Remove use of isinstance

README.md

@@ -64,7 +64,7 @@ with open("path_to_file/conf.toml", "rb") as f:
     toml_dict = tomli.load(f)
 ```
 
-Opening the file in binary mode (with the `"rb"` flag) is highly encouraged.
+The file must be opened in binary mode (with the `"rb"` flag).
 Binary mode will enforce decoding the file as UTF-8 with universal newlines disabled,
 both of which are required to correctly parse TOML.
 Support for text file objects is deprecated for removal in the next major release.

tomli/_parser.py

@@ -63,11 +63,11 @@ class TOMLDecodeError(ValueError):
 
 
 def load(fp: IO, *, parse_float: ParseFloat = float) -> Dict[str, Any]:
-    """Parse TOML from a file object."""
+    """Parse TOML from a binary file object."""
     s = fp.read()
-    if isinstance(s, bytes):
+    try:
         s = s.decode()
-    else:
+    except AttributeError:
         warnings.warn(
             "Text file object support is deprecated in favor of binary file objects."
             ' Use `open("foo.toml", "rb")` to open the file in binary mode.',