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I have a top level class Car with Tire class as a property. When I update Tire from a top-level instantiation of the class (honda in example below), it also updates the other instance (bmw in example below).

Is there a way to make the assert statement below True? I could make Tire inherit from NamedTuple, but then it would be immutable, and I would not be able to simple update a property such as with honda.tire.tread_count=44.

Example Program with failing assert:

class Tire():
    def __init__(self, tread_count: int, size: int):
        self.tread_count = tread_count
        self.size = size

class Car():
    def __init__(self, color: str = "red", tire: Tire = Tire(tread_count=50, size=20)):
        self.color = color
        self.tire = tire

# Instantiate honda and change a property.
honda = Car()
print(honda.tire.tread_count)
honda.tire.tread_count=44
print(honda.tire.tread_count)
# Instantiate bmw.
bmw = Car()
print(bmw.tire.tread_count)
assert bmw.tire.tread_count == 50
# Question -- How can we make bmw not get updated by updating honda?

Output of example program:

50
44
44
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
     17 bmw = Car()
     18 print(bmw.tire.tread_count)
---> 19 assert bmw.tire.tread_count == 50
     20 # Question -- How can we make bmw not get updated by updating honda?
AssertionError:
ddetone
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    Don't use a default argument here; the default is created _once_ and part of the method definition. See the duplicate. – Martijn Pieters Feb 16 '21 at 21:43
  • Replace your argument with `tire: Optional[Tire] = None`, and in the body of the method, use `if tire is None: tire = Tire(tread_count=50, size=20)`. – Martijn Pieters Feb 16 '21 at 21:44

0 Answers0