appending to a list of objects within a dictionary of objects in python

Question

I am creating a class that has a list of a second object and a total of the amount value from the second object. This in itself works. However I need to group them by category so I created a dictionary of them. Which seems simple however whenever I append an object to a list it appends to all of the lists in the dictionary.

class objA:
    amount = 0
    listBs = []
    category = ""
    def __init__(self,category):
        self.category = category
class objB:
    amount = 0
    category = ""
    otherproperty = ""

mydict = {}

b1 = objB()
b1.amount = 1
b1.category = "foo"
b1.otherproperty = "abc"

if(b1.category not in mydict.keys()):
    mydict[b1.category] = objA(b1.category)
mydict[b1.category].listBs.append(b1)
mydict[b1.category].amount += b1.amount


b2 = objB()
b2.amount = 2
b2.category = "bar"
b2.otherproperty = "xyz"

if(b2.category not in mydict.keys()):
    mydict[b2.category] = objA(b2.category)
mydict[b2.category].listBs.append(b2)
mydict[b2.category].amount += b2.amount

print("foo amount: " + str(mydict["foo"].amount) + " - foo len: " + str(len(mydict["foo"].listBs)))
print("bar amount: " + str(mydict["bar"].amount) + " - bar len: " + str(len(mydict["bar"].listBs)))

when I run the above code I get the expected amount of 1 for foo and 2 for bar but both lists have a len of 2 since they both contain b1 and b2.

I took the class objects out and the same pricipal works just appending to a dict of lists so the following works

dictoflists = {}
dictoflists["key1"] = []
dictoflists["key1"].append("k1v1")
dictoflists["key2"] = []
dictoflists["key2"].append("k2v1")
dictoflists["key2"].append("k2v2")
print(dictoflists)

output:

{'key1': ['k1v1'], 'key2': ['k2v1', 'k2v2']}

Is there a way to get this to work or a better solution?

objA.listBs has been defined on the class itself. Every time you reference listBs from any instance of objA it will be a reference to the same list — Iain Shelvington
– Iain Shelvington, Commented Dec 29, 2019 at 5:38
@IainShelvington How do I achieve the desired behavior giving each instance of objA it's own listBs? Is there some other way to define it that I'm not aware of? — DTATSO
– DTATSO, Commented Dec 29, 2019 at 6:21

Iain Shelvington · Accepted Answer · 2019-12-29 06:36:42Z

1

Change the definition of the objA class so that the variables are defined pre-instance instead of on the class itself

class objA:
    def __init__(self, category):
        self.category = category
        self.amount = 0
        self.listBs = []

answered Dec 29, 2019 at 6:36

Iain Shelvington

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1 Comment

DTATSO Over a year ago

I just realized I forgot to mark this as the correct answer. Thank you for your help.

shiva2492 · Accepted Answer · 2019-12-29 07:24:34Z

So, I have initialized the variable inside constructor -

class objA:
        amount = 0

        category = ""
        def __init__(self,category):
            self.category = category
      #Initialize the variable inside constructor     
            self.listBs = []
class objB:
    amount = 0
    category = ""
    otherproperty = ""

mydict = {}

b1 = objB()
b1.amount = 1
b1.category = "foo"
b1.otherproperty = "abc"

if(b1.category not in mydict.keys()):
    mydict[b1.category] = objA(b1.category)
mydict[b1.category].listBs.append(b1)
mydict[b1.category].amount += b1.amount


b2 = objB()
b2.amount = 2
b2.category = "bar"
b2.otherproperty = "xyz"

if(b2.category not in mydict.keys()):
    mydict[b2.category] = objA(b2.category)
mydict[b2.category].listBs.append(b2)
mydict[b2.category].amount += b2.amount

print("foo amount: " + str(mydict["foo"].amount) + " - foo len: " + str(len(mydict["foo"].listBs)))
print("bar amount: " + str(mydict["bar"].amount) + " - bar len: " + str(len(mydict["bar"].listBs)))

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