sequence multiplied by -1

Steven D'Aprano steve at
Sat Oct 2 07:03:17 CEST 2010

On Fri, 01 Oct 2010 14:56:52 +0200, Antoon Pardon wrote:

> Think about the following possibility.
> Someone provides you with a library of functions that act on sequences.
> They rely on the fact that '+' concatenates.
> Someone else provides you with a library of functions that act on
> numbers. They rely on the fact that '+' provides addition.
> Now however you write your sequence like numbers or number like
> sequences one of those libraries will be useless for it.

And? So what? Sequences aren't numbers. Numbers aren't sequences. You 
can't expect every library to correctly work with every data type 
regardless of the polymorphism of operators.

If you have no idea what x is, you can't possibly expect to know what 
function(x) does, *for any function*. 

(Actually, there may be one or two exceptions, like id(x). But fewer than 
you think -- even str(x) is not guaranteed to work for arbitrary types.)

If you don't know whether x is a number or a sequence, you can't know 
what x.sort() will do, or math.sqrt(x), or x+x. Why single out x+x as 
more disturbing than the other examples?

>> -- which would it do with only one
>> symbol?
> But why limit ourselves to one symbol for different kind of operations,
> to begin with?

Because the more symbols you have, the higher the learning curve to 
become proficient in the language, and the more like line-noise the code 
becomes. If you want APL or Perl, you know where to find them -- they're 
perfectly reasonable languages, but they have made design choices that 
are not Python's design choices.

We use operators, because for certain common operations it is more 
convenient and natural to use infix notation than function notation. And 
polymorphism is useful, because it reduces the size of namespaces and 
therefore the burden on the programmer. Putting these two factors 
together, instead of:


plus mixed-operand versions of at least some of them, we have a single 
symbol, +, for all of these. Arguing about whether it should be a single 
operator + or two operators + & is a comparatively trivial matter. I 
believe that the similarity between concatenation and addition is close 
enough that it is appropriate to use one symbol for both, and likewise 
between repetition and multiplication. I'm glad that Guido apparently 
agrees with me.


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