Thread-safe atomic counters

Feature or enhancement

Python doesn't currently have a thread-safe atomic counter. CPython currently internally uses a few different patterns to implement thread-safe counters. For example, some test code uses appends to a shared list because the implementation of list.append() is atomic.

We should provide a counter that is explicitly thread-safe so that people don't have to rely on other idioms. The minimum initial API should provide:

• atomic increment
• get value (i.e., inspect)

The threading module may be a good place for it.

cc @rhettinger

まず threading モジュールと、issue で説明されている既存のスレッドセーフなカウンターのパターンを確認します。アトミックなインクリメントと値の検査に必要な最小限の API を定義してテストし、並行アクセス時の動作も含めます。