read_excel and Excel File Engine conflict (#26736)

Author: alimcmaster1

doc/source/whatsnew/v0.25.0.rst

@@ -665,6 +665,7 @@ I/O
 - Bug in :func:`read_json` where date strings with ``Z`` were not converted to a UTC timezone (:issue:`26168`)
 - Added ``cache_dates=True`` parameter to :meth:`read_csv`, which allows to cache unique dates when they are parsed (:issue:`25990`)
 - :meth:`DataFrame.to_excel` now raises a ``ValueError`` when the caller's dimensions exceed the limitations of Excel (:issue:`26051`)
+- :func:`read_excel` now raises a ``ValueError`` when input is of type :class:`pandas.io.excel.ExcelFile` and ``engine`` param is passed since :class:`pandas.io.excel.ExcelFile` has an engine defined (:issue:`26566`)
 
 Plotting
 ^^^^^^^^

pandas/io/excel/_base.py

@@ -289,6 +289,9 @@ def read_excel(io,
 
     if not isinstance(io, ExcelFile):
         io = ExcelFile(io, engine=engine)
+    elif engine and engine != io.engine:
+        raise ValueError("Engine should not be specified when passing "
+                         "an ExcelFile - ExcelFile already has the engine set")
 
     return io.parse(
         sheet_name=sheet_name,
@@ -777,6 +780,7 @@ def __init__(self, io, engine=None):
         if engine not in self._engines:
             raise ValueError("Unknown engine: {engine}".format(engine=engine))
 
+        self.engine = engine
         # could be a str, ExcelFile, Book, etc.
         self.io = io
         # Always a string

pandas/tests/io/excel/test_readers.py

@@ -834,3 +834,14 @@ def test_reader_closes_file(self, read_ext):
             pd.read_excel(xlsx, 'Sheet1', index_col=0)
 
         assert f.closed
+
+    @pytest.mark.parametrize('excel_engine', [
+        'xlrd',
+        None
+    ])
+    def test_read_excel_engine_value(self, read_ext, excel_engine):
+        # GH 26566
+        xl = ExcelFile("test1" + read_ext, engine=excel_engine)
+        msg = "Engine should not be specified when passing an ExcelFile"
+        with pytest.raises(ValueError, match=msg):
+            pd.read_excel(xl, engine='openpyxl')