# Comparison of functions

Discussion in 'Python' started by Steven D'Aprano, Jul 30, 2005.

1. ### Steven D'ApranoGuest

Playing around with comparisons of functions (don't ask), I discovered an
interesting bit of unintuitive behaviour:

>>> (lambda y: y) < (lambda y: y)

False

Do the comparison again and things become even more bizarre:

>>> (lambda y: y) < (lambda y: y)

True
>>> (lambda y: y) < (lambda y: y)

False

This behaviour should be easy for experienced Pythonisters to answer, but
will probably confuse beginners greatly.

For the benefit of beginners, imagine we expand the first line into two,
and commit what some people call an abuse of lambdas: we bind them to
names.

>>> a = lambda y: y
>>> b = lambda y: y
>>> a

<function <lambda> at 0xf70598ec>
>>> b

<function <lambda> at 0xf7059844>

a and b are now bound to two objects, each of which is an anonymous
function that just happens to do the same thing. But each time you create
a lambda function, you get a different object at some unpredictable
location in memory.

This is where my level of Python knowledge fails me. I don't understand
how Python is comparing the two objects since neither a nor b have any
rich comparison methods or even the old-style __cmp__ method.

>>> a < b

False

So I'm puzzled about how Python compares the two.

If we compare a and b again, we will always get the same answer. But if we
create a new pair of anonymous functions with lambda, and compare them, it
is the luck of the draw each time whether the first compares bigger or
smaller than the second.

--
Steven.

Steven D'Aprano, Jul 30, 2005

2. ### Peter HansenGuest

Steven D'Aprano wrote:
> Playing around with comparisons of functions (don't ask), I discovered an
> interesting bit of unintuitive behaviour:
>
>>>>(lambda y: y) < (lambda y: y)

> False
>
> Do the comparison again and things become even more bizarre:
>
>>>>(lambda y: y) < (lambda y: y)

> True
>
>>>>(lambda y: y) < (lambda y: y)

> False
>
> This behaviour should be easy for experienced Pythonisters to answer, but
> will probably confuse beginners greatly.

Beginners should not be comparing lambdas.

Neither should you. ;-)

-Peter

Peter Hansen, Jul 30, 2005

3. ### tiissaGuest

Steven D'Aprano wrote:
> Playing around with comparisons of functions (don't ask), I discovered an
> interesting bit of unintuitive behaviour:
>
>>>>a = lambda y: y
>>>>b = lambda y: y
>>>>a

> <function <lambda> at 0xf70598ec>
>>>>b

> <function <lambda> at 0xf7059844>
>>>>a < b

> False
>
> So I'm puzzled about how Python compares the two.

Seems to me the object addresses are compared in this case. But I'm too
lazy to check it in the source.

However, the doc [1] warns you about such comparisons:
"""Most other types compare unequal unless they are the same object; the
choice whether one object is considered smaller or larger than another
one is made arbitrarily but consistently within one execution of a
program."""

[1] http://docs.python.org/ref/comparisons.html

tiissa, Jul 30, 2005

Steven D'Aprano ha scritto:
> On Sat, 30 Jul 2005 08:13:26 -0400, Peter Hansen wrote:
>
>
>>Beginners should not be comparing lambdas.
>>
>>Neither should you. ;-)

>
>
> Actually, yes I should, because I'm trying to make sense of the mess that
> is Python's handling of comparisons. At least two difference senses of
> comparisons is jammed into one, leading to such such warts as these:
>
>
>>>>L = []
>>>>L.sort() # we can sort lists
>>>>L.append(1+1j)
>>>>L.sort() # even if they include a complex number
>>>>L.append(1)
>>>>L.sort() # but not any more

>
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: cannot compare complex numbers using <, <=, >, >=
>
> Um, I didn't ask to compare complex numbers using comparison operators. I
> asked to sort a list. And please don't tell me that that sorting is
> implemented with comparison operators. That just means that the
> implementation is confusing numeric ordering with sort order.
>
> Then there is this:
>
>
>>>>1 > 0

>
> True
>
>>>>1+0j == 1

>
> True
>
>>>>1+0j == 1 > 0

>
> True
>
>>>>1+0j > 0

>
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: cannot compare complex numbers using <, <=, >, >=
>
> I applaud that Python has got rich comparisons for those who need them.
> But by confusing the question "which comes first in a sorted list?" with
> "which is larger?", you get all sorts of warts like being unable to sort
> lists with some objects, while being able to make meaningless
> comparisons like ''.join >= [].append.
>
> I'm not sure what the solution to this ugly state of affairs is. I'm not
> even sure if there is a solution. But I'm willing to make a good effort to
> *look*, and even though you were joking, I don't appreciate being told
> not to.

