ENH: loosen XLS signature

doc/source/whatsnew/v1.3.0.rst

@@ -195,7 +195,7 @@ Other enhancements
 - Improved integer type mapping from pandas to SQLAlchemy when using :meth:`DataFrame.to_sql` (:issue:`35076`)
 - :func:`to_numeric` now supports downcasting of nullable ``ExtensionDtype`` objects (:issue:`33013`)
 - Add support for dict-like names in :class:`MultiIndex.set_names` and :class:`MultiIndex.rename` (:issue:`20421`)
-- :func:`pandas.read_excel` can now auto detect .xlsb files (:issue:`35416`)
+- :func:`pandas.read_excel` can now auto detect .xlsb files (:issue:`35416`) and older .xls files (:issue:`41225`)
 - :class:`pandas.ExcelWriter` now accepts an ``if_sheet_exists`` parameter to control the behaviour of append mode when writing to existing sheets (:issue:`40230`)
 - :meth:`.Rolling.sum`, :meth:`.Expanding.sum`, :meth:`.Rolling.mean`, :meth:`.Expanding.mean`, :meth:`.ExponentialMovingWindow.mean`, :meth:`.Rolling.median`, :meth:`.Expanding.median`, :meth:`.Rolling.max`, :meth:`.Expanding.max`, :meth:`.Rolling.min`, and :meth:`.Expanding.min` now support ``Numba`` execution with the ``engine`` keyword (:issue:`38895`, :issue:`41267`)
 - :meth:`DataFrame.apply` can now accept NumPy unary operators as strings, e.g. ``df.apply("sqrt")``, which was already the case for :meth:`Series.apply` (:issue:`39116`)
@@ -224,7 +224,6 @@ Other enhancements
 - :meth:`.GroupBy.any` and :meth:`.GroupBy.all` return a ``BooleanDtype`` for columns with nullable data types (:issue:`33449`)
 - Constructing a :class:`DataFrame` or :class:`Series` with the ``data`` argument being a Python iterable that is *not* a NumPy ``ndarray`` consisting of NumPy scalars will now result in a dtype with a precision the maximum of the NumPy scalars; this was already the case when ``data`` is a NumPy ``ndarray`` (:issue:`40908`)
 - Add keyword ``sort`` to :func:`pivot_table` to allow non-sorting of the result (:issue:`39143`)
--
 
 .. ---------------------------------------------------------------------------
 
@@ -831,6 +830,8 @@ I/O
 - Bug in :meth:`DataFrame.to_string` misplacing the truncation column when ``index=False`` (:issue:`40907`)
 - Bug in :func:`read_orc` always raising ``AttributeError`` (:issue:`40918`)
 - Bug in the conversion from pyarrow to pandas (e.g. for reading Parquet) with nullable dtypes and a pyarrow array whose data buffer size is not a multiple of dtype size (:issue:`40896`)
+- Bug in read_excel would raise an error when pandas could not determine the file type, even when user specified the ``engine`` argument (:issue:`41225`)
+-
 
 Period
 ^^^^^^

pandas/io/excel/_base.py

@@ -1014,16 +1014,21 @@ def close(self):
         return content
 
 
-XLS_SIGNATURE = b"\xD0\xCF\x11\xE0\xA1\xB1\x1A\xE1"
+XLS_SIGNATURES = (
+    b"\x09\x00\x04\x00\x07\x00\x10\x00",  # BIFF2
+    b"\x09\x02\x06\x00\x00\x00\x10\x00",  # BIFF3
+    b"\x09\x04\x06\x00\x00\x00\x10\x00",  # BIFF4
+    b"\xD0\xCF\x11\xE0\xA1\xB1\x1A\xE1",  # Compound File Binary
+)
 ZIP_SIGNATURE = b"PK\x03\x04"
-PEEK_SIZE = max(len(XLS_SIGNATURE), len(ZIP_SIGNATURE))
+PEEK_SIZE = max(map(len, XLS_SIGNATURES + (ZIP_SIGNATURE, )))
 
 
 @doc(storage_options=_shared_docs["storage_options"])
 def inspect_excel_format(
     content_or_path: FilePathOrBuffer,
     storage_options: StorageOptions = None,
-) -> str:
+) -> str | None:
     """
     Inspect the path or content of an excel file and get its format.
 
