added FutureWarning to empty Series without dtype and adjusted the tests and docs so that no unnecessary warnings are thrown

Author: SaturnFromTitan

doc/source/user_guide/missing_data.rst

@@ -190,15 +190,15 @@ The sum of an empty or all-NA Series or column of a DataFrame is 0.
 
    pd.Series([np.nan]).sum()
 
-   pd.Series([]).sum()
+   pd.Series([], dtype="float64").sum()
 
 The product of an empty or all-NA Series or column of a DataFrame is 1.
 
 .. ipython:: python
 
    pd.Series([np.nan]).prod()
 
-   pd.Series([]).prod()
+   pd.Series([], dtype="float64").prod()
 
 
 NA values in GroupBy

doc/source/user_guide/scale.rst

@@ -358,6 +358,7 @@ results will fit in memory, so we can safely call ``compute`` without running
 out of memory. At that point it's just a regular pandas object.
 
 .. ipython:: python
+   :okwarning:
 
    @savefig dask_resample.png
    ddf[['x', 'y']].resample("1D").mean().cumsum().compute().plot()

doc/source/whatsnew/v0.19.0.rst

@@ -707,6 +707,7 @@ A ``Series`` will now correctly promote its dtype for assignment with incompat v
 
 
 .. ipython:: python
+   :okwarning:
 
    s = pd.Series()
 

doc/source/whatsnew/v0.21.0.rst

@@ -428,6 +428,7 @@ Note that this also changes the sum of an empty ``Series``. Previously this alwa
 but for consistency with the all-NaN case, this was changed to return NaN as well:
 
 .. ipython:: python
+   :okwarning:
 
    pd.Series([]).sum()
 

doc/source/whatsnew/v0.22.0.rst

@@ -55,6 +55,7 @@ The default sum for empty or all-*NA* ``Series`` is now ``0``.
 *pandas 0.22.0*
 
 .. ipython:: python
+   :okwarning:
 
    pd.Series([]).sum()
    pd.Series([np.nan]).sum()
@@ -67,6 +68,7 @@ pandas 0.20.3 without bottleneck, or pandas 0.21.x), use the ``min_count``
 keyword.
 
 .. ipython:: python
+   :okwarning:
 
    pd.Series([]).sum(min_count=1)
 
@@ -85,6 +87,7 @@ required for a non-NA sum or product.
 returning ``1`` instead.
 
 .. ipython:: python
+   :okwarning:
 
    pd.Series([]).prod()
    pd.Series([np.nan]).prod()

doc/source/whatsnew/v1.0.0.rst

@@ -356,6 +356,23 @@ When :class:`Categorical` contains ``np.nan``,
 
    pd.Categorical([1, 2, np.nan], ordered=True).min()
 
+
+Default dtype of empty :class:`pandas.core.series.Series`
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Initialising an empty :class:`pandas.core.series.Series` without specifying a dtype will raise a `FutureWarning` now
+(:issue:`17261`). The default dtype will change from ``float64`` to ``object`` in future releases so that it is
+consistent with the behaviour of :class:`DataFrame` and :class:`Index`.
+
+*pandas 1.0.0*
+
+.. code-block:: ipython
+
+   In [1]: pd.Series()
+   Out[2]:
+   FutureWarning: The default dtype for empty Series will be 'object' instead of 'float64' in the next version. Specify a dtype explicitly to silence this warning.
+   Series([], dtype: float64)
+
 .. _whatsnew_1000.api_breaking.deps:
 
 Increased minimum versions for dependencies
@@ -484,7 +501,7 @@ Removal of prior version deprecations/changes
 
 Previously, pandas would register converters with matplotlib as a side effect of importing pandas (:issue:`18720`).
 This changed the output of plots made via matplotlib plots after pandas was imported, even if you were using
-matplotlib directly rather than rather than :meth:`~DataFrame.plot`.
+matplotlib directly rather than :meth:`~DataFrame.plot`.
 
 To use pandas formatters with a matplotlib plot, specify
 

pandas/compat/pickle_compat.py

@@ -64,7 +64,7 @@ def __new__(cls) -> "Series":  # type: ignore
             stacklevel=6,
         )
 
-        return Series()
+        return Series(dtype=object)
 
 
 class _LoadSparseFrame:

pandas/core/algorithms.py

@@ -601,7 +601,7 @@ def _factorize_array(
 )
 @Appender(_shared_docs["factorize"])
 def factorize(
-    values, sort: bool = False, na_sentinel: int = -1, size_hint: Optional[int] = None,
+    values, sort: bool = False, na_sentinel: int = -1, size_hint: Optional[int] = None
 ) -> Tuple[np.ndarray, Union[np.ndarray, ABCIndex]]:
     # Implementation notes: This method is responsible for 3 things
     # 1.) coercing data to array-like (ndarray, Index, extension array)

pandas/core/apply.py

@@ -15,6 +15,8 @@
 )
 from pandas.core.dtypes.generic import ABCMultiIndex, ABCSeries
 
+from pandas.core.construction import create_series_with_explicit_dtype
+
 if TYPE_CHECKING:
     from pandas import DataFrame, Series, Index
 
