BUG: don't raise on empty usecols

Title is self-explanatory.

Author: gfyoung <[email protected]>

Closes #13402 from gfyoung/empty-usecols-bug and squashes the following commits:

8eed8d1 [gfyoung] BUG: don't raise on empty usecols

Author: gfyoung

doc/source/whatsnew/v0.18.2.txt

@@ -358,6 +358,7 @@ Bug Fixes
 - Bug in ``pd.read_csv()`` with ``engine='python'`` in which infinities of mixed-case forms were not being interpreted properly (:issue:`13274`)
 - Bug in ``pd.read_csv()`` with ``engine='python'`` in which trailing ``NaN`` values were not being parsed (:issue:`13320`)
 - Bug in ``pd.read_csv()`` that prevents ``usecols`` kwarg from accepting single-byte unicode strings (:issue:`13219`)
+- Bug in ``pd.read_csv()`` that prevents ``usecols`` from being an empty set (:issue:`13402`)
 
 
 

pandas/io/parsers.py

@@ -944,7 +944,8 @@ def _validate_usecols_arg(usecols):
 
     if usecols is not None:
         usecols_dtype = lib.infer_dtype(usecols)
-        if usecols_dtype not in ('integer', 'string', 'unicode'):
+        if usecols_dtype not in ('empty', 'integer',
+                                 'string', 'unicode'):
             raise ValueError(msg)
 
     return usecols

pandas/io/tests/parser/usecols.py

@@ -354,3 +354,10 @@ def test_usecols_with_multibyte_unicode_characters(self):
 
         df = self.read_csv(StringIO(s), usecols=[u'あああ', u'いい'])
         tm.assert_frame_equal(df, expected)
+
+    def test_empty_usecols(self):
+        # should not raise
+        data = 'a,b,c\n1,2,3\n4,5,6'
+        expected = DataFrame()
+        result = self.read_csv(StringIO(data), usecols=set([]))
+        tm.assert_frame_equal(result, expected)