(or smallest) item, which he expects to get by sorting, then grabbing the
first element:

sorted(alist)[0]

That requires sorting the entire list, only to throw away everything
except the first item. A better solution is to use min() and max(),
neither of which sort the list, so they are much faster.

 

[Steven D'Aprano]

[snip] We
understand that lists are mutable and tuples are not, but we're a
little lost as to why the two were kept separate from the start. They
both perform a very similar job as far as we can tell.

My guess is that it was probably for optimization reasons long ago. I've
never heard a *good* reason why Python needs both.

Suppose you could do this:

key = [1, 2]
adict = {key: 'x', [1, 1]: 'y'}

This would work as expected:

adict[ [1, 2] ]
=> returns 'x'

adict[ [1, 1] ]
=> returns 'y'

But what happens if you mutate the key?

key[1] = 0
adict[ [1, 2] ]
=> raises KeyError

Okay, that's bad. What's even worse is this:

key[1] = 1
adict[ [1, 1] ]
=> should it return 'x' or 'y'?


The solution is to disallow mutable objects like lists from being used as
keys in dicts, and to allow immutable list-like tuples instead.