As far as I recall from Math Analysis, which I studied two months ago,
you can't sort complex numbers. It makes no sense. The reason being
(reading from my book), it's not possible to define an order that
preserves the properties of arithmetical operations on complex numbers.
So you can't order them, and you can't compare them.

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005

> As far as I recall from Math Analysis, which I studied two months ago,
> you can't sort complex numbers. It makes no sense. The reason being
> (reading from my book), it's not possible to define an order that
> preserves the properties of arithmetical operations on complex numbers.
> So you can't order them, and you can't compare them.
>

Sorry, that should have been "you can't sort them, and you can't compare
them with greater than, lesser than, etc. Of course, using == will work.

But tell me, how do you think sort works if not with <, >, ==, <= and >=
? I'm really interested.

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005
6. ### Reinhold BirkenfeldGuest

Steven D'Aprano wrote:
> On Sat, 30 Jul 2005 08:13:26 -0400, Peter Hansen wrote:
>
>> Beginners should not be comparing lambdas.
>>
>> Neither should you. ;-)

>
> Actually, yes I should, because I'm trying to make sense of the mess that
> is Python's handling of comparisons. At least two difference senses of
> comparisons is jammed into one, leading to such such warts as these:
>
>>>> L = []
>>>> L.sort() # we can sort lists
>>>> L.append(1+1j)
>>>> L.sort() # even if they include a complex number
>>>> L.append(1)
>>>> L.sort() # but not any more

> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: cannot compare complex numbers using <, <=, >, >=
>
> Um, I didn't ask to compare complex numbers using comparison operators. I
> asked to sort a list. And please don't tell me that that sorting is
> implemented with comparison operators. That just means that the
> implementation is confusing numeric ordering with sort order.

Sorting is implemented with comparison operators. How should it be otherwise?
Would you prefer a __sort__ method to specify sort order?

And how would you sort a list of complex numbers?

> Then there is this:
>
>>>> 1 > 0

> True

Okay.

>>>> 1+0j == 1

> True

Okay.

>>>> 1+0j == 1 > 0

> True

(1+0j == 1) yields True, which is comparable to 0.

>>>> 1+0j > 0

> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: cannot compare complex numbers using <, <=, >, >=

But complex numbers are not greater or littler than others. You can't order them,
not on a one-dimensional scale.

> I applaud that Python has got rich comparisons for those who need them.
> But by confusing the question "which comes first in a sorted list?" with
> "which is larger?", you get all sorts of warts like being unable to sort
> lists with some objects, while being able to make meaningless
> comparisons like ''.join >= [].append.

That's a wart indeed, and intended to be removed for 3.0, if I'm informed correctly.

Reinhold

Reinhold Birkenfeld, Jul 30, 2005
7. ### Steven D'ApranoGuest

On Sat, 30 Jul 2005 08:13:26 -0400, Peter Hansen wrote:

> Beginners should not be comparing lambdas.
>
> Neither should you. ;-)

Actually, yes I should, because I'm trying to make sense of the mess that
is Python's handling of comparisons. At least two difference senses of
comparisons is jammed into one, leading to such such warts as these:

>>> L = []
>>> L.sort() # we can sort lists
>>> L.append(1+1j)
>>> L.sort() # even if they include a complex number
>>> L.append(1)
>>> L.sort() # but not any more

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: cannot compare complex numbers using <, <=, >, >=

Um, I didn't ask to compare complex numbers using comparison operators. I
asked to sort a list. And please don't tell me that that sorting is
implemented with comparison operators. That just means that the
implementation is confusing numeric ordering with sort order.

Then there is this:

>>> 1 > 0

True
>>> 1+0j == 1

True
>>> 1+0j == 1 > 0

True
>>> 1+0j > 0

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: cannot compare complex numbers using <, <=, >, >=

I applaud that Python has got rich comparisons for those who need them.
But by confusing the question "which comes first in a sorted list?" with
"which is larger?", you get all sorts of warts like being unable to sort
lists with some objects, while being able to make meaningless
comparisons like ''.join >= [].append.

I'm not sure what the solution to this ugly state of affairs is. I'm not
even sure if there is a solution. But I'm willing to make a good effort to
*look*, and even though you were joking, I don't appreciate being told
not to.

--
Steven.

