BUG: Ignore the BOM in BOM UTF-8 CSV files

closes #4793
closes #13855

doc/source/whatsnew/v0.19.0.txt

@@ -43,8 +43,8 @@ The following are now part of this API:
 
 .. _whatsnew_0190.enhancements.asof_merge:
 
-:func:`merge_asof` for asof-style time-series joining
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``merge_asof`` for asof-style time-series joining
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 A long-time requested feature has been added through the :func:`merge_asof` function, to
 support asof style joining of time-series. (:issue:`1870`, :issue:`13695`, :issue:`13709`). Full documentation is
@@ -192,8 +192,8 @@ default of the index) in a DataFrame.
 
 .. _whatsnew_0190.enhancements.read_csv_dupe_col_names_support:
 
-:func:`read_csv` has improved support for duplicate column names
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``read_csv`` has improved support for duplicate column names
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 :ref:`Duplicate column names <io.dupe_names>` are now supported in :func:`read_csv` whether
 they are in the file or passed in as the ``names`` parameter (:issue:`7160`, :issue:`9424`)
@@ -307,48 +307,6 @@ Google BigQuery Enhancements
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - The :func:`pandas.io.gbq.read_gbq` method has gained the ``dialect`` argument to allow users to specify whether to use BigQuery's legacy SQL or BigQuery's standard SQL. See the :ref:`docs <io.bigquery_reader>` for more details (:issue:`13615`).
 
-.. _whatsnew_0190.sparse:
-
-Sparse changes
-~~~~~~~~~~~~~~
-
-These changes allow pandas to handle sparse data with more dtypes, and for work to make a smoother experience with data handling.
-
-- Sparse data structure now can preserve ``dtype`` after arithmetic ops (:issue:`13848`)
-
-.. ipython:: python
-
-   s = pd.SparseSeries([0, 2, 0, 1], fill_value=0, dtype=np.int64)
-   s.dtype
-
-   s + 1
-
-- Sparse data structure now support ``astype`` to convert internal ``dtype`` (:issue:`13900`)
-
-.. ipython:: python
-
-   s = pd.SparseSeries([1., 0., 2., 0.], fill_value=0)
-   s
-   s.astype(np.int64)
-
-``astype`` fails if data contains values which cannot be converted to specified ``dtype``.
-Note that the limitation is applied to ``fill_value`` which default is ``np.nan``.
-
-.. code-block:: ipython
-
-   In [7]: pd.SparseSeries([1., np.nan, 2., np.nan], fill_value=np.nan).astype(np.int64)
-   Out[7]:
-   ValueError: unable to coerce current fill_value nan to int64 dtype
-
-- Subclassed ``SparseDataFrame`` and ``SparseSeries`` now preserve class types when slicing or transposing. (:issue:`13787`)
-- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing may raise ``IndexError`` (:issue:`13144`)
-- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing result may have normal ``Index`` (:issue:`13144`)
-- Bug in ``SparseDataFrame`` in which ``axis=None`` did not default to ``axis=0`` (:issue:`13048`)
-- Bug in ``SparseSeries`` and ``SparseDataFrame`` creation with ``object`` dtype may raise ``TypeError`` (:issue:`11633`)
-- Bug in ``SparseDataFrame`` doesn't respect passed ``SparseArray`` or ``SparseSeries`` 's dtype and ``fill_value``  (:issue:`13866`)
-- Bug in ``SparseArray`` and ``SparseSeries`` don't apply ufunc to ``fill_value`` (:issue:`13853`)
-- Bug in ``SparseSeries.abs`` incorrectly keeps negative ``fill_value`` (:issue:`13853`)
-
 .. _whatsnew_0190.enhancements.other:
 
 Other enhancements
@@ -684,8 +642,8 @@ New Behavior:
 
 .. _whatsnew_0190.api.autogenerated_chunksize_index:
 
-:func:`read_csv` called with ``chunksize`` will progressively enumerate chunks
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``read_csv`` will progressively enumerate chunks
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 When :func:`read_csv` is called with ``chunksize='n'`` and without specifying an index,
 each chunk used to have an independently generated index from `0`` to ``n-1``.
@@ -716,10 +674,52 @@ New behaviour:
 
    pd.concat(pd.read_csv(StringIO(data), chunksize=2))
 
