Address comments of @gfyoung in pandas-dev#25153

pandas/io/excel/_base.py

@@ -590,9 +590,8 @@ def __new__(cls, path, engine=None, **kwargs):
                     if engine == 'auto':
                         engine = _get_default_writer(ext)
                 except KeyError:
-                    error = ValueError("No engine for filetype: '{ext}'"
-                                       .format(ext=ext))
-                    raise error
+                    raise ValueError("No engine for filetype: '{ext}'"
+                                     .format(ext=ext))
             cls = get_writer(engine)
 
         return object.__new__(cls)

pandas/io/excel/_util.py

@@ -7,7 +7,9 @@
 
 from pandas.core import config
 
-_writer_extensions = ["xlsx", "xls", "xlsm"]
+
+# the following extensions are already registered in pandas/core/config_init.py
+_registered_writer_extensions = ["xlsx", "xls", "xlsm"]
 
 
 _writers = {}
@@ -24,13 +26,15 @@ def register_writer(klass):
     for ext in klass.supported_extensions:
         if ext.startswith('.'):
             ext = ext[1:]
-        if ext not in _writer_extensions:
+        if ext not in _registered_writer_extensions:
             config.register_option("io.excel.{ext}.writer".format(ext=ext),
                                    engine_name, validator=str)
-            _writer_extensions.append(ext)
+            _registered_writer_extensions.append(ext)
 
 
 def _get_default_writer(ext):
+    """Return the default writer per extension. This default engine is used
+    unles another engine is explicitly defined."""
     _default_writers = {'xlsx': 'openpyxl', 'xlsm': 'openpyxl', 'xls': 'xlwt'}
     try:
         import xlsxwriter  # noqa
@@ -230,8 +234,6 @@ def _fill_mi_header(row, control_row):
 
     return _maybe_convert_to_string(row), control_row
 
-# fill blank if index_col not None
-
 
 def _pop_header_name(row, index_col):
     """