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

Ivan Pozdeev vano at mail.mipt.ru
Wed Jul 4 22:29:51 EDT 2018

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 (xy := ...)[0]: x, y = xy; ...". 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 emptyvalues: nodataval = chunk break ... "nodataval = 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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