How to do a deepequals on a object in python

Question

Is there a function so that I can do

class Test():
    def __init__(self):
        self.value_1 = 42

x = Test()
y = Test()
deepequals(x, y) == True
x.value = 7
deepequals(x, y) == False
y.value = 7
deepequals(x, y) == True

However, by default, it would always be false because x and y are different instances of Test

Define `__eq__` of your class `Test`. – Hyperboreus Mar 11 '14 at 18:00 — Hyperboreus, Mar 11 '14 at 18:00

jonrsharpe · Accepted Answer · 2020-09-20T11:05:41.593

1

You can implement the __eq__ (equals) "magic method":

class Test():
    def __init__(self):
        self.value_1 = 42
    def __eq__(self, other):
        return self.__dict__ == other.__dict__

where __dict__ holds all of the instance attributes. This will return True when two objects have all the same values for all the same attributes. This gives the result you want:

>>> x = Test()
>>> y = Test()
>>> x == y
True
>>> x.value = 7
>>> x == y
False
>>> y.value = 7
>>> x == y
True

To support comparisons with objects that do not have a __dict__ attribute (like those defined in C or using __slots__ instead) you can either check for that attribute first using hasattr:

return hasattr(other, '__dict__') and self.__dict__ == other.__dict__

or access it safely using getattr with a default:

return self.__dict__ == getattr(other, '__dict__', None)

edited Sep 20 '20 at 11:05

answered Mar 11 '14 at 18:01

jonrsharpe

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`return hasattr(other, '__dict__') and self.__dict__ == other.__dict__` – v_b Sep 20 '20 at 09:12
@v_b yes, that would be safer in cases where you're dealing with objects that may not have a `__dict__`, like those using `__slots__` instead. – jonrsharpe Sep 20 '20 at 10:12
What I mean is, `x == None`, `x == 1`, or `x == {}` would fail for `x = Test()`. – v_b Sep 20 '20 at 10:50
@v_b we're agreeing, those are other examples of objects that don't have a `__dict__`. – jonrsharpe Sep 20 '20 at 10:51

score 1 · Answer 2 · answered Sep 20 '20 at 11:19

class Test:
    def __init__(self):
        self.value_1 = 42
        
    def __eq__(self, other):
        return (
             self.__class__ == other.__class__ and
             self.value_1 == other.value_1)

t1 = Test()
t2 = Test()
print(t1 == t2)

output

True

score 0 · Answer 3 · answered Mar 11 '14 at 18:02

You might want to implement __eq__ of your class. Then you can use the standard comparison operator:

class Test():
    def __init__(self):
        self.value = 42

    def __eq__ (self, other):
        return self.value == other.value

x = Test()
y = Test()
print (x == y)
x.value = 7
print (x == y)
y.value = 7
print (x == y)

How to do a deepequals on a object in python

3 Answers3