Support nested ReadTransaction (#59)

* Support nested ReadTransaction

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/pycrdt/transaction.py

@@ -14,9 +14,9 @@ class Transaction:
     _txn: _Transaction | None
     _nb: int
 
-    def __init__(self, doc: Doc) -> None:
+    def __init__(self, doc: Doc, _txn: _Transaction | None = None) -> None:
         self._doc = doc
-        self._txn = None
+        self._txn = _txn
         self._nb = 0
 
     def __enter__(self) -> Transaction:
@@ -45,19 +45,4 @@ def __exit__(
 
 
 class ReadTransaction(Transaction):
-    def __init__(self, doc: Doc, _txn: _Transaction) -> None:
-        self._doc = doc
-        self._txn = _txn
-
-    def __enter__(self) -> Transaction:
-        self._doc._txn = self
-        return self
-
-    def __exit__(
-        self,
-        exc_type: type[BaseException] | None,
-        exc_val: BaseException | None,
-        exc_tb: TracebackType | None,
-    ) -> None:
-        self._txn = None
-        self._doc._txn = None
+    pass

tests/test_transaction.py

@@ -15,7 +15,12 @@ def test_callback_transaction():
     events = []
 
     def callback(event):
-        events.append(str(event.target))
+        target = event.target
+        doc = target.doc
+        with doc.transaction():
+            events.append(target.to_py())
+            with doc.transaction():
+                events.append(str(target))
 
     text.observe(callback)
     array.observe(callback)
@@ -25,4 +30,11 @@ def callback(event):
         text += " world"
     array.append(1)
     map_["foo"] = "bar"
-    assert events == ["hello world", "[1.0]", '{"foo":"bar"}']
+    assert events == [
+        "hello world",
+        "hello world",
+        [1.0],
+        "[1.0]",
+        {"foo": "bar"},
+        '{"foo":"bar"}',
+    ]