@@ -1037,8 +1042,8 @@ def inspect_excel_format(
 
     Returns
     -------
-    str
-        Format of file.
+    str or None
+        Format of file if it can be determined.
 
     Raises
     ------
@@ -1063,10 +1068,10 @@ def inspect_excel_format(
             peek = buf
         stream.seek(0)
 
-        if peek.startswith(XLS_SIGNATURE):
+        if any(peek.startswith(sig) for sig in XLS_SIGNATURES):
             return "xls"
         elif not peek.startswith(ZIP_SIGNATURE):
-            raise ValueError("File is not a recognized excel file")
+            return None
 
         # ZipFile typing is overly-strict
         # https://github.com/python/typeshed/issues/4212
@@ -1174,8 +1179,12 @@ def __init__(
                 ext = inspect_excel_format(
                     content_or_path=path_or_buffer, storage_options=storage_options
                 )
+                if ext is None:
+                    raise ValueError(
+                        "Excel file format cannot be determined, you must specify "
+                        "an engine manually."
+                    )
 
-            # ext will always be valid, otherwise inspect_excel_format would raise
             engine = config.get_option(f"io.excel.{ext}.reader", silent=True)
             if engine == "auto":
                 engine = get_default_engine(ext, mode="reader")
@@ -1190,12 +1199,13 @@ def __init__(
                         path_or_buffer, storage_options=storage_options
                     )
 
-            if ext != "xls" and xlrd_version >= "2":
+            # Pass through if ext is None, otherwise check if ext valid for xlrd
+            if ext and ext != "xls" and xlrd_version >= "2":
                 raise ValueError(
                     f"Your version of xlrd is {xlrd_version}. In xlrd >= 2.0, "
                     f"only the xls format is supported. Install openpyxl instead."
                 )
-            elif ext != "xls":
+            elif ext and ext != "xls":
                 caller = inspect.stack()[1]
                 if (
                     caller.filename.endswith(

pandas/tests/io/excel/test_readers.py

@@ -718,9 +718,19 @@ def test_missing_file_raises(self, read_ext):
 
     def test_corrupt_bytes_raises(self, read_ext, engine):
         bad_stream = b"foo"
-        if engine is None or engine == "xlrd":
+        if engine is None:
             error = ValueError
-            msg = "File is not a recognized excel file"
+            msg = (
+                "Excel file format cannot be determined, you must "
+                "specify an engine manually."
+            )
+        elif engine == "xlrd":
+            from xlrd import XLRDError
+            error = XLRDError
+            msg = (
+                "Unsupported format, or corrupt file: Expected BOF "
+                "record; found b'foo'"
+            )
         else:
             error = BadZipFile
             msg = "File is not a zip file"

pandas/tests/io/excel/test_xlrd.py

@@ -1,3 +1,4 @@
+import io
 import pytest
 
 from pandas.compat._optional import import_optional_dependency
@@ -7,6 +8,7 @@
 from pandas.tests.io.excel import xlrd_version
 
 from pandas.io.excel import ExcelFile
+from pandas.io.excel._base import inspect_excel_format
 
 xlrd = pytest.importorskip("xlrd")
 xlwt = pytest.importorskip("xlwt")
@@ -77,3 +79,18 @@ def test_read_excel_warning_with_xlsx_file(datapath):
     else:
         with tm.assert_produces_warning(None):
             pd.read_excel(path, "Sheet1", engine=None)
+
+
+@pytest.mark.parametrize(
+    "file_header",
+    [
+        b"\x09\x00\x04\x00\x07\x00\x10\x00",
+        b"\x09\x02\x06\x00\x00\x00\x10\x00",
+        b"\x09\x04\x06\x00\x00\x00\x10\x00",
+        b"\xd0\xcf\x11\xe0\xa1\xb1\x1a\xe1",
+    ]
+)
+def test_read_old_xls_files(file_header):
+    # GH 41226
+    f = io.BytesIO(file_header)
+    assert inspect_excel_format(f) == "xls"