Steven D'Aprano, Jul 30, 2005
8. ### Steven D'ApranoGuest

On Sat, 30 Jul 2005 14:20:50 +0200, tiissa wrote:

> Steven D'Aprano wrote:
>> Playing around with comparisons of functions (don't ask), I discovered an
>> interesting bit of unintuitive behaviour:
>>
>>>>>a = lambda y: y
>>>>>b = lambda y: y
>>>>>a

>> <function <lambda> at 0xf70598ec>
>>>>>b

>> <function <lambda> at 0xf7059844>
>>>>>a < b

>> False
>>
>> So I'm puzzled about how Python compares the two.

>
> Seems to me the object addresses are compared in this case. But I'm too
> lazy to check it in the source.

Strangely enough, I'm lazy enough to not check the source too

Actually, more to the point, I don't read C, so even if I did check the
source, I wouldn't know what it was trying to tell me.

> However, the doc [1] warns you about such comparisons: """Most other
> types compare unequal unless they are the same object; the choice
> whether one object is considered smaller or larger than another one is
> made arbitrarily but consistently within one execution of a program."""

I am aware of that. That's a wart.

--
Steven.

Steven D'Aprano, Jul 30, 2005
9. ### Georg NeisGuest

* Reinhold Birkenfeld <> wrote:
> Steven D'Aprano wrote:
>>>>> 1+0j == 1 > 0

>> True

>
> (1+0j == 1) yields True, which is comparable to 0.

"a == b > c" is equivalent to "a == b and b > c":

>>> 1 == 1+0j > 0

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: cannot compare complex numbers using <, <=, >, >=

Georg Neis, Jul 30, 2005

Steven D'Aprano ha scritto:

> It was easy. I never once asked myself whether some complex number was
> greater or less than another, I just asked "which one comes first in a
> lexicographic sort?"
>
> The two questions are NOT the same, and it is an ugliness in an otherwise
> beautiful language that Python treats them as the same.
>
> Mathematically, 1 == 1.0 == 1+0j but in the dictionary "1" should sort
> before "1.0" which sorts before "1.0+0.0j".

Because, of course, when I sort numbers the first thing I think of is
looking at the ascii table... Here I was, thinking maths was useful.
Sorting numbers "lexicographically" might make sense to you, but it's
totally unintuitive to me. For example, why or how would I guess that
"3-3j" is bigger (or smaller) than "3+3j"?

You'll still want to sort complex numbers lexicographically. It'll still
have no meaning whatsoever, so you might as well leave the list
unsorted. You might think you sorted something. 100? 200? years of maths
say you didn't.

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005

Steven D'Aprano ha scritto:
> On Sat, 30 Jul 2005 13:30:20 +0000, Adriano Varoli Piazza wrote:
>
>
>>But tell me, how do you think sort works if not with <, >, ==, <= and >=
>>? I'm really interested.

>
>
> How do you sort words in a dictionary? Why does five come before four
> when the number five is larger than the number four? Why does hundred
> come before ten? What does it mean to say that elephant is less than
> mouse?
>
> When you can answer those questions, you will be enlightened.
>

>>> 'a' > 'A'

True
>>> 'a' > '-'

True
>>> 'a' > 'h'

False

Whiiiiii. I just discovered the Ascii table! wooot! (Your EBCDIC mileage
might vary)

What does it have to do with complex numbers, pray tell? and how do you
think any string sort works if not by comparing the numerical value of
each character?

Once more:

>>> 'five' < 'four'

True
>>> 'five' < 'Four'

False
>>> 'Five' < 'Four'

True

Ooohhh! magic of ascii!

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005
12. ### Steven D'ApranoGuest

On Sat, 30 Jul 2005 13:22:47 +0000, Adriano Varoli Piazza wrote:

> As far as I recall from Math Analysis, which I studied two months ago,
> you can't sort complex numbers. It makes no sense. The reason being
> (reading from my book), it's not possible to define an order that
> preserves the properties of arithmetical operations on complex numbers.
> So you can't order them, and you can't compare them.

You are confusing mathematical ordering with sorting a list. Here, I will
sort some mixed complex and real numbers for you. If you look at them
closely, you will even be able to work out the algorithm I used to sort
them.

1
1+0j
1+7j
2
2+3j
3+3j
3-3j
3+4j
4
4+2j

It was easy. I never once asked myself whether some complex number was
greater or less than another, I just asked "which one comes first in a
lexicographic sort?"

The two questions are NOT the same, and it is an ugliness in an otherwise
beautiful language that Python treats them as the same.