@@ -203,15 +205,15 @@ def apply_empty_result(self):
 
         if not should_reduce:
             try:
-                r = self.f(Series([]))
+                r = self.f(Series([], dtype=np.float64))
             except Exception:
                 pass
             else:
                 should_reduce = not isinstance(r, Series)
 
         if should_reduce:
             if len(self.agg_axis):
-                r = self.f(Series([]))
+                r = self.f(Series([], dtype=np.float64))
             else:
                 r = np.nan
 
@@ -346,14 +348,25 @@ def apply_series_generator(self) -> Tuple[ResType, "Index"]:
     def wrap_results(
         self, results: ResType, res_index: "Index"
     ) -> Union["Series", "DataFrame"]:
+        from pandas import Series
 
         # see if we can infer the results
         if len(results) > 0 and 0 in results and is_sequence(results[0]):
 
             return self.wrap_results_for_axis(results, res_index)
 
         # dict of scalars
-        result = self.obj._constructor_sliced(results)
+
+        # the default dtype of an empty Series will be `object`, but this
+        # code can be hit by df.mean() where the result should have dtype
+        # float64 even if it's an empty Series.
+        constructor_sliced = self.obj._constructor_sliced
+        if constructor_sliced is Series:
+            result = create_series_with_explicit_dtype(
+                results, dtype_if_empty=np.float64
+            )
+        else:
+            result = constructor_sliced(results)
         result.index = res_index
 
         return result

pandas/core/base.py

@@ -34,6 +34,7 @@
 from pandas.core.accessor import DirNamesMixin
 from pandas.core.algorithms import duplicated, unique1d, value_counts
 from pandas.core.arrays import ExtensionArray
+from pandas.core.construction import create_series_with_explicit_dtype
 import pandas.core.nanops as nanops
 
 _shared_docs: Dict[str, str] = dict()
@@ -1132,9 +1133,14 @@ def _map_values(self, mapper, na_action=None):
                 # convert to an Series for efficiency.
                 # we specify the keys here to handle the
                 # possibility that they are tuples
-                from pandas import Series
 
-                mapper = Series(mapper)
+                # The return value of mapping with an empty mapper is
+                # expected to be pd.Series(np.nan, ...). As np.nan is
+                # of dtype float64 the return value of this method should
+                # be float64 as well
+                mapper = create_series_with_explicit_dtype(
+                    mapper, dtype_if_empty=np.float64
+                )
 
         if isinstance(mapper, ABCSeries):
             # Since values were input this means we came from either

pandas/core/construction.py

@@ -565,3 +565,38 @@ def _try_cast(
         else:
             subarr = np.array(arr, dtype=object, copy=copy)
     return subarr
+
+
+# see gh-17261
+def is_empty_data(data):
+    """
+    Utility to check if a Series is instantiated with empty data
+    """
+    is_none = data is None
+    is_simple_empty = isinstance(data, (list, tuple, dict)) and not data
+    return is_none or is_simple_empty
+
+
+def create_series_with_explicit_dtype(
+    data=None,
+    index=None,
+    dtype=None,
+    name=None,
+    copy=False,
+    fastpath=False,
+    dtype_if_empty=object,
+):
+    """
+    Helper to pass an explicit dtype when instantiating an empty Series.
+
+    The signature of this function mirrors the signature of Series.__init__
+    but adds the additional keyword argument `dtype_if_empty`.
+
+    This silences a FutureWarning described in the GitHub issue
+    mentioned above.
+    """
+    from pandas.core.series import Series
+
+    if is_empty_data(data) and dtype is None:
+        dtype = dtype_if_empty
+    return Series(data, index, dtype, name, copy, fastpath)

pandas/core/frame.py

@@ -7954,7 +7954,7 @@ def quantile(self, q=0.5, axis=0, numeric_only=True, interpolation="linear"):
             cols = Index([], name=self.columns.name)
             if is_list_like(q):
                 return self._constructor([], index=q, columns=cols)
-            return self._constructor_sliced([], index=cols, name=q)
+            return self._constructor_sliced([], index=cols, name=q, dtype=np.float64)
 
         result = data._data.quantile(
             qs=q, axis=1, interpolation=interpolation, transposed=is_transposed

pandas/core/generic.py

@@ -72,6 +72,7 @@
 import pandas.core.algorithms as algos
 from pandas.core.base import PandasObject, SelectionMixin
 import pandas.core.common as com
+from pandas.core.construction import create_series_with_explicit_dtype
 from pandas.core.index import (
     Index,
     InvalidIndexError,
@@ -6110,9 +6111,7 @@ def fillna(
 
             if self.ndim == 1:
                 if isinstance(value, (dict, ABCSeries)):
-                    from pandas import Series
-
-                    value = Series(value)
+                    value = create_series_with_explicit_dtype(value)
                 elif not is_list_like(value):
                     pass
                 else:
@@ -7064,7 +7063,7 @@ def asof(self, where, subset=None):
                 if not is_series:
                     from pandas import Series
 
-                    return Series(index=self.columns, name=where)
+                    return Series(index=self.columns, name=where, dtype=np.float64)
                 return np.nan
 
