[Python] Assignment Expressions 「:=」

NAME := expr

Assign to variables within an expression using the notation.

使用例

if Statement

sex = 0
if sex == 0: print("boy")

⇒ if (sex := 0) == 0: print("boy")

while Statement

例１：讀取檔案

file = open("...")
while True:
  line = file.readline()
  if not line: break
print(line.strip())

⇒ file = open("...")
while (line := file.readline()): print(line.strip())

例２：輸入密碼

while True:
  p = input("Enter the password: ")
  if p == "*****": break

⇒ while (p := input("Enter the password: ")) != "*****": continue

錯誤例：

while (p := 0) < n:
  ...
  p += 1

⇒ 無限迴圈

Comprehension 推導式

members = [{"name": "Adam", "age": 23,
            "height": 1.75, "weight": 72}, # bmi = 23.51
           {"name": "Ben", "age": 17,
            "height": 1.72, "weight": 63}, # bmi = 21.30
           {"name": "Charlie", "age": 20,
            "height": 1.78, "weight": 82}, # bmi = 25.88]
 
def bmi(info):
  height = info["height"]
  weight = info["weight"]
  return weight / (height ** 2)
 
fat_guys = {info["name"]: bmi(info)
            for info in members if bmi(info) > 24}
※ { ... for ... if ... } is a Comprehension

bmi() 會被呼叫 4 次 (計算 if 的時候 3 次 + 宣告 dict 時 1 次)

使用 := 解決問題：
fat_guys = {info["name"]: _bmi
            for info in members if (_bmi := bmi(info)) > 24}

cannot use assignment expressions with attribute/subscribe

在 walrus operator (:=) 的左邊，只能是 single NAME
請參考 PEP 572 # Differences Between Assignment Expressions and Assignment Statements

Single assignment targets other than a single NAME are not supported:

# No equivalent
a[i] = x
self.rest = []

替代例

使用 setattr(object, name, value)

if object.name := value:
 if (setattr(object, name, temp := value), temp)[1]:

Last Updated on 2024/12/05 by A1go