On 8/17/2016 2:39 AM, Steven D'Aprano wrote:
On Wednesday 17 August 2016 06:59, Lawrence D’Oliveiro wrote:
Here
<http://ldo17.tumblr.com/post/17544419704/conditional-expressions-in-python>
are some examples of that varying mileage.
Quote:
The Python syntax for conditional expressions (introduced in Python 2.5) is
trueval if cond else falseval
I think this is bloody awful.
When this was initially considered, this was the majority view.
Why couldn’t they have adopted the standard C
syntax, as used in a whole bunch of other C-derivative languages?
cond ? trueval : falseval
That particular syntax was not really considered. At least 10 versions
using 'if', 'then', 'else', and other tokens were.
They all had the problem of requiring a new keyword such as 'then' or
some other innovation.
Because the C syntax is horrifically ugly, whereas the Python syntax is very
close to real English syntax.
"What will you do tonight?"
"Go to the movies, if I finish work on time, otherwise just go home."
"If I finish work on on time, go to the movies, otherwise just go home."
is also real English syntax, and to me, more graceful. It is certainly
more neutral among the alternatives. The inverted version implies a
clear preference for the first alternative.
It would be an interesting exercise to see which order for ternary
expressions is more common in some large corpus of English text.
Every time you read the C syntax, you lose another three minutes off your
lifespan. That's how ugly it is.
Every time I write or read the Python syntax chosen, I lose time
rearranging the terms.
The background to the Python ternary operator is documented here:
https://www.python.org/dev/peps/pep-0308/
What the *current* version removed from an earlier version is that there
was a clear community consensus against the condition-in-the-middle
syntax Guido proposed and for some version of "if condition then
True-alternative else False-alternative". Where consensus was lacking
was which of multiple 'normal order' alternatives to choose. Part of
the problem was a lack of knowledge of which alternative Guido might
accept. In any case, a runoff vote among the top contenders was not
allowed.
Rather than ask why Python uses `trueval if cond else falseval`, you should ask
why C uses `cond ? trueval : falseval`. Is that documented anywhere?
The ordering of the if-then-else terms is obvious. '?' ending a
question (in English) is obvious. ';' was already used to end
statememts and could not be used here. Could ',' have been used instead
of ':'? I am not sure, but it is used elsewhere in C. ':' is the only
other within-Englich-sentence separator available.
The ':' naturally translates to 'else', which was already a keyword.
There was consensus on this.
The problem for a pythonic (wordy) version of the C expression is that
the word marker for questions, 'if', normally begins rather than ending
a question*. So "cond if trueval else falseval" is likely to be
misinterpreted. Hence proposals for "cond then trueval else falseval"
and "if cond then trueval else falseval" and other variations.
* In a construction like "The cock crows? If so, I must go, else I
would tarry with thee longer." the 'if' follows the question, yet
cannot standalone but must be followed by something referring back to
the question.
--
Terry Jan Reedy
--
https://mail.python.org/mailman/listinfo/python-list
