New python object affected by other instance properties

Question

I'm building a Class in Python and I found an unexpected behavior. It's actually easy to solve, I just think I have a misconception about Python fundamentals and would like to understand what it is.

Expected: All properties inside __init__ should be exclusive to an object.
Actual Result: The second object somehow inherits properties from the first one.
Goal: Understand my misconception about Python fundamentals.

The code is easily reproducible and self explanatory.

class MyClass:
    def __init__(self, custom_list=[]):
        self.object_list = custom_list
    def add_to_list(self, item):
        self.object_list.append(item)

p1 = MyClass()
print("p1 starts with empty list", p1.object_list) # [] ✅
p1.add_to_list("p1_item")
print("Now p1 list is populated:", p1.object_list) # ['p1_item'] ✅

p2 = MyClass()
print("p2 should start with empty list", p2.object_list) # ['p1_item'] ❌

This is one way to easily fix the issue (even though I don't understand why):

class MyClass:
    def __init__(self, custom_list=[]):
        if custom_list:
            self.object_list = custom_list
        else:
            self.object_list = []

Lastly, it seems to affect a more complex object, like a list or a dictionary, but it won't affect a simple variable, like a float. Those code are analogous:

# ✅ This works fine
class MyFloat:
    def __init__(self, a=0):
        self.a = a
    def increment_a(self, x):
        self.a+=x

# ❌ This also fails
class MyDict:
    def __init__(self, custom_dict={}):
        self.object_list = custom_dict
    def add_to_dict(self, item):
        self.object_list.update(item)

New python object affected by other instance properties

0 Answers0