了解新 python := 运算符的原因
Understanding the reason for the new python := operator
这是一个元编程问题：我想了解为什么 python 开发人员使用新的 := 引入了另一个运算符。我知道这是干什么的。但是，我想知道为什么开发人员选择了一个新符号而不是重新使用例如as 运算符。
即为什么写
更可取
while (command := input("> ")) != "quit":
print("You entered:", command)
而不是
while input("> ") as command != "quit":
print("You entered:", command)
这在 PEP 572
中得到了回答
Alternative spellings
Broadly the same semantics as the current
proposal, but spelled differently.
EXPR as NAME:
stuff = [[f(x) as y, x/y] for x in range(5)]
Since EXPR as NAME already has meaning in import, except and with
statements (with different semantics), this would create unnecessary
confusion or require special-casing (e.g. to forbid assignment within
the headers of these statements).
(Note that with EXPR as VAR does not simply assign the value of EXPR
to VAR -- it calls EXPR.__enter__() and assigns the result of that to
VAR.)
Additional reasons to prefer := over this spelling include:
In if f(x) as y the assignment target doesn't jump out at you -- it
just reads like if f x blah blah and it is too similar visually to if
f(x) and y.
In all other situations where an as clause is allowed, even readers
with intermediary skills are led to anticipate that clause (however
optional) by the keyword that starts the line, and the grammar ties
that keyword closely to the as clause:
import foo as bar
except Exc as var
with ctxmgr() as var
To the contrary, the assignment expression does not belong to the if or while
that starts the line, and we intentionally allow assignment
expressions in other contexts as well.
The parallel cadence between
NAME = EXPR
if NAME := EXPR
reinforces the visual recognition of
assignment expressions.
这是一个元编程问题：我想了解为什么 python 开发人员使用新的 := 引入了另一个运算符。我知道这是干什么的。但是，我想知道为什么开发人员选择了一个新符号而不是重新使用例如as 运算符。
即为什么写
更可取while (command := input("> ")) != "quit":
print("You entered:", command)
而不是
while input("> ") as command != "quit":
print("You entered:", command)
这在 PEP 572
中得到了回答Alternative spellings
Broadly the same semantics as the current proposal, but spelled differently.
EXPR as NAME:
stuff = [[f(x) as y, x/y] for x in range(5)]Since
EXPR as NAMEalready has meaning inimport,exceptandwithstatements (with different semantics), this would create unnecessary confusion or require special-casing (e.g. to forbid assignment within the headers of these statements).(Note that
with EXPR as VARdoes not simply assign the value ofEXPRtoVAR-- it callsEXPR.__enter__()and assigns the result of that toVAR.)Additional reasons to prefer
:=over this spelling include:In
if f(x) as ythe assignment target doesn't jump out at you -- it just reads likeif f x blah blahand it is too similar visually toif f(x) and y.In all other situations where an
asclause is allowed, even readers with intermediary skills are led to anticipate that clause (however optional) by the keyword that starts the line, and the grammar ties that keyword closely to theasclause:import foo as bar except Exc as var with ctxmgr() as varTo the contrary, the assignment expression does not belong to the
iforwhilethat starts the line, and we intentionally allow assignment expressions in other contexts as well.The parallel cadence between
NAME = EXPR if NAME := EXPRreinforces the visual recognition of assignment expressions.