Fix: Improve error message for invalid data parameter with AsyncClient

httpx/_content.py

@@ -202,6 +202,19 @@ def encode_request(
         # However for compat with requests, we *do* still support
         # `data=<bytes...>` usages. We deal with that case here, treating it
         # as if `content=<...>` had been supplied instead.
+
+        # Validate that data is bytes-like or an iterable of bytes, not other types
+        if isinstance(data, (list, tuple)):
+            # Check if it's a list/tuple of bytes
+            for item in data:
+                if not isinstance(item, (bytes, bytearray, memoryview)):
+                    item_type = type(item).__name__
+                    raise TypeError(
+                        f"Expected bytes-like object in 'data' sequence, "
+                        f"got {item_type}. Use 'json=' for JSON data or "
+                        f"'data={{...}}' for form data."
+                    )
+
         message = "Use 'content=<...>' to upload raw bytes/text content."
         warnings.warn(message, DeprecationWarning, stacklevel=2)
         return encode_content(data)

tests/test_content.py

@@ -364,6 +364,27 @@ def test_invalid_argument():
         httpx.Request(method, url, content={"a": "b"})  # type: ignore
 
 
+def test_invalid_data_list_of_dicts():
+    """Test that passing a list of dicts to data= produces a clear error message.
+
+    This was previously raising a confusing 'Attempted to send a sync request
+    with an AsyncClient instance' error with AsyncClient. Now it should give
+    a clear error for both sync and async clients.
+    """
+    with pytest.raises(TypeError, match="Expected bytes-like object.*got dict"):
+        httpx.Request(method, url, data=[{"a": "b"}])  # type: ignore
+
+
+@pytest.mark.anyio
+async def test_invalid_data_list_of_dicts_async():
+    """Test that AsyncClient produces clear error for invalid data parameter.
+
+    Regression test for issue #3471.
+    """
+    with pytest.raises(TypeError, match="Expected bytes-like object.*got dict"):
+        httpx.Request(method, url, data=[{"a": "b"}])  # type: ignore
+
+
 @pytest.mark.anyio
 async def test_multipart_multiple_files_single_input_content():
     files = [