Python change dict value

Question

I create a class with a dictionary variable, and a method for editing it:

class Test:
   TestDict = {"TestKey": "OldValue"}

   def ChangeDictionary(self):
       self.TestDict["TestKey"] = "NewValue"

Then, i create a two objects:

ObjectOne = Test()
ObjectTwo = Test()

Print their dictionaries before ChangeDictionary method call, and after:

# Print values before method call
print(ObjectOne.TestDict)
print(ObjectTwo.TestDict)

# Call method
ObjectTwo.ChangeDictionary()

# Print values after method call
print(ObjectOne.TestDict)
print(ObjectTwo.TestDict)

And i get this output:

{'TestKey': 'OldValue'}
{'TestKey': 'OldValue'}
    * * Method call * *
{'TestKey': 'NewValue'}
{'TestKey': 'NewValue'}

So method ChangeDictionary which was called from second object changed value also and in first object.
Is it Python language bug or reasons of it somewhere else?

Updated: If class variable will be any other type (For example - String) - In first object variable will have old value:

class Test:
    TestString = "OldValue"

    def ChangeString(self):
        self.TestString = "OldValue"

@tevemadar If i get it right - No. In topic which link you posted talks about changing variables of class from object. My question is why dictionary value of object changing after i call a method from other object (Instanse of same class), if i will change variable type to any other (For example string) values will be stay different. — Komp Tip, Aug 15 '21 at 01:06
`TestDict` is shared across all objects since its not defined inside `__init__`. To fix this, create a `self.TestDict` and initialize it inside the `__init__` function. Check my answer for details. — Akshay Sehgal, Aug 15 '21 at 01:09
@AkshaySehgal But why its all works only with dictionary? If variable will have string type, after i change it - First object will have old value. — Komp Tip, Aug 15 '21 at 01:14
it works for all types of variables. please update the question (add the updated class) with the case that you are mentioning. — Akshay Sehgal, Aug 15 '21 at 01:15
"Is it Python language bug" this should never be your first inclination for something like this, almost certainly, you are just not understanding something — juanpa.arrivillaga, Aug 15 '21 at 01:36

Akshay Sehgal · Accepted Answer · 2021-08-15T01:39:31.283

As defined in this top answer:

Elements outside the init method are static elements; they belong to the class.
Elements inside the init method are elements of the object (self); they don't belong to the class.

To fix this behavior, try this -

class Test:
    def __init__(self):                          #<-------
        self.TestDict = {"TestKey": "OldValue"}  #<-------
    
    def ChangeDictionary(self):
        self.TestDict["TestKey"] = "NewValue"
        
        
ObjectOne = Test()
ObjectTwo = Test()

# Print values before method call
print(ObjectOne.TestDict)
print(ObjectTwo.TestDict)

# Call method
ObjectTwo.ChangeDictionary()
print("** Method call **")

# Print values after method call
print(ObjectOne.TestDict)
print(ObjectTwo.TestDict)

{'TestKey': 'OldValue'}
{'TestKey': 'OldValue'}
** Method call **
{'TestKey': 'OldValue'}
{'TestKey': 'NewValue'}

EDIT: As mentioned by @juanpa.arrivillaga, modifications you are making with your class function are defining an immutable object separate from the one that you have before.

Note the different in the 2 methods below -

#### ORIGINAL STRING ####

class Test:
    TestString = "OldValue"

    def ChangeString(self):
        self.TestString = "NewValue"
        
        
ObjectOne = Test()
ObjectTwo = Test()

# Print values before method call
print(ObjectOne.TestString)
print(ObjectTwo.TestString)

# Call method
ObjectTwo.ChangeString()
print("** Method call **")

# Print values after method call
print(ObjectOne.TestString)
print(ObjectTwo.TestString)

print("** Self dicts **")
print(ObjectOne.__dict__)
print(ObjectTwo.__dict__)

OldValue
OldValue

** Method call **
OldValue
NewValue

** Self dicts **
{}
{'TestString': 'NewValue'}

AND, with __init__


### FIXED STRING ###

class Test:
    def __init__(self):
        self.TestString = 'OldValue'
    
    def ChangeString(self):
        self.TestString = "NewValue"
               
ObjectOne = Test()
ObjectTwo = Test()

# Print values before method call
print(ObjectOne.TestString)
print(ObjectTwo.TestString)

# Call method
ObjectTwo.ChangeString()
print("** Method call **")

# Print values after method call
print(ObjectOne.TestString)
print(ObjectTwo.TestString)

print("** Self dicts **")
print(ObjectOne.__dict__)
print(ObjectTwo.__dict__)

OldValue
OldValue
** Method call **
OldValue
NewValue
** Self dicts **
{'TestString': 'OldValue'}
{'TestString': 'NewValue'}

Thanks, but why is it works only with dictionary type variables? If variable will be a string - Even if they will define ouside __init__ - They will changed only in their object. — Komp Tip, Aug 15 '21 at 01:35
"Variable set outside `__init__` belong to the class." Not exactly. Variables set in the class block *belong to the class*. Instance variables are any attribute you assign to an instance. — juanpa.arrivillaga, Aug 15 '21 at 01:36
@KompTip no, no they won't. `str` objects *are immutable* so you cannot mutate them. You are probably *assigning another string* inside your method, which shadows your class variable — juanpa.arrivillaga, Aug 15 '21 at 01:37
@juanpa.arrivillaga Thanks, so when we edit defiened out of ```__init__``` list/dictionary variable - We are change value of class variable, when we change a string variable value - We redefine/recreate it? — Komp Tip, Aug 15 '21 at 01:47
@KompTip no, the *type of the object is irrelevant*. If you assign a variable to `self` inside a method, i.e. the value of the first positional argument, that is assigning the attribute *to the instance*. Consider something like `class Foo: pass`, then if you do `foo = Foo(); foo.bar = 42` then you *created an instance variable*. That is **the exact same thing** that happens in a method, `def some_method(self): self.bar = 42`, `self` is just a naming convention, when you call `some_method` on an instance, the instance itself is passed as the first argument\ — juanpa.arrivillaga, Aug 15 '21 at 01:50

Python change dict value

So method ChangeDictionary which was called from second object changed value also and in first object. Is it Python language bug or reasons of it somewhere else?

1 Answers1

So method ChangeDictionary which was called from second object changed value also and in first object.
Is it Python language bug or reasons of it somewhere else?