TYP: Add types to top-level funcs, step 2

pandas/_typing.py

@@ -21,9 +21,10 @@
     from pandas.core.arrays.base import ExtensionArray  # noqa: F401
     from pandas.core.dtypes.dtypes import ExtensionDtype  # noqa: F401
     from pandas.core.indexes.base import Index  # noqa: F401
-    from pandas.core.series import Series  # noqa: F401
     from pandas.core.generic import NDFrame  # noqa: F401
     from pandas import Interval  # noqa: F401
+    from pandas.core.series import Series  # noqa: F401
+    from pandas.core.frame import DataFrame  # noqa: F401
 
 # array-like
 
@@ -41,7 +42,19 @@
 
 Dtype = Union[str, np.dtype, "ExtensionDtype"]
 FilePathOrBuffer = Union[str, Path, IO[AnyStr]]
+
+# FrameOrSeriesUnion  means either a DataFrame or a Series. E.g.
+# `def func(a: FrameOrSeriesUnion) -> FrameOrSeriesUnion: ...` means that if a Series
+# is passed in, either a Series or DataFrame is returned, and if a DataFrame is passed
+# in, either a DataFrame or a Series is returned.
+FrameOrSeriesUnion = Union["DataFrame", "Series"]
+
+# FrameOrSeries is stricter and ensures that the same subclass of NDFrame always is
+# used. E.g. `def func(a: FrameOrSeries) -> FrameOrSeries: ...` means that if a
+# Series is passed into a function, a Series is always returned and if a DataFrame is
+# passed in, a DataFrame is always returned.
 FrameOrSeries = TypeVar("FrameOrSeries", bound="NDFrame")
+
 Axis = Union[str, int]
 Ordered = Optional[bool]
 JSONSerializable = Union[PythonScalar, List, Dict]

pandas/core/algorithms.py

@@ -3,7 +3,7 @@
 intended for public consumption
 """
 from textwrap import dedent
-from typing import Dict, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Dict, Optional, Tuple, Union
 from warnings import catch_warnings, simplefilter, warn
 
 import numpy as np
@@ -50,6 +50,9 @@
 from pandas.core.construction import array, extract_array
 from pandas.core.indexers import validate_indices
 
+if TYPE_CHECKING:
+    from pandas import Series
+
 _shared_docs: Dict[str, str] = {}
 
 
@@ -651,7 +654,7 @@ def value_counts(
     normalize: bool = False,
     bins=None,
     dropna: bool = True,
-) -> ABCSeries:
+) -> "Series":
     """
     Compute a histogram of the counts of non-null values.
 
@@ -793,7 +796,7 @@ def duplicated(values, keep="first") -> np.ndarray:
     return f(values, keep=keep)
 
 
-def mode(values, dropna: bool = True) -> ABCSeries:
+def mode(values, dropna: bool = True) -> "Series":
     """
     Returns the mode(s) of an array.
 

pandas/core/frame.py

@@ -5878,7 +5878,7 @@ def groupby(
 
     @Substitution("")
     @Appender(_shared_docs["pivot"])
-    def pivot(self, index=None, columns=None, values=None):
+    def pivot(self, index=None, columns=None, values=None) -> "DataFrame":
         from pandas.core.reshape.pivot import pivot
 
         return pivot(self, index=index, columns=columns, values=values)
@@ -6025,7 +6025,7 @@ def pivot_table(
         dropna=True,
         margins_name="All",
         observed=False,
-    ):
+    ) -> "DataFrame":
         from pandas.core.reshape.pivot import pivot_table
 
         return pivot_table(

pandas/core/reshape/concat.py

@@ -2,10 +2,12 @@
 concat routines
 """
 
-from typing import Hashable, List, Optional
+from typing import Hashable, List, Mapping, Optional, Sequence, Union, overload
 
 import numpy as np
 
+from pandas._typing import FrameOrSeriesUnion
+
 from pandas import DataFrame, Index, MultiIndex, Series
 from pandas.core.arrays.categorical import (
     factorize_from_iterable,
@@ -26,8 +28,27 @@
 # Concatenate DataFrame objects
 
 
+@overload
+def concat(
+    objs: Union[Sequence["DataFrame"], Mapping[Optional[Hashable], "DataFrame"]],
+    axis=0,
+    join: str = "outer",
+    ignore_index: bool = False,
+    keys=None,
+    levels=None,
+    names=None,
+    verify_integrity: bool = False,
+    sort: bool = False,
+    copy: bool = True,
+) -> "DataFrame":
+    ...
+
+
+@overload
 def concat(
-    objs,
+    objs: Union[
+        Sequence[FrameOrSeriesUnion], Mapping[Optional[Hashable], FrameOrSeriesUