[Python-Dev] Assignment expression and coding style: the while True case (original) (raw)

Victor Stinner vstinner at redhat.com
Wed Jul 4 18:51:37 EDT 2018

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 % (510*(len(str(abs(coeff)))-p-1)): break places += 3

NOT replaced with:

while not (coeff := _dlog10(c, e, places)) % (510*(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

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