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Ternary operator associativity

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  Ternary operator associativity candide <c.candide@laposte.net> - 2014-03-06 03:34 -0800
    Re: Ternary operator associativity Michael Torrie <torriem@gmail.com> - 2014-03-06 10:47 -0700
    Re: Ternary operator associativity Terry Reedy <tjreedy@udel.edu> - 2014-03-06 13:52 -0500
    Re: Ternary operator associativity Tim Chase <python.list@tim.thechases.com> - 2014-03-06 12:59 -0600
    Re: Ternary operator associativity Terry Reedy <tjreedy@udel.edu> - 2014-03-06 15:15 -0500

#67933 — Ternary operator associativity

According to the official documentation, the ternary operator has left-to-right associativity :

-------------------
Operators in the same box group left to right (except for comparisons, including tests, which all have the same precedence and chain from left to right -- see section Comparisons -- and exponentiation, which groups from right to left).
-------------------


Nevertheless, the ternary operator grouping seems to be from right to left, compare :


>>> p = 0 if 1 else 0 if 0 else 1
>>> p
0
>>> left_to_right = (0 if 1 else 0) if 0 else 1
>>> right_to_left = 0 if 1 else (0 if 0 else 1)
>>> p == left_to_right
False
>>> p == right_to_left
True
>>> 

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#67945

On 03/06/2014 04:34 AM, candide wrote:
> According to the official documentation, the ternary operator has left-to-right associativity :
> 
> -------------------
> Operators in the same box group left to right (except for comparisons, including tests, which all have the same precedence and chain from left to right -- see section Comparisons -- and exponentiation, which groups from right to left).
> -------------------
> 
> 
> Nevertheless, the ternary operator grouping seems to be from right to left, compare :

I was reading a blog about PHP the other day and it mentioned PHP was
the only language he knew of that had ternary operator precedence going
left to right.  All other languages use right to left.  So I assume that
Python also uses right to left and that the documentation is a bug.

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#67947

On 3/6/2014 6:34 AM, candide wrote:
> According to the official documentation, the ternary operator has
> left-to-right associativity :

The proper terms is 'conditional expression', which goes back to "The C 
Programming Language" (K&R). There are many unary operators, many binary 
operators, and there could be other ternary operators.

> ------------------- Operators in the same box group left to right
> (except for comparisons, including tests, which all have the same
> precedence and chain from left to right -- see section Comparisons --
> and exponentiation, which groups from right to left).
> -------------------
>
> Nevertheless, the ternary operator grouping seems to be from right to
> left, compare :
>
>>>> p = 0 if 1 else 0 if 0 else 1 p
> 0
>>>> left_to_right = (0 if 1 else 0) if 0 else 1
 >>>> right_to_left = 0 if 1 else (0 if 0 else 1)
 >>>> p == left_to_right
> False
>>>> p == right_to_left
> True

This behavior is specified in the grammar as given in the C-E section.

The doc is also inconsistent about evaluation order and precedence. I 
opened  http://bugs.python.org/issue20859 .

-- 
Terry Jan Reedy

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#67948

On 2014-03-06 03:34, candide wrote:
> According to the official documentation, the ternary operator has
> left-to-right associativity 
>
> >>> left_to_right = (0 if 1 else 0) if 0 else 1
> >>> right_to_left = 0 if 1 else (0 if 0 else 1)

I'd never want to rely on my own ability to remember the language
spec, so I strongly advocate for making it explicit with parens
regardless of what the language defines.  And that's if I ever created
such a mess in the first place.  If you have more than one pair of
conditional expressions in a single assignment, I'd suggest that it's
a code-smell that could use refactoring for clarity/disambiguity.

-tkc

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#67949

On 3/6/2014 1:59 PM, Tim Chase wrote:
> On 2014-03-06 03:34, candide wrote:
>> According to the official documentation, the ternary operator has
>> left-to-right associativity
>>
>>>>> left_to_right = (0 if 1 else 0) if 0 else 1
>>>>> right_to_left = 0 if 1 else (0 if 0 else 1)
>
> I'd never want to rely on my own ability to remember the language
> spec, so I strongly advocate for making it explicit with parens
> regardless of what the language defines.  And that's if I ever created
> such a mess in the first place.  If you have more than one pair of
> conditional expressions in a single assignment, I'd suggest that it's
> a code-smell that could use refactoring for clarity/disambiguity.

It is intended and part of the PEP discussion that one be able to write 
chained conditional expression in Python

x = (a1 if c1 else
      a2 if c2 else
      a3 if c3 else
      a4)

without extra ()s, similar to what one can write in C (if I remember 
correctly)

x = c1 ? a1 :
     c2 ? a2 :
     c3 ? a3 :
          a4

both being abbreviations for chained if-elses

if   c1: x = a1
elif c2: x = a2
elif c3: x = a3
else:    x = a4

In all three cases, the conditions are evaluated in 1,2,3 order, each 
before the corresponding expression.

-- 
Terry Jan Reedy

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