[Python-Dev] Assignment expression and coding style: the while True case

[Victor Stinner]

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.

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? :-)


== 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.

Victor