On 05.07.2018 1:51, Victor Stinner wrote:
Hi,
Let's say that the PEP 572 (assignment expression) is going to be
approved. Let's move on and see how it can be used in the Python
stdlib.
I propose to start the discussion about "coding style" (where are
assignment expressions appropriate or not?) with the "while True"
case.
I wrote a WIP pull request to use assignment expressions in "while True":
https://github.com/python/cpython/pull/8095/files
In short, replace:
while True:
x = expr
if not x:
break
...
with:
while (x := expr):
...
My question is now: for which "while True" patterns are the assignment
expression appropriate? There identified different patterns.
== Pattern 1, straighforward ==
while True:
line = input.readline()
if not line:
break
...
IMHO here assingment expression is appropriate here. The code remains
straighfoward to read.
while (line := input.readline()):
...
== Pattern 2, condition ==
Condition more complex than just "not line":
while True:
q = c//n
if n <= q:
break
...
replaced with:
while (q := c//n) < n:
...
IMHO it's still acceptable to use assignement expression... Maybe only
for basic conditions? (see above)
== Pattern 3, double condition ==
while True:
s = self.__read(1)
if not s or s == NUL:
break
....
replaced with:
while (s := self.__read(1)) and s != NUL:
...
Honestly, here, I don't know if it's appropriate...
At the first look, "s != NUL" is surprising, since "s" is not defined
before the while, it's only defined in the first *test* (defining a
variable inside a test is *currently* uncommon in Python).
== Pattern 4, while (...): pass ==
Sometimes, the loop body is replaced by "pass".
while True:
tarinfo = self.next()
if tarinfo is None:
break
replaced with:
while (tarinfo := self.next()) is not None:
pass
It reminds me the *surprising* "while (func());" or "while (func())
{}" in C (sorry for theorical C example, I'm talking about C loops
with an empty body).
Maybe it's acceptable here, I'm not sure.
Would be more readable with a more descriptive variable name:
while (chunk := self.next()) is not None:
pass
tarinfo = chunk
Note: such loop is rare (see my PR).
== Pattern 5, two variables ==
while True:
m = match()
if not m:
break
j = m.end()
if i == j:
break
...
replaced with:
while (m := match()) and (j := m.end()) == i:
...
Maybe we reached here the maximum acceptable complexity of a single
Python line? :-)
Would be more readable with additional parentheses:
while (m := match()) and ((j := m.end()) == i):
== Other cases ==
I chose to not use assignment expressions for the following while loops.
(A)
while True:
name, token = _getname(g)
if not name:
break
...
"x, y := ..." is invalid. It can be tricked using "while (x_y :=
...)[0]: x, y = x_y; ...". IMHO it's not worth it.
(B)
while True:
coeff = _dlog10(c, e, places)
# assert len(str(abs(coeff)))-p >= 1
if coeff % (5*10**(len(str(abs(coeff)))-p-1)):
break
places += 3
NOT replaced with:
while not (coeff := _dlog10(c, e, places)) % (5*10**(len(str(abs(coeff)))-p-1)):
places += 3
^-- Tim Peters, I'm looking at you :-)
coeff is defined and then "immediately" used in "y" expression of
x%y... Yeah, it's valid code, but it looks too magic to me...
(C)
while True:
chunk = self.raw.read()
if chunk in empty_values:
nodata_val = chunk
break
...
"nodata_val = chunk" cannot be put into the "chunk := self.raw.read()"
assignment expression combined with a test. At least, I don't see how.
(D)
while 1:
u1 = random()
if not 1e-7 < u1 < .9999999:
continue
...
Again, I don't see how to use assignment expression here.
Here, unlike the previous example, the assignment is a subexpression of
the conditions, so it _can_ be inlined:
while (u1:=random()) < 1e-7 or u1 > .9999999:
Victor
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--
Regards,
Ivan
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