Dev Blog

Identity and class design ​

Identity, equality, and the lifecycle ​

By default, two separately created instances of an ordinary class do not become equal merely because their fields match. is checks identity, while the meaning of == can be defined with a special method in Topic 10. Assignment other = obj does not copy the object: both names reference the same state.

py
class Label:
    def __init__(self, text: str) -> None:
        self.text = text


a = Label("A")
b = Label("A")
c = a
print(a == b, a is b, a is c)
c.text = "B"
print(a.text, b.text)
False False True
B A

del a removes a name binding; it does not order immediate destruction of an object that still has references. Garbage collection frees unreachable objects, but finalization timing is not a universal language guarantee. Do not use __del__ as the primary way to close a file or commit an operation. Resources need explicit management, including a with context manager.

Design and working in PyCharm ​

Start with the class's responsibility: what entity it represents, which states are valid, and which operations a client can perform. Avoid a class that simultaneously reads keyboard input, calculates a rate, draws a table, and saves a file. The model returns data; the interface decides how to receive and display it. This lets you test the model without launching a menu and reuse it in another application.

In PyCharm, Structure (Alt+7) shows classes, methods, and properties, while File Structure (Ctrl+F12) lets you quickly navigate to a member. The Generate menu (Alt+Insert) depends on context; check the actual Python actions rather than expecting Java or C# entries. Help: https://www.jetbrains.com/help/pycharm/generating-code.html.

Screenshot

BankAccount editor; Alt+7; show methods, property and fields.

Figure 8.5. Class members in the Structure window

UML support in PyCharm depends on the available features and components of the installed product. Check for Diagrams → Show Diagram in the context menu. Its absence does not prevent completing the work: you can draw UML manually using the same rules. Official instructions: https://www.jetbrains.com/help/pycharm/class-diagram.html.

Screenshot

Pause after cart.add; expand cart, items, Product and mangled discount field.

Figure 8.6. Cart state in the debugger

Common mistakes include forgetting self, sharing a class-level list, recursively calling a setter forever, partially changing state before validation, and duplicating a derived value in two fields. For each method, test an ordinary case, a boundary, and a rejection. After rejection, compare the state with its previous value; the exception text alone is insufficient. For two instances, check that their collections are independent.