Raise TypeError if content is passed a 'dict' instance. (#2495)

httpx/_content.py

@@ -114,7 +114,11 @@ def encode_content(
         headers = {"Content-Length": str(content_length)} if body else {}
         return headers, ByteStream(body)
 
-    elif isinstance(content, Iterable):
+    elif isinstance(content, Iterable) and not isinstance(content, dict):
+        # `not isinstance(content, dict)` is a bit oddly specific, but it
+        # catches a case that's easy for users to make in error, and would
+        # otherwise pass through here, like any other bytes-iterable,
+        # because `dict` happens to be iterable. See issue #2491.
         content_length_or_none = peek_filelike_length(content)
 
         if content_length_or_none is None:

tests/test_content.py

@@ -360,6 +360,9 @@ def test_invalid_argument():
     with pytest.raises(TypeError):
         httpx.Request(method, url, content=123)  # type: ignore
 
+    with pytest.raises(TypeError):
+        httpx.Request(method, url, content={"a": "b"})  # type: ignore
+
 
 @pytest.mark.asyncio
 async def test_multipart_multiple_files_single_input_content():