             # It's always much faster to use a *while* loop here for

pandas/core/groupby/generic.py

@@ -51,6 +51,7 @@
 import pandas.core.algorithms as algorithms
 from pandas.core.base import DataError, SpecificationError
 import pandas.core.common as com
+from pandas.core.construction import create_series_with_explicit_dtype
 from pandas.core.frame import DataFrame
 from pandas.core.generic import ABCDataFrame, ABCSeries, NDFrame, _shared_docs
 from pandas.core.groupby import base
@@ -259,7 +260,7 @@ def aggregate(self, func=None, *args, **kwargs):
                 result = self._aggregate_named(func, *args, **kwargs)
 
             index = Index(sorted(result), name=self.grouper.names[0])
-            ret = Series(result, index=index)
+            ret = create_series_with_explicit_dtype(result, index=index)
 
         if not self.as_index:  # pragma: no cover
             print("Warning, ignoring as_index=True")
@@ -407,7 +408,7 @@ def _wrap_transformed_output(
     def _wrap_applied_output(self, keys, values, not_indexed_same=False):
         if len(keys) == 0:
             # GH #6265
-            return Series([], name=self._selection_name, index=keys)
+            return Series([], name=self._selection_name, index=keys, dtype=np.float64)
 
         def _get_index() -> Index:
             if self.grouper.nkeys > 1:
@@ -493,7 +494,7 @@ def _transform_general(self, func, *args, **kwargs):
 
             result = concat(results).sort_index()
         else:
-            result = Series()
+            result = Series(dtype=np.float64)
 
         # we will only try to coerce the result type if
         # we have a numeric dtype, as these are *always* user-defined funcs
@@ -1205,9 +1206,17 @@ def first_not_none(values):
             if v is None:
                 return DataFrame()
             elif isinstance(v, NDFrame):
+
+                # this is to silence a FutureWarning
+                # TODO: Remove when default dtype of empty Series is object
+                kwargs = v._construct_axes_dict()
+                if v._constructor is Series:
+                    is_empty = "data" not in kwargs or not kwargs["data"]
+                    if "dtype" not in kwargs and is_empty:
+                        kwargs["dtype"] = object
+
                 values = [
-                    x if x is not None else v._constructor(**v._construct_axes_dict())
-                    for x in values
+                    x if (x is not None) else v._constructor(**kwargs) for x in values
                 ]
 
             v = values[0]

pandas/core/series.py

@@ -54,7 +54,12 @@
 from pandas.core.arrays.categorical import Categorical, CategoricalAccessor
 from pandas.core.arrays.sparse import SparseAccessor
 import pandas.core.common as com
-from pandas.core.construction import extract_array, sanitize_array
+from pandas.core.construction import (
+    create_series_with_explicit_dtype,
+    extract_array,
+    is_empty_data,
+    sanitize_array,
+)
 from pandas.core.index import (
     Float64Index,
     Index,
@@ -175,6 +180,18 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
     def __init__(
         self, data=None, index=None, dtype=None, name=None, copy=False, fastpath=False
     ):
+        if is_empty_data(data) and dtype is None:
+            # Empty Series should have dtype object to be consistent
+            # with the behaviour of DataFrame and Index
+            warnings.warn(
+                "The default dtype for empty Series will be 'object' instead"
+                " of 'float64' in the next version. Specify a dtype explicitly"
+                " to silence this warning.",
+                FutureWarning,
+                stacklevel=2,
+            )
+            # uncomment the line below when removing the FutureWarning
+            # dtype = np.dtype(object)
 
         # we are called internally, so short-circuit
         if fastpath:
@@ -328,7 +345,9 @@ def _init_dict(self, data, index=None, dtype=None):
             keys, values = [], []
 
         # Input is now list-like, so rely on "standard" construction:
-        s = Series(values, index=keys, dtype=dtype)
+        s = create_series_with_explicit_dtype(
+            values, index=keys, dtype=dtype, dtype_if_empty=np.float64
+        )
 
         # Now we just make sure the order is respected, if any
         if data and index is not None:

pandas/core/tools/datetimes.py

@@ -145,7 +145,8 @@ def _maybe_cache(arg, format, cache, convert_listlike):
     """
     from pandas import Series
 
-    cache_array = Series()
+    cache_array = Series(dtype=object)
+
     if cache:
         # Perform a quicker unique check
         if not should_cache(arg):

pandas/io/html.py

@@ -14,7 +14,7 @@
 
 from pandas.core.dtypes.common import is_list_like
 
-from pandas import Series
+from pandas.core.construction import create_series_with_explicit_dtype
 
 from pandas.io.common import _is_url, _validate_header_arg, urlopen
 from pandas.io.formats.printing import pprint_thing
@@ -762,7 +762,8 @@ def _parse_tfoot_tr(self, table):
 
 
 def _expand_elements(body):
-    lens = Series([len(elem) for elem in body])
+    data = [len(elem) for elem in body]
+    lens = create_series_with_explicit_dtype(data)
     lens_max = lens.max()
     not_max = lens[lens != lens_max]