Python-使用字符串列表作为类数据成员。 当字符串包含变量时，变量不会更新

Question

Is there a way to change variables within lists within classes after the class instance has been initialized? (Sorry, sounds more complicated than it is!)

I'm writing a game and storing all of my display text as array elements in instances of a "Text" class. I wrote the following example program to test what I'm trying to do:

#****************************************#
#INITIALIZE CLASS (to hold text component)
#****************************************#
class Text:
        def __init__(self, array, maxIndex):

            #list of text
            self.list = array

            #number of items in list
            self.max = maxIndex

#set global variable to 7
globalv = 7

#****************************************#
#INITIALIZE VARIABLE-ENHANCED TEXT
#necessary because variables inside class
#data member arrays seem to have a local
#scope, and can't be changed after they
#are initialized
#****************************************#
varLine = Text(["This line contains globalv, which holds: {}".format(globalv)], 0)

print varLine.list[varLine.max]
#prints 7

#CHANGE VALUE OF globalv:
print "Directly accessing globalv: {}".format(globalv)
#prints 7
print "\nUpdate value of globalv to 9"
globalv = 9
print "Directly accessing globalv: {}".format(globalv)
#prints 9

#Try to change and re-print variable
#doesn't work, despite the fact that "global" has changed outside of the class
print "\nfirst try:"

#call print function on globalv directly:
    print "Directly accessing globalv: {}".format(globalv)
    #prints 9

    #print it from the class:
    print "Printing from class:"
    print varLine.list[varLine.max])
    #prints 7

#Change variable by re-initializing class
print "\nsecond try:"
varLine = Text(["This line contains globalv, which holds: {}".format(globalv)], 0)

    #call print function on globalv directly:
    print "Directly accessing globalv: {}".format(globalv)
    #prints 9

    #print it from the class:
    print "Printing from class:"
    print varLine.list[varLine.max])
    #prints 9

Answer 1

When you create that string you are concatinating the current value of globalv into a string. Changing globalv after that string been created has no effect on the value of the string. You can make setter and print methods to accomplish this:

class Text:
    def __init__(self, strList, valueList, maxIndex):

        #list of text/values
        self.strList = strList
        self.valueList = valueList

        #number of items in list
        self.max = maxIndex

    def printStrAtIndex(self, idx):
        print(self.strList[idx] + str(self.valueList[idx]))

    def setValAtIndex(self, val, idx):
        self.valueList[idx] = val
        return self

    # add new str value pair to the the class
    def addStrAndValue(self, newStr, newValue):
        self.strList.append(newStr)
        self.valueList.append(newValue)
        self.max = len(self.valueList)-1

Then just call it like this:

varLine = Text(["This line is tied to the value: "], [7], 0)
varLine.printStrAtIndex(0)
varLine.setValAtIndex(9,0)
varLine.printStrAtIndex(0)

Answer 2

Answer is easy.

str.format()

Function returns copy of string with intrerpolated values . So Your Class contains :

vaeLine.list = ["Copy of formating string with resolved value"]

When You change global var to soemthing this string isn't reevaluated

And this string copy is immutable so Your class is holding array which is muteable but its holding just an already resolved string with value 7. So changing global doesnt affect this stirng.

Answer 3

The code is working correctly. I'm not sure what you're expecting.

You convert globalv to text and store it in an object's text. When you come back later, that text string is just as you left it. Are you expecting it to magically change because you changed the value of globalv ? This is somewhat like taking a photograph of yourself, changing clothes, and expecting the photo to change to match your new outfit. Variables don't work that way.

Perhaps what you need is a two-step approach:

1. Create an attribute to hold the value, such as self.value
2. Write a method to display an object's attribute; within that method, acces the current value of self.value, using that in the display string that you build.

Does that help?