Mathematically, 1 == 1.0 == 1+0j but in the dictionary "1" should sort
before "1.0" which sorts before "1.0+0.0j".

--
Steven.

Steven D'Aprano, Jul 30, 2005
13. ### Steven D'ApranoGuest

On Sat, 30 Jul 2005 13:30:20 +0000, Adriano Varoli Piazza wrote:

> But tell me, how do you think sort works if not with <, >, ==, <= and >=
> ? I'm really interested.

How do you sort words in a dictionary? Why does five come before four
when the number five is larger than the number four? Why does hundred
come before ten? What does it mean to say that elephant is less than
mouse?

When you can answer those questions, you will be enlightened.

--
Steven.

Steven D'Aprano, Jul 30, 2005

Steven D'Aprano ha scritto:
> On Sat, 30 Jul 2005 13:22:47 +0000, Adriano Varoli Piazza wrote:
>
>
>>As far as I recall from Math Analysis, which I studied two months ago,
>>you can't sort complex numbers. It makes no sense. The reason being
>>(reading from my book), it's not possible to define an order that
>>preserves the properties of arithmetical operations on complex numbers.
>>So you can't order them, and you can't compare them.

>
>
> You are confusing mathematical ordering with sorting a list. Here, I will
> sort some mixed complex and real numbers for you. If you look at them
> closely, you will even be able to work out the algorithm I used to sort
> them.
>
> 1
> 1+0j
> 1+7j
> 2
> 2+3j
> 3+3j
> 3-3j
> 3+4j
> 4
> 4+2j
>
>
> It was easy. I never once asked myself whether some complex number was
> greater or less than another, I just asked "which one comes first in a
> lexicographic sort?"
>
> The two questions are NOT the same, and it is an ugliness in an otherwise
> beautiful language that Python treats them as the same.
>
> Mathematically, 1 == 1.0 == 1+0j but in the dictionary "1" should sort
> before "1.0" which sorts before "1.0+0.0j".
>

If you want to treat numbers as strings, why not convert them before
sorting them? Python is just saying "He's trying to sort complex
numbers. No can do". You're trying to make it guess that you want them
sorted as strings, not as numbers. I don't see how Python treats things
the same way. I see that real numbers and strings can be compared and
sorted (asciibetically? don't remember). It has nothing to do with
because they don't have an order as numbers, and Py is not intelligent
enough to know that you want them asciibetized

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005
15. ### Steven D'ApranoGuest

On Sat, 30 Jul 2005 15:32:45 +0200, Reinhold Birkenfeld wrote:

>> Um, I didn't ask to compare complex numbers using comparison operators. I
>> asked to sort a list. And please don't tell me that that sorting is
>> implemented with comparison operators. That just means that the
>> implementation is confusing numeric ordering with sort order.

>
> Sorting is implemented with comparison operators. How should it be otherwise?

That's a good question, and that's why I'm exploring different comparisons
in Python, trying to understand what Python already does, and the pros and
cons thereof, before I suggest anything new.

> Would you prefer a __sort__ method to specify sort order?

Well, there are an infinite number of sort orders. Most of them are pretty
much useless, but even if we limit ourselves to common, useful sort
orders, there are still a good half dozen or more.

So at this time, I'd rather not be pinned down to some half-baked
"solution" before I have even understood the problem.

> And how would you sort a list of complex numbers?

>> Then there is this:
>>
>>>>> 1 > 0

>> True

>
> Okay.
>
>>>>> 1+0j == 1

>> True

>
> Okay.
>
>>>>> 1+0j == 1 > 0

>> True

>
> (1+0j == 1) yields True, which is comparable to 0.

No, 1+0j == 1 > 0 is calculated as 1+0j == 1 and 1 > 0.

>>>>> 1+0j > 0

>> Traceback (most recent call last):
>> File "<stdin>", line 1, in ?
>> TypeError: cannot compare complex numbers using <, <=, >, >=

>
> But complex numbers are not greater or littler than others. You can't order them,
> not on a one-dimensional scale.

Of course you can order them. You are confusing order with magnitude. The
two are not identical, although they are similar enough in some contexts
as to cause confusion. I admit that I haven't fully grasped all the
subtleties of the general ordering problem. Fortunately, my needs are much
less lofty.

--
Steven.

