存储并 return 同一变量的先前值

Question

我正在尝试存储变量的先前值，该变量已通过对话框更改，然后如果对话框被取消并且 returns，则将该变量的值转换回先前设置的值None。例如，考虑：

letter_height = 30 letter_width = 50

我希望用户通过这个改变值，这确实有效：

def NewLetterDimensions():
    global letter_height
    letter_height = (numinput("New Letter Height", "Please set the new letter height: ", minval = 10, maxval = 170))
    if letter_height == None:
        listen()

    global letter_width
    letter_width = (numinput("New Letter Width", "Please set the new    letter width: ", minval = 10, maxval = 170))
    if letter_width == None:
        listen()

调用的是：

onkey(NewLetterDimensions, "k")

但是，如果对话框被取消，我希望 "if" 语句能够 return 存储的先前值，以便将变量更改为先前值而不是 None。那么，我将如何着手实施呢？所以，我想要这个：

def NewLetterDimensions():
    global letter_height
    letter_height = (numinput("New Letter Height", "Please set the new letter height: ", minval = 10, maxval = 170))
    if letter_height == None:
        [] <- # Return previously set letter height (whether from previous dialog or not)
        listen()

global letter_width
letter_width = (numinput("New Letter Width", "Please set the new    letter width: ", minval = 10, maxval = 170))
if letter_width == None:
    [] <- # Return previously set letter width (whether from previous dialog or not)
    listen()

是的，这是海龟图形。

Answer 1

怎么样：

user_input = (numinput("New Letter Height", "Please set the new letter height: ",
                             minval=10, maxval=170))
letter_height = letter_height if user_input is None else user_input

仅当 letter_height 不是 None 时才将 letter_height 更改为 user_input，否则保留旧值。

顺便说一句，总是将 None 与 is 测试进行比较，因为 PEP8 建议：

Comparisons to singletons like None should always be done with is or is not, never the equality operators.

存储并 return 同一变量的先前值

Store and return a previous value for same variable

python

turtle-graphics

python-3.x