Fix writing with pre-opened file (#54)

* Fixing #53

* Add regression test for the failing case.

src/pydantic_yaml/dumper.py

@@ -22,7 +22,7 @@ def _chk_model(model: Any) -> BaseModel:
     raise TypeError(f"We can currently only write `pydantic.BaseModel`, but recieved: {model!r}")
 
 
-def _write_yaml_model(stream: IOBase, model: BaseModel, **kwargs):
+def _write_yaml_model(stream: IOBase, model: BaseModel, **kwargs) -> None:
     """Write YAML model to the stream object.
 
     This uses JSON dumping as an intermediary.
@@ -88,6 +88,7 @@ def to_yaml_file(file: Union[Path, str, IOBase], model: BaseModel, **kwargs) ->
     model = _chk_model(model)
     if isinstance(file, IOBase):
         _write_yaml_model(file, model, **kwargs)
+        return
 
     if isinstance(file, str):
         file = Path(file).resolve()

src/test/test_basic.py

@@ -1,12 +1,13 @@
 """Tests for basic functionality."""
 
+from pathlib import Path
 from typing import Type
 
 import pydantic
 import pytest
 from pydantic import BaseModel
 
-from pydantic_yaml import parse_yaml_file_as, parse_yaml_raw_as, to_yaml_str
+from pydantic_yaml import parse_yaml_file_as, parse_yaml_raw_as, to_yaml_str, to_yaml_file
 from pydantic_yaml.examples.base_models import (
     A,
     B,
@@ -77,3 +78,9 @@ def test_secret_yes_rt():
     mdl = parse_yaml_raw_as(SecretTstModelDumpable, raw)
     assert mdl.ss.get_secret_value() == "123"
     assert mdl.sb.get_secret_value() == b"321"
+
+
+def test_write_open_file(tmpdir):
+    """Test writing to a pre-opened file."""
+    with (Path(tmpdir) / "test_write_open_file.yaml").open(mode="w") as f:
+        to_yaml_file(f, A(a="a"))