BUG: read_fwf: incorrect error message with no colspecs or widths

Author: jtratner

doc/source/release.rst

@@ -335,6 +335,7 @@ See :ref:`Internal Refactoring<whatsnew_0130.refactoring>`
   - Bug in setting with ``loc/ix`` a single indexer with a multi-index axis and a numpy array, related to (:issue:`3777`)
   - Bug in concatenation with duplicate columns across dtypes not merging with axis=0 (:issue:`4771`)
   - Bug in ``iloc`` with a slice index failing (:issue:`4771`)
+  - Incorrect error message with no colspecs or width in ``read_fwf``. (:issue:`4774`)
 
 pandas 0.12
 ===========

pandas/io/parsers.py

@@ -414,7 +414,9 @@ def parser_f(filepath_or_buffer,
 @Appender(_read_fwf_doc)
 def read_fwf(filepath_or_buffer, colspecs=None, widths=None, **kwds):
     # Check input arguments.
-    if bool(colspecs is None) == bool(widths is None):
+    if colspecs is None and widths is None:
+        raise ValueError("Must specify either colspecs or widths")
+    elif colspecs is not None and widths is not None:
         raise ValueError("You must specify only one of 'widths' and "
                          "'colspecs'")
 

pandas/io/tests/test_parsers.py

@@ -1995,8 +1995,11 @@ def test_fwf(self):
             StringIO(data3), colspecs=colspecs, delimiter='~', header=None)
         tm.assert_frame_equal(df, expected)
 
-        self.assertRaises(ValueError, read_fwf, StringIO(data3),
-                          colspecs=colspecs, widths=[6, 10, 10, 7])
+        with tm.assertRaisesRegexp(ValueError, "must specify only one of"):
+            read_fwf(StringIO(data3), colspecs=colspecs, widths=[6, 10, 10, 7])
+
+        with tm.assertRaisesRegexp(ValueError, "Must specify either"):
+            read_fwf(StringIO(data3))
 
     def test_fwf_regression(self):
         # GH 3594