Steven D'Aprano, Jul 30, 2005
16. ### Kay SchluehrGuest

Some indications:

>>> for i in range(5):

.... x = lambda x:x
.... y = lambda y:y
.... print x,y,x<y,id(x)<id(y)
....
<function <lambda> at 0x00EE83F0> <function <lambda> at 0x00EE8FB0>
True True
<function <lambda> at 0x00EE8AB0> <function <lambda> at 0x00EE83F0>
False False
<function <lambda> at 0x00EE8FB0> <function <lambda> at 0x00EE8AB0>
False False
<function <lambda> at 0x00EE83F0> <function <lambda> at 0x00EE8FB0>
True True
<function <lambda> at 0x00EE8AB0> <function <lambda> at 0x00EE83F0>
False False

Regards,
Kay

Kay Schluehr, Jul 30, 2005
17. ### tiissaGuest

Steven D'Aprano wrote:
> It was easy. I never once asked myself whether some complex number was
> greater or less than another, I just asked "which one comes first in a
> lexicographic sort?"
>
> The two questions are NOT the same, and it is an ugliness in an otherwise
> beautiful language that Python treats them as the same.

The point is Python does not.
The order you proposed above can be implemented using a comparison
function you can pass to the sort function of lists [1] or the sorted
builtin [2].
If the elements can be compared, Python offers you not to pass the cmp
function to sort if you want to use this default order.

Python allows you to use the default ordering to sort a list but does
not treat both questions in the same manner. However, the fact is
ordering a list using the default '<' happens pretty often.

In the case of complex numbers, no mathematically sound comparison
function exists and Python does not impose some default function that
would be called a wart.

[1] http://docs.python.org/lib/typesseq-mutable.html
[2] http://docs.python.org/lib/built-in-funcs.html

tiissa, Jul 30, 2005
18. ### Reinhold BirkenfeldGuest

Georg Neis wrote:
> * Reinhold Birkenfeld <> wrote:
>> Steven D'Aprano wrote:
>>>>>> 1+0j == 1 > 0
>>> True

>>
>> (1+0j == 1) yields True, which is comparable to 0.

>
> "a == b > c" is equivalent to "a == b and b > c":

Right. Stupid me Doesn't do much to the point, though.

>>>> 1 == 1+0j > 0

> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> TypeError: cannot compare complex numbers using <, <=, >, >=

Reinhold

Reinhold Birkenfeld, Jul 30, 2005
19. ### Reinhold BirkenfeldGuest

Steven D'Aprano wrote:
> On Sat, 30 Jul 2005 15:32:45 +0200, Reinhold Birkenfeld wrote:
>
>>> Um, I didn't ask to compare complex numbers using comparison operators. I
>>> asked to sort a list. And please don't tell me that that sorting is
>>> implemented with comparison operators. That just means that the
>>> implementation is confusing numeric ordering with sort order.

>>
>> Sorting is implemented with comparison operators. How should it be otherwise?

>
> That's a good question, and that's why I'm exploring different comparisons
> in Python, trying to understand what Python already does, and the pros and
> cons thereof, before I suggest anything new.
>
>> Would you prefer a __sort__ method to specify sort order?

>
> Well, there are an infinite number of sort orders. Most of them are pretty
> much useless, but even if we limit ourselves to common, useful sort
> orders, there are still a good half dozen or more.

That's why the sort method does have certain keyword arguments with which you can
customize sorting to your liking. But unless the standard sort without arguments
should be disallowed, it has to resort to comparison.

> So at this time, I'd rather not be pinned down to some half-baked
> "solution" before I have even understood the problem.

That's wise.

>> But complex numbers are not greater or littler than others. You can't order them,
>> not on a one-dimensional scale.

>
> Of course you can order them. You are confusing order with magnitude. The
> two are not identical, although they are similar enough in some contexts
> as to cause confusion.

Well, you can order everything according to some specs, but you can't find a default
sort order for everything. That's where custom comparison functions can help.

Correct me if I'm wrong, but is there a default order for complex number?

Reinhold

Reinhold Birkenfeld, Jul 30, 2005

Steven D'Aprano ha scritto:

> Do you understand the difference between partial and total ordering, or
> weakly and strongly ordered? When you do understand them, come back and
> tell me again whether you still think lexicographic sorting has no meaning
> whatsoever.
>

I think I answered this in another post... If you want to order text
strings (as complex numbers in an index in a book, or in a lexicographic
sort are) do so. I understand the importance it has to you, but it's
hardly reasonable to argue that Python should do the most unintuitive
thing with complex numbers by default just because it suits you.

--
The Inside Out: http://moranar.com.ar
MSN:
ICQ: 4410132

Adriano Varoli Piazza, Jul 30, 2005