BUG: ExcelWriter's engine and supported_extensions are properties (#46444)

Author: twoertwein

pandas/io/excel/_base.py

@@ -945,9 +945,9 @@ class ExcelWriter(metaclass=abc.ABCMeta):
     # - Mandatory
     #   - ``write_cells(self, cells, sheet_name=None, startrow=0, startcol=0)``
     #     --> called to write additional DataFrames to disk
-    #   - ``supported_extensions`` (tuple of supported extensions), used to
+    #   - ``_supported_extensions`` (tuple of supported extensions), used to
     #      check that engine supports the given extension.
-    #   - ``engine`` - string that gives the engine name. Necessary to
+    #   - ``_engine`` - string that gives the engine name. Necessary to
     #     instantiate class directly and bypass ``ExcelWriterMeta`` engine
     #     lookup.
     #   - ``save(self)`` --> called to save file to disk
@@ -961,6 +961,10 @@ class ExcelWriter(metaclass=abc.ABCMeta):
     # You also need to register the class with ``register_writer()``.
     # Technically, ExcelWriter implementations don't need to subclass
     # ExcelWriter.
+
+    _engine: str
+    _supported_extensions: tuple[str, ...]
+
     def __new__(
         cls,
         path: FilePath | WriteExcelBuffer | ExcelWriter,
@@ -983,7 +987,6 @@ def __new__(
             )
 
         # only switch class if generic(ExcelWriter)
-
         if cls is ExcelWriter:
             if engine is None or (isinstance(engine, str) and engine == "auto"):
                 if isinstance(path, str):
@@ -1027,16 +1030,14 @@ def __new__(
     _path = None
 
     @property
-    @abc.abstractmethod
-    def supported_extensions(self) -> tuple[str, ...] | list[str]:
+    def supported_extensions(self) -> tuple[str, ...]:
         """Extensions that writer engine supports."""
-        pass
+        return self._supported_extensions
 
     @property
-    @abc.abstractmethod
     def engine(self) -> str:
         """Name of engine."""
-        pass
+        return self._engine
 
     @property
     @abc.abstractmethod
@@ -1292,12 +1293,7 @@ def check_extension(cls, ext: str) -> Literal[True]:
         """
         if ext.startswith("."):
             ext = ext[1:]
-        # error: "Callable[[ExcelWriter], Any]" has no attribute "__iter__" (not
-        #  iterable)
-        if not any(
-            ext in extension
-            for extension in cls.supported_extensions  # type: ignore[attr-defined]
-        ):
+        if not any(ext in extension for extension in cls._supported_extensions):
             raise ValueError(f"Invalid extension for engine '{cls.engine}': '{ext}'")
         else:
             return True

pandas/io/excel/_odswriter.py

@@ -25,8 +25,8 @@
 
 
 class ODSWriter(ExcelWriter):
-    engine = "odf"
-    supported_extensions = (".ods",)
+    _engine = "odf"
+    _supported_extensions = (".ods",)
 
     def __init__(
         self,

pandas/io/excel/_openpyxl.py

@@ -36,8 +36,8 @@
 
 
 class OpenpyxlWriter(ExcelWriter):
-    engine = "openpyxl"
-    supported_extensions = (".xlsx", ".xlsm")
+    _engine = "openpyxl"
+    _supported_extensions = (".xlsx", ".xlsm")
 
     def __init__(
         self,

pandas/io/excel/_util.py

@@ -41,9 +41,7 @@ def register_writer(klass: ExcelWriter_t) -> None:
     """
     if not callable(klass):
         raise ValueError("Can only register callables as engines")
-    engine_name = klass.engine
-    # for mypy
-    assert isinstance(engine_name, str)
+    engine_name = klass._engine
     _writers[engine_name] = klass
 
 

pandas/io/excel/_xlsxwriter.py

@@ -169,8 +169,8 @@ def convert(cls, style_dict, num_format_str=None):
 
 
 class XlsxWriter(ExcelWriter):
-    engine = "xlsxwriter"
-    supported_extensions = (".xlsx",)
+    _engine = "xlsxwriter"
+    _supported_extensions = (".xlsx",)
 
     def __init__(
         self,

pandas/io/excel/_xlwt.py

@@ -25,8 +25,8 @@
 
 
 class XlwtWriter(ExcelWriter):
-    engine = "xlwt"
-    supported_extensions = (".xls",)
+    _engine = "xlwt"
+    _supported_extensions = (".xls",)
 
     def __init__(
         self,

pandas/tests/io/excel/test_writers.py

@@ -1319,8 +1319,8 @@ class DummyClass(ExcelWriter):
             called_save = False
             called_write_cells = False
             called_sheets = False
-            supported_extensions = ["xlsx", "xls"]
-            engine = "dummy"
+            _supported_extensions = ("xlsx", "xls")
+            _engine = "dummy"
 
             def book(self):
                 pass