ENH/TST/DOC: set infer_nrows for read_fwf (GH15138)

doc/source/whatsnew/v0.24.0.rst

@@ -286,6 +286,7 @@ Other Enhancements
 - New attribute :attr:`__git_version__` will return git commit sha of current build (:issue:`21295`).
 - Compatibility with Matplotlib 3.0 (:issue:`22790`).
 - Added :meth:`Interval.overlaps`, :meth:`IntervalArray.overlaps`, and :meth:`IntervalIndex.overlaps` for determining overlaps between interval-like objects (:issue:`21998`)
+- :func:`read_fwf` now accepts keyword `infer_nrows` (:issue:`15138`).
 - :func:`~DataFrame.to_parquet` now supports writing a ``DataFrame`` as a directory of parquet files partitioned by a subset of the columns when ``engine = 'pyarrow'`` (:issue:`23283`)
 - :meth:`Timestamp.tz_localize`, :meth:`DatetimeIndex.tz_localize`, and :meth:`Series.tz_localize` have gained the ``nonexistent`` argument for alternative handling of nonexistent times. See :ref:`timeseries.timezone_nonexistent` (:issue:`8917`)
 - :meth:`Index.difference` now has an optional ``sort`` parameter to specify whether the results should be sorted if possible (:issue:`17839`)

pandas/io/parsers.py

@@ -501,6 +501,7 @@ def _read(filepath_or_buffer, kwds):
 
 _fwf_defaults = {
     'colspecs': 'infer',
+    'infer_nrows': 100,
     'widths': None,
 }
 
@@ -718,8 +719,8 @@ def parser_f(filepath_or_buffer,
                       )(read_table)
 
 
-def read_fwf(filepath_or_buffer, colspecs='infer',
-             widths=None, **kwds):
+def read_fwf(filepath_or_buffer, colspecs='infer', widths=None,
+             infer_nrows=100, **kwds):
 
     r"""
     Read a table of fixed-width formatted lines into DataFrame.
@@ -752,6 +753,11 @@ def read_fwf(filepath_or_buffer, colspecs='infer',
     widths : list of int, optional
         A list of field widths which can be used instead of 'colspecs' if
         the intervals are contiguous.
+    infer_nrows : int, default 100
+        The number of rows to consider when letting the parser determine the
+        `colspecs`.
+
+        .. versionadded:: 0.24.0
     **kwds : optional
         Optional keyword arguments can be passed to ``TextFileReader``.
 
@@ -786,6 +792,7 @@ def read_fwf(filepath_or_buffer, colspecs='infer',
             col += w
 
     kwds['colspecs'] = colspecs
+    kwds['infer_nrows'] = infer_nrows
     kwds['engine'] = 'python-fwf'
     return _read(filepath_or_buffer, kwds)
 
@@ -3442,13 +3449,15 @@ class FixedWidthReader(BaseIterator):
     A reader of fixed-width lines.
     """
 
-    def __init__(self, f, colspecs, delimiter, comment, skiprows=None):
+    def __init__(self, f, colspecs, delimiter, comment, skiprows=None,
+                 infer_nrows=100):
         self.f = f
         self.buffer = None
         self.delimiter = '\r\n' + delimiter if delimiter else '\n\r\t '
         self.comment = comment
         if colspecs == 'infer':
-            self.colspecs = self.detect_colspecs(skiprows=skiprows)
+            self.colspecs = self.detect_colspecs(infer_nrows=infer_nrows,
+                                                 skiprows=skiprows)
         else:
             self.colspecs = colspecs
 
@@ -3464,19 +3473,20 @@ def __init__(self, f, colspecs, delimiter, comment, skiprows=None):
                 raise TypeError('Each column specification must be '
                                 '2 element tuple or list of integers')
 
-    def get_rows(self, n, skiprows=None):
+    def get_rows(self, infer_nrows, skiprows=None):
         """
         Read rows from self.f, skipping as specified.
 
-        We distinguish buffer_rows (the first <= n lines)
-        from the rows returned to detect_colspecs because
-        it's simpler to leave the other locations with
-        skiprows logic alone than to modify them to deal
-        with the fact we skipped some rows here as well.
+        We distinguish buffer_rows (the first <= infer_nrows
+        lines) from the rows returned to detect_colspecs
+        because it's simpler to leave the other locations
+        with skiprows logic alone than to modify them to
+        deal with the fact we skipped some rows here as
+        well.
 
         Parameters
         ----------
-        n : int
+        infer_nrows : int
             Number of rows to read from self.f, not counting
             rows that are skipped.
         skiprows: set, optional
@@ -3496,16 +3506,16 @@ def get_rows(self, n, skiprows=None):
             if i not in skiprows:
                 detect_rows.append(row)
             buffer_rows.append(row)
-            if len(detect_rows) >= n:
+            if len(detect_rows) >= infer_nrows:
                 break
         self.buffer = iter(buffer_rows)
         return detect_rows
 
-    def detect_colspecs(self, n=100, skiprows=None):
+    def detect_colspecs(self, infer_nrows=100, skiprows=None):
         # Regex escape the delimiters
         delimiters = ''.join(r'\%s' % x for x in self.delimiter)
         pattern = re.compile('([^%s]+)' % delimiters)
-        rows = self.get_rows(n, skiprows)
+        rows = self.get_rows(infer_nrows, skiprows)
         if not rows:
             raise EmptyDataError("No rows from which to infer column width")
         max_len = max(map(len, rows))
@@ -3544,8 +3554,10 @@ class FixedWidthFieldParser(PythonParser):
     def __init__(self, f, **kwds):
         # Support iterators, convert to a list.
         self.colspecs = kwds.pop('colspecs')
+        self.infer_nrows = kwds.pop('infer_nrows')
         PythonParser.__init__(self, f, **kwds)
 
     def _make_reader(self, f):
         self.data = FixedWidthReader(f, self.colspecs, self.delimiter,
-                                     self.comment, self.skiprows)
+                                     self.comment, self.skiprows,
+                                     self.infer_nrows)

pandas/tests/io/parser/test_read_fwf.py

@@ -140,6 +140,22 @@ def test_fwf_colspecs_None(self):
         expected = DataFrame([[123456, 456], [456789, 789]])
         tm.assert_frame_equal(result, expected)
 
+    def test_fwf_colspecs_infer_nrows(self):
+        # GH 15138
+        data = """\
+  1  2
+123 98
+"""
+        # infer_nrows == 1 should have colspec == [(2, 3), (5, 6)]
+        df = read_fwf(StringIO(data), header=None, infer_nrows=1)
+        expected = pd.DataFrame([[1, 2], [3, 8]])
+        tm.assert_frame_equal(df, expected)
+
+        # test for infer_nrows > number of rows
+        df = read_fwf(StringIO(data), header=None, infer_nrows=10)
+        expected = pd.DataFrame([[1, 2], [123, 98]])
+        tm.assert_frame_equal(df, expected)
+
     def test_fwf_regression(self):
         # GH 3594
         # turns out 'T060' is parsable as a datetime slice!