+.. _whatsnew_0190.sparse:
+
+Sparse Changes
+^^^^^^^^^^^^^^
+
+These changes allow pandas to handle sparse data with more dtypes, and for work to make a smoother experience with data handling.
+
+- Sparse data structure now can preserve ``dtype`` after arithmetic ops (:issue:`13848`)
+
+.. ipython:: python
+
+   s = pd.SparseSeries([0, 2, 0, 1], fill_value=0, dtype=np.int64)
+   s.dtype
+
+   s + 1
+
+- Sparse data structure now support ``astype`` to convert internal ``dtype`` (:issue:`13900`)
+
+.. ipython:: python
+
+   s = pd.SparseSeries([1., 0., 2., 0.], fill_value=0)
+   s
+   s.astype(np.int64)
+
+``astype`` fails if data contains values which cannot be converted to specified ``dtype``.
+Note that the limitation is applied to ``fill_value`` which default is ``np.nan``.
+
+.. code-block:: ipython
+
+   In [7]: pd.SparseSeries([1., np.nan, 2., np.nan], fill_value=np.nan).astype(np.int64)
+   Out[7]:
+   ValueError: unable to coerce current fill_value nan to int64 dtype
+
+- Subclassed ``SparseDataFrame`` and ``SparseSeries`` now preserve class types when slicing or transposing. (:issue:`13787`)
+- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing may raise ``IndexError`` (:issue:`13144`)
+- Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing result may have normal ``Index`` (:issue:`13144`)
+- Bug in ``SparseDataFrame`` in which ``axis=None`` did not default to ``axis=0`` (:issue:`13048`)
+- Bug in ``SparseSeries`` and ``SparseDataFrame`` creation with ``object`` dtype may raise ``TypeError`` (:issue:`11633`)
+- Bug in ``SparseDataFrame`` doesn't respect passed ``SparseArray`` or ``SparseSeries`` 's dtype and ``fill_value``  (:issue:`13866`)
+- Bug in ``SparseArray`` and ``SparseSeries`` don't apply ufunc to ``fill_value`` (:issue:`13853`)
+- Bug in ``SparseSeries.abs`` incorrectly keeps negative ``fill_value`` (:issue:`13853`)
+
 .. _whatsnew_0190.deprecations:
 
 Deprecations
-^^^^^^^^^^^^
+~~~~~~~~~~~~
 - ``Categorical.reshape`` has been deprecated and will be removed in a subsequent release (:issue:`12882`)
 - ``Series.reshape`` has been deprecated and will be removed in a subsequent release (:issue:`12882`)
 
@@ -738,7 +738,7 @@ Deprecations
 .. _whatsnew_0190.prior_deprecations:
 
 Removal of prior version deprecations/changes
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 - The ``SparsePanel`` class has been removed (:issue:`13778`)
 - The ``pd.sandbox`` module has been removed in favor of the external library ``pandas-qt`` (:issue:`13670`)
 - The ``pandas.io.data`` and ``pandas.io.wb`` modules are removed in favor of
@@ -797,6 +797,7 @@ Bug Fixes
 
 - Bug in ``groupby().shift()``, which could cause a segfault or corruption in rare circumstances when grouping by columns with missing values (:issue:`13813`)
 - Bug in ``pd.read_csv()``, which may cause a segfault or corruption when iterating in large chunks over a stream/file under rare circumstances (:issue:`13703`)
+- Bug in ``pd.read_csv()``, which caused BOM files to be incorrectly parsed by not ignoring the BOM (:issue:`4793`)
 - Bug in ``io.json.json_normalize()``, where non-ascii keys raised an exception (:issue:`13213`)
 - Bug when passing a not-default-indexed ``Series`` as ``xerr`` or ``yerr`` in ``.plot()`` (:issue:`11858`)
 - Bug in matplotlib ``AutoDataFormatter``; this restores the second scaled formatting and re-adds micro-second scaled formatting (:issue:`13131`)

pandas/io/parsers.py

@@ -11,7 +11,8 @@
 import numpy as np
 
 from pandas import compat
-from pandas.compat import range, lrange, StringIO, lzip, zip, string_types, map
+from pandas.compat import (range, lrange, StringIO, lzip,
+                           zip, string_types, map, u)
 from pandas.types.common import (is_integer, _ensure_object,
                                  is_list_like, is_integer_dtype,
                                  is_float,
@@ -40,6 +41,12 @@
     'N/A', 'NA', '#NA', 'NULL', 'NaN', '-NaN', 'nan', '-nan', ''
 ])
 
+# BOM character (byte order mark)
+# This exists at the beginning of a file to indicate endianness
+# of a file (stream). Unfortunately, this marker screws up parsing,
+# so we need to remove it if we see it.
+_BOM = u('\ufeff')
+
 _parser_params = """Also supports optionally iterating or breaking of the file
 into chunks.
 
@@ -2161,6 +2168,67 @@ def _buffered_line(self):
         else:
             return self._next_line()
 
+    def _check_for_bom(self, first_row):
+        """
+        Checks whether the file begins with the BOM character.
+        If it does, remove it. In addition, if there is quoting
+        in the field subsequent to the BOM, remove it as well
+        because it technically takes place at the beginning of
+        the name, not the middle of it.
+        """
+        # first_row will be a list, so we need to check
+        # that that list is not empty before proceeding.
+        if not first_row:
+            return first_row
+
+        # The first element of this row is the one that could have the
+        # BOM that we want to remove. Check that the first element is a
+        # string before proceeding.
+        if not isinstance(first_row[0], compat.string_types):
+            return first_row
+
+        # Check that the string is not empty, as that would
+        # obviously not have a BOM at the start of it.
+        if not first_row[0]:
+            return first_row
+
+        # Since the string is non-empty, check that it does
+        # in fact begin with a BOM.
+        first_elt = first_row[0][0]
+
+        # This is to avoid warnings we get in Python 2.x if
+        # we find ourselves comparing with non-Unicode
+        if compat.PY2 and not isinstance(first_elt, unicode):  # noqa
+            try:
+                first_elt = u(first_elt)
+            except UnicodeDecodeError:
+                return first_row
+
+        if first_elt != _BOM:
+            return first_row
+
+        first_row = first_row[0]
+
+        if len(first_row) > 1 and first_row[1] == self.quotechar:
+            start = 2
+            quote = first_row[1]
+            end = first_row[2:].index(quote) + 2
+
+            # Extract the data between the quotation marks
+            new_row = first_row[start:end]
+
+            # Extract any remaining data after the second
+            # quotation mark.
+            if len(first_row) > end + 1:
+                new_row += first_row[end + 1:]
+            return [new_row]
+        elif len(first_row) > 1:
+            return [first_row[1:]]
+        else:
+            # First row is just the BOM, so we
+            # return an empty string.
+            return [""]
+
     def _empty(self, line):
         return not line or all(not x for x in line)
 
@@ -2212,6 +2280,12 @@ def _next_line(self):
                         line = ret[0]
                         break
 
+        # This was the first line of the file,
+        # which could contain the BOM at the
+        # beginning of it.
+        if self.pos == 1:
+            line = self._check_for_bom(line)
+
         self.line_pos += 1
         self.buf.append(line)
         return line

pandas/io/tests/parser/common.py

@@ -1517,3 +1517,54 @@ def test_null_byte_char(self):
             msg = "NULL byte detected"
             with tm.assertRaisesRegexp(csv.Error, msg):
                 self.read_csv(StringIO(data), names=cols)
+
+    def test_utf8_bom(self):
+        # see gh-4793
+        bom = u('\ufeff')
+        utf8 = 'utf-8'
+
+        def _encode_data_with_bom(_data):
+            bom_data = (bom + _data).encode(utf8)
+            return BytesIO(bom_data)
+
+        # basic test
+        data = 'a\n1'
+        expected = DataFrame({'a': [1]})
+
+        out = self.read_csv(_encode_data_with_bom(data),
+                            encoding=utf8)
+        tm.assert_frame_equal(out, expected)
+
+        # test with "regular" quoting
+        data = '"a"\n1'
+        expected = DataFrame({'a': [1]})
+
+        out = self.read_csv(_encode_data_with_bom(data),
+                            encoding=utf8, quotechar='"')
+        tm.assert_frame_equal(out, expected)
+
+        # test in a data row instead of header
+        data = 'b\n1'
+        expected = DataFrame({'a': ['b', '1']})
+
+        out = self.read_csv(_encode_data_with_bom(data),
+                            encoding=utf8, names=['a'])
+        tm.assert_frame_equal(out, expected)
+
+        # test in empty data row with skipping
+        data = '\n1'
+        expected = DataFrame({'a': [1]})
+
+        out = self.read_csv(_encode_data_with_bom(data),
+                            encoding=utf8, names=['a'],
+                            skip_blank_lines=True)
+        tm.assert_frame_equal(out, expected)
+
+        # test in empty data row without skipping
+        data = '\n1'
+        expected = DataFrame({'a': [np.nan, 1.0]})
+
+        out = self.read_csv(_encode_data_with_bom(data),
+                            encoding=utf8, names=['a'],
+                            skip_blank_lines=False)
+        tm.assert_frame_equal(out, expected)

pandas/src/parser/tokenizer.c

@@ -704,6 +704,11 @@ static int parser_buffer_bytes(parser_t *self, size_t nbytes) {
     self->datapos = i;                                                  \
     TRACE(("_TOKEN_CLEANUP: datapos: %d, datalen: %d\n", self->datapos, self->datalen));
 
+#define CHECK_FOR_BOM()                                                   \
+    if (*buf == '\xef' && *(buf + 1) == '\xbb' && *(buf + 2) == '\xbf') { \
+        buf += 3;                                                         \
+        self->datapos += 3;                                               \
+    }
 
 int skip_this_line(parser_t *self, int64_t rownum) {
     if (self->skipset != NULL) {
@@ -736,6 +741,10 @@ int tokenize_bytes(parser_t *self, size_t line_limit)
 
     TRACE(("%s\n", buf));
 
+    if (self->file_lines == 0) {
+        CHECK_FOR_BOM();
+    }
+
     for (i = self->datapos; i < self->datalen; ++i)
     {
         // next character in file