chore: use a dataclass to return values from `prepare_send_data`

I found the tuple of three values confusing. So instead use a
dataclass to return the three values. It is still confusing but a
little bit less so.

Also add some unit tests

4 files changed, 62 insertions, 15 deletions

diff --git a/gitlab/_backends/requests_backend.py b/gitlab/_backends/requests_backend.py
index fd40baa..d70cf42 100644
--- a/gitlab/_backends/requests_backend.py
+++ b/gitlab/_backends/requests_backend.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
-from typing import Any, Dict, Optional, Tuple, TYPE_CHECKING, Union
+import dataclasses
+from typing import Any, Dict, Optional, TYPE_CHECKING, Union
 
 import requests
 from requests.structures import CaseInsensitiveDict
@@ -9,6 +10,20 @@ from requests_toolbelt.multipart.encoder import MultipartEncoder  # type: ignore
 from . import protocol
 
 
+@dataclasses.dataclass
+class SendData:
+    content_type: str
+    data: Optional[Union[Dict[str, Any], MultipartEncoder]] = None
+    json: Optional[Union[Dict[str, Any], bytes]] = None
+
+    def __post_init__(self) -> None:
+        if self.json is not None and self.data is not None:
+            raise ValueError(
+                f"`json` and `data` are mutually exclusive. Only one can be set. "
+                f"json={self.json!r}  data={self.data!r}"
+            )
+
+
 class RequestsResponse(protocol.BackendResponse):
     def __init__(self, response: requests.Response) -> None:
         self._response: requests.Response = response
@@ -50,11 +65,7 @@ class RequestsBackend(protocol.Backend):
         files: Optional[Dict[str, Any]] = None,
         post_data: Optional[Union[Dict[str, Any], bytes]] = None,
         raw: bool = False,
-    ) -> Tuple[
-        Optional[Union[Dict[str, Any], bytes]],
-        Optional[Union[Dict[str, Any], MultipartEncoder]],
-        str,
-    ]:
+    ) -> SendData:
         if files:
             if post_data is None:
                 post_data = {}
@@ -70,12 +81,12 @@ class RequestsBackend(protocol.Backend):
             post_data["avatar"] = files.get("avatar")
 
             data = MultipartEncoder(post_data)
-            return (None, data, data.content_type)
+            return SendData(data=data, content_type=data.content_type)
 
         if raw and post_data:
-            return (None, post_data, "application/octet-stream")
+            return SendData(data=post_data, content_type="application/octet-stream")
 
-        return (post_data, None, "application/json")
+        return SendData(json=post_data, content_type="application/json")
 
     def http_request(
         self,
diff --git a/gitlab/client.py b/gitlab/client.py
index c3982f3..495ed44 100644
--- a/gitlab/client.py
+++ b/gitlab/client.py
@@ -716,10 +716,8 @@ class Gitlab:
             retry_transient_errors = self.retry_transient_errors
 
         # We need to deal with json vs. data when uploading files
-        json, data, content_type = self._backend.prepare_send_data(
-            files, post_data, raw
-        )
-        opts["headers"]["Content-type"] = content_type
+        send_data = self._backend.prepare_send_data(files, post_data, raw)
+        opts["headers"]["Content-type"] = send_data.content_type
 
         cur_retries = 0
         while True:
@@ -727,8 +725,8 @@ class Gitlab:
                 result = self._backend.http_request(
                     method=verb,
                     url=url,
-                    json=json,
-                    data=data,
+                    json=send_data.json,
+                    data=send_data.data,
                     params=params,
                     timeout=timeout,
                     verify=verify,
diff --git a/tests/unit/_backends/__init__.py b/tests/unit/_backends/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tests/unit/_backends/__init__.py
diff --git a/tests/unit/_backends/test_requests_backend.py b/tests/unit/_backends/test_requests_backend.py
new file mode 100644
index 0000000..d83703d
--- /dev/null
+++ b/tests/unit/_backends/test_requests_backend.py
@@ -0,0 +1,38 @@
+import pytest
+from requests_toolbelt.multipart.encoder import MultipartEncoder  # type: ignore
+
+from gitlab._backends import requests_backend
+
+
+class TestSendData:
+    def test_senddata_json(self) -> None:
+        result = requests_backend.SendData(
+            json={"a": 1}, content_type="application/json"
+        )
+        assert result.data is None
+
+    def test_senddata_data(self) -> None:
+        result = requests_backend.SendData(
+            data={"b": 2}, content_type="application/octet-stream"
+        )
+        assert result.json is None
+
+    def test_senddata_json_and_data(self) -> None:
+        with pytest.raises(ValueError, match=r"json={'a': 1}  data={'b': 2}"):
+            requests_backend.SendData(
+                json={"a": 1}, data={"b": 2}, content_type="application/json"
+            )
+
+
+class TestRequestsBackend:
+    def test_prepare_send_data_str_parentid(self) -> None:
+        file = "12345"
+        files = {"file": ("file.tar.gz", file, "application/octet-stream")}
+        post_data = {"parent_id": "12"}
+
+        result = requests_backend.RequestsBackend.prepare_send_data(
+            files=files, post_data=post_data, raw=False
+        )
+        assert result.json is None
+        assert result.content_type.startswith("multipart/form-data")
+        assert isinstance(result.data, MultipartEncoder)