输出背后的原因是什么？

Question

输入：

x, y = 20, 60
y, x, y = x, y-10, x+10
print(x, y)

输出：

50 30

  

我期望的是？

• x = 20

• y = 60

• y = x = 20

• x = y-10 = 20-10 = 10

• y = x + 10 = 20

预期输出：

10 20

为什么不是这种情况？是因为首先对表达式求值，然后才给变量赋值？

Answer 1

右侧完全排在左侧之前。然后从左到右评估左侧。

x, y = 20, 60
# x = 20, y = 60

# ----------------------

y, x, y = x, y-10, x+10
# Evaulate the right first:
# x, y-10, x+10 = 20, 50, 30
# So now we have
# y, x, y = 20, 50, 30

# Now it goes left to right so:
# y = 20
# x = 50
# y = 30 --> note this overwrote the first y assignment

print(x, y)

因此

50 30