On 04/22/2018 10:44 PM, Tim Peters wrote:
[Guido]
In reality there often are other conditions being applied to the match for
which `if expr as name` is inadequate. The simplest would be something like
if ...:
<something>
elif (m := re.match('(.*):(.*)', line)) and m.group(1) == m.group(2):
<whatever>
And the match() call may not even be the first thing to check -- e.g. we
could have
elif line is not None and (m := re.match('(.*):(.*)', line)) and
m.group(1) == m.group(2):
I find myself warming more to binding expressions the more I keep them
in mind while writing new code. And I think it may be helpful to
continue showing real examples where they would help.
Today's example: I happened to code this a few hours ago:
diff = x - x_base
if diff:
g = gcd(diff, n)
if g > 1:
return g
It's not really hard to follow, but two levels of nesting "feels
excessive", as does using the names "diff" and "g" three times each.
It's _really_ an "and" test: if the diff isn't 0 and gcd(diff, n) >
1, return the gcd. That's how I _thought_ of it from the start.
Which this alternative expresses directly:
if (diff := x - x_base) and (g := gcd(diff, n)) > 1:
return g
So I really like being able to make the assignment in the expression, but I have a really hard time parsing it with the
name first.
Attempting to read just the names first:
if
diff
scan for ending right paren
found
and
g
scan for ending right paren
oops, found opening left paren
scan for ending right paren
found
resume scanning for final right paren
found
> 1:
return g
Attempting to read expressions first:
if
x - x_base
and
gcd(diff, n)
what's diff?
scan backwards
diff is x - x_base
> 1:
return g
what's g?
scan up and backwards
g is gcd(diff, n)
Attempting to read interleaved:
if
skip diff
x - x_base
back to diff
as diff
and
skip g
gcd(diff, n)
back to g
as g
> 1:
return g
On the other hand, if it were using the "as" keyword:
if (x - xbase as diff) and (gcd(diff, n) as g) > 1:
return g
I would parse as:
if
x - x_base
as diff
and
gcd(diff, n)
as g
> 1:
return g
For me at least, the last is much more readable. Thinking about it some more, the problem (or maybe just my problem) is
that I see an "if" or "while" and the I look for the thing that is True or False, and using the ":=" syntax the first
thing I see is a placeholder for a result that doesn't exist yet, making me constantly scan backwards and forwards to
put all the pieces in the correct place.
With "as", it just flows forwards.
--
~Ethan~
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
