API: add dtype param to read_excel (pandas-dev#14786)

* API add dtype param to read_excel

* doc fixup

Author: chris-b1

doc/source/io.rst

@@ -2538,6 +2538,20 @@ missing data to recover integer dtype:
    cfun = lambda x: int(x) if x else -1
    read_excel('path_to_file.xls', 'Sheet1', converters={'MyInts': cfun})
 
+dtype Specifications
+++++++++++++++++++++
+
+.. versionadded:: 0.20
+
+As an alternative to converters, the type for an entire column can
+be specified using the `dtype` keyword, which takes a dictionary
+mapping column names to types.  To interpret data with
+no type inference, use the type ``str`` or ``object``.
+
+.. code-block:: python
+
+   read_excel('path_to_file.xls', dtype={'MyInts': 'int64', 'MyText': str})
+
 .. _io.excel_writer:
 
 Writing Excel Files

doc/source/whatsnew/v0.20.0.txt

@@ -22,8 +22,8 @@ New features
 ~~~~~~~~~~~~
 
 
-``read_csv`` supports ``dtype`` keyword for python engine
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``dtype`` keyword for data io
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The ``dtype`` keyword argument in the :func:`read_csv` function for specifying the types of parsed columns
  is now supported with the ``'python'`` engine (:issue:`14295`). See the :ref:`io docs <io.dtypes>` for more information.
@@ -35,7 +35,7 @@ The ``dtype`` keyword argument in the :func:`read_csv` function for specifying t
    pd.read_csv(StringIO(data), engine='python', dtype={'a':'float64', 'b':'object'}).dtypes
 
 The ``dtype`` keyword argument is also now supported in the :func:`read_fwf` function for parsing
-fixed-width text files.
+fixed-width text files, and :func:`read_excel` for parsing Excel files.
 
 .. ipython:: python
 

pandas/io/excel.py

@@ -87,6 +87,14 @@
     either be integers or column labels, values are functions that take one
     input argument, the Excel cell content, and return the transformed
     content.
+dtype : Type name or dict of column -> type, default None
+    Data type for data or columns. E.g. {'a': np.float64, 'b': np.int32}
+    Use `str` or `object` to preserve and not interpret dtype.
+    If converters are specified, they will be applied INSTEAD
+    of dtype conversion.
+
+    .. versionadded:: 0.20.0
+
 true_values : list, default None
     Values to consider as True
 
@@ -184,8 +192,8 @@ def read_excel(io, sheetname=0, header=0, skiprows=None, skip_footer=0,
                index_col=None, names=None, parse_cols=None, parse_dates=False,
                date_parser=None, na_values=None, thousands=None,
                convert_float=True, has_index_names=None, converters=None,
-               true_values=None, false_values=None, engine=None, squeeze=False,
-               **kwds):
+               dtype=None, true_values=None, false_values=None, engine=None,
+               squeeze=False, **kwds):
 
     if not isinstance(io, ExcelFile):
         io = ExcelFile(io, engine=engine)
@@ -195,7 +203,7 @@ def read_excel(io, sheetname=0, header=0, skiprows=None, skip_footer=0,
         index_col=index_col, parse_cols=parse_cols, parse_dates=parse_dates,
         date_parser=date_parser, na_values=na_values, thousands=thousands,
         convert_float=convert_float, has_index_names=has_index_names,
-        skip_footer=skip_footer, converters=converters,
+        skip_footer=skip_footer, converters=converters, dtype=dtype,
         true_values=true_values, false_values=false_values, squeeze=squeeze,
         **kwds)
 
@@ -318,7 +326,7 @@ def _parse_excel(self, sheetname=0, header=0, skiprows=None, names=None,
                      parse_cols=None, parse_dates=False, date_parser=None,
                      na_values=None, thousands=None, convert_float=True,
                      true_values=None, false_values=None, verbose=False,
-                     squeeze=False, **kwds):
+                     dtype=None, squeeze=False, **kwds):
 
         skipfooter = kwds.pop('skipfooter', None)
         if skipfooter is not None:
@@ -501,6 +509,7 @@ def _parse_cell(cell_contents, cell_typ):
                                     skiprows=skiprows,
                                     skipfooter=skip_footer,
                                     squeeze=squeeze,
+                                    dtype=dtype,
                                     **kwds)
 
                 output[asheetname] = parser.read()

pandas/io/parsers.py

@@ -18,7 +18,7 @@
 from pandas.types.common import (is_integer, _ensure_object,
                                  is_list_like, is_integer_dtype,
                                  is_float, is_dtype_equal,
-                                 is_object_dtype,
+                                 is_object_dtype, is_string_dtype,
                                  is_scalar, is_categorical_dtype)
 from pandas.types.missing import isnull
 from pandas.types.cast import _astype_nansafe
@@ -1329,7 +1329,7 @@ def _convert_to_ndarrays(self, dct, na_values, na_fvalues, verbose=False,
                     try_num_bool=False)
             else:
                 # skip inference if specified dtype is object
-                try_num_bool = not (cast_type and is_object_dtype(cast_type))
+                try_num_bool = not (cast_type and is_string_dtype(cast_type))
 
                 # general type inference and conversion
                 cvals, na_count = self._infer_types(

pandas/io/tests/data/testdtype.xls

@@ -373,6 +373,33 @@ def test_reader_converters(self):
         actual = self.get_exceldf(basename, 'Sheet1', converters=converters)
         tm.assert_frame_equal(actual, expected)
 
+    def test_reader_dtype(self):
+        # GH 8212
+        basename = 'testdtype'
+        actual = self.get_exceldf(basename)
+
+        expected = DataFrame({
+            'a': [1, 2, 3, 4],
+            'b': [2.5, 3.5, 4.5, 5.5],
+            'c': [1, 2, 3, 4],
+            'd': [1.0, 2.0, np.nan, 4.0]}).reindex(
+                columns=['a', 'b', 'c', 'd'])
+
+        tm.assert_frame_equal(actual, expected)
+
+        actual = self.get_exceldf(basename,
+                                  dtype={'a': 'float64',
+                                         'b': 'float32',
+                                         'c': str})
+
+        expected['a'] = expected['a'].astype('float64')
+        expected['b'] = expected['b'].astype('float32')
+        expected['c'] = ['001', '002', '003', '004']
+        tm.assert_frame_equal(actual, expected)
+
+        with tm.assertRaises(ValueError):
+            actual = self.get_exceldf(basename, dtype={'d': 'int64'})
+
     def test_reading_all_sheets(self):
         # Test reading all sheetnames by setting sheetname to None,
         # Ensure a dict is returned.