BUG: DataFrame.first_valid_index() fails if there is no valid entry. (#17488)

Closes #17400

Author: skwbc

doc/source/whatsnew/v0.21.0.txt

@@ -518,6 +518,7 @@ Indexing
 - Bug in ``CategoricalIndex`` reindexing in which specified indices containing duplicates were not being respected (:issue:`17323`)
 - Bug in intersection of ``RangeIndex`` with negative step (:issue:`17296`)
 - Bug in ``IntervalIndex`` where performing a scalar lookup fails for included right endpoints of non-overlapping monotonic decreasing indexes (:issue:`16417`, :issue:`17271`)
+- Bug in :meth:`DataFrame.first_valid_index` and :meth:`DataFrame.last_valid_index` when no valid entry (:issue:`17400`)
 
 I/O
 ^^^

pandas/core/frame.py

@@ -4063,23 +4063,27 @@ def update(self, other, join='left', overwrite=True, filter_func=None,
  # ----------------------------------------------------------------------
  # Misc methods
 
+ def _get_valid_indices(self):
+ is_valid = self.count(1) > 0
+ return self.index[is_valid]
+
+ @Appender(_shared_docs['valid_index'] % {
+ 'position': 'first', 'klass': 'DataFrame'})
  def first_valid_index(self):
- """
- Return label for first non-NA/null value
- """
  if len(self) == 0:
  return None
 
- return self.index[self.count(1) > 0][0]
+ valid_indices = self._get_valid_indices()
+ return valid_indices[0] if len(valid_indices) else None
 
+ @Appender(_shared_docs['valid_index'] % {
+ 'position': 'first', 'klass': 'DataFrame'})
  def last_valid_index(self):
- """
- Return label for last non-NA/null value
- """
  if len(self) == 0:
  return None
 
- return self.index[self.count(1) > 0][-1]
+ valid_indices = self._get_valid_indices()
+ return valid_indices[-1] if len(valid_indices) else None
 
  # ----------------------------------------------------------------------
  # Data reshaping

pandas/core/generic.py

@@ -6757,6 +6757,22 @@ def transform(self, func, *args, **kwargs):
 
  cls.transform = transform
 
+ # ----------------------------------------------------------------------
+ # Misc methods
+
+ _shared_docs['valid_index'] = """
+ Return index for %(position)s non-NA/null value.
+
+ Notes
+ --------
+ If all elements are non-NA/null, returns None.
+ Also returns None for empty %(klass)s.
+
+ Returns
+ --------
+ scalar : type of index
+ """
+
 
 def _doc_parms(cls):
  """Return a tuple of the doc parms."""

pandas/core/series.py

@@ -2825,10 +2825,9 @@ def dropna(self, axis=0, inplace=False, **kwargs):
  valid = lambda self, inplace=False, **kwargs: self.dropna(inplace=inplace,
  **kwargs)
 
+ @Appender(generic._shared_docs['valid_index'] % {
+ 'position': 'first', 'klass': 'Series'})
  def first_valid_index(self):
- """
- Return label for first non-NA/null value
- """
  if len(self) == 0:
  return None
 
@@ -2839,10 +2838,9 @@ def first_valid_index(self):
  else:
  return self.index[i]
 
+ @Appender(generic._shared_docs['valid_index'] % {
+ 'position': 'last', 'klass': 'Series'})
  def last_valid_index(self):
- """
- Return label for last non-NA/null value
- """
  if len(self) == 0:
  return None
 

pandas/tests/frame/test_timeseries.py

@@ -440,6 +440,11 @@ def test_first_last_valid(self):
  assert empty.last_valid_index() is None
  assert empty.first_valid_index() is None
 
+ # GH17400: no valid entries
+ frame[:] = nan
+ assert frame.last_valid_index() is None
+ assert frame.first_valid_index() is None
+
  def test_at_time_frame(self):
  rng = date_range('1/1/2000', '1/5/2000', freq='5min')
  ts = DataFrame(np.random.randn(len(rng), 2), index=rng)