A for loop calls iter() once and next() until StopIteration. Write __iter__ and __next__ yourself, avoid the iterator that only works once, and know when a generator in __iter__ does the same job in fewer lines.
If a class mostly holds data, @dataclass writes __init__, a readable printout and equality for you. Use field(default_factory=list) for lists, __post_init__ for checks and frozen=True for values that must not change.
Write a context manager when something must be undone no matter how the block ends. Build one as a class and with @contextmanager, keep yield inside try/finally, and don't swallow errors by accident.
Getters compute a value when you read it, so totals never go stale. Setters check a value when you assign it, so a negative price throws instead of saving. With private fields, nobody can go around them.
Stop checking error messages with includes(). Extend Error, set name, add the fields the handler needs, catch with instanceof, and wrap low-level errors with cause so the real reason isn't lost.
Trace a wrong callback receiver, bind one stable listener, and verify start, stop and restart without duplicate or stale subscriptions.
Run the same class with the private keyword and with a hash field, and compare the output, the errors and what outside code can still reach. After this, you can pick one and say why.