Removing unncessary class states in bdb.Breakpoint

Feature or enhancement

Proposal:

According to the comment in the Breakpoint class

Keeping state in the class is a mistake -- this means you cannot have more than one active Bdb instance.

I can support this idea further. Currently, Breakpoint has three class states, next, bplist, and bpbynumber.

class Breakpoint:
    ...
    next = 1
    bplist = {}
    bpbynumber = [None]

These states are used to enable the reuse of previously set breakpoint instances across multiple interactive sessions or other use cases. However, relying on class states is not the only way to achieve this goal.

Using class states in this case has several clear limitations. As noted in the comment, it makes it difficult to maintain more than one active Bdb instance. Additionally, logic dependent on class state can make the behavior of a new Bdb instance unpredictable in many scenarios unless the exact states of Breakpoint is fully known. Another drawback of maintaining states in the Breakpoint class is that it tightly couples the behavior of all Bdb instances. For example, the deleteMe method must be called periodically by a Bdb instance to ensure the Breakpoint class remains in a valid state.

# example deleteMe calls in Bdb methods

class Bdb:
    ...
    def clear_break(self, filename, lineno):
        ...
        for bp in Breakpoint.bplist[filename, lineno][:]:
            bp.deleteMe()  # here
        self._prune_breaks(filename, lineno)
        return None
        
    def clear_bpbynumber(self, arg):
        ...
        try:
            bp = self.get_bpbynumber(arg)
        except ValueError as err:
            return str(err)
        bp.deleteMe()  # here
        self._prune_breaks(bp.file, bp.line)
        return None

As an alternative, instance-level state can be maintained within Bdb or derived classes such as Pdb, allowing each instance to manage its own breakpoint data independently. This approach can still support the reuse of previously set breakpoint instances across multiple interactive sessions.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-127410

Lib/bdb.py、特に Breakpoint クラスと issue に示されている Bdb メソッドから始め、リンクされている PR gh-127410 を確認します。現在、Breakpoint の状態が Bdb インスタンス間でどのように共有されているか、また提案されているインスタンスレベルの所有権が Bdb と Pdb にどのような影響を与えるかを明らかにします。完了の条件は、共有された Breakpoint クラスの状態に依存せず、独立したアクティブなデバッガーインスタンスを設計がサポートすることです。