Brython - Python in the browser

Steven D'Aprano steve+comp.lang.python at
Fri Dec 21 03:05:03 CET 2012

On Thu, 20 Dec 2012 18:59:39 -0500, Terry Reedy wrote:

>> On Thu, Dec 20, 2012 at 8:37 PM, Pierre Quentel
>> <pierre.quentel at> wrote:
>>> I'm afraid I am going to disagree. The document is a tree structure,
>>> and today Python doesn't have a syntax for easily manipulating trees.
> What Python does have is 11 versions of the augmented assignment
> statement: +=, -=, *=, /=, //=, %=, **=, >>=, <<=, &=, ^=, |=. Moreover,
> these are *intended* to be implemented in place, by mutation, for
> mutable objects, with possibly class-specific meanings.

I don't believe that is the case. The problem is that augmented 
assignment that mutates can be rather surprising to anyone who expects 
"a += b" to be a short cut for "a = a + b".

py> a = [1, 2, 3]; b = [99]; another = a
py> a = a + b
py> print(a, another)  # What I expect.
[1, 2, 3, 99] [1, 2, 3]

py> a = [1, 2, 3]; b = [99]; another = a
py> a += b
py> print(a, another)  # Surprise!
[1, 2, 3, 99] [1, 2, 3, 99]

Whichever behaviour you pick, you're going to surprise somebody. So I 
wouldn't say that mutate in place is *intended* or preferred in any way, 
only that it is *allowed* as an optimization if the class designer 
prefers so.

One might even have a class where (say) __iadd__ is defined but __add__ 
is not.

> <= is a comparison expression operator, which is completely different.

<= is a comparison operator for ints, floats, strings, lists, ... but not 
necessarily for *everything*. That's the beauty and horror of operator 
overloading. Any operator can mean anything.

If it were intended to only return a flag, then 1) Python would enforce 
that rule, and 2) the numpy people would be most upset.

I have no opinion on the usefulness or sensibility of using <= as an in-
place mutator method in this context, but I will say that if I were 
designing my own mini-DSL, I would not hesitate to give "comparison 
operators" some other meaning. Syntax should be judged in the context of 
the language you are using, not some other language. If you are using a 
DSL, then normal Python rules don't necessarily apply. <= in particular 
looks just like a left-pointing arrow and is an obvious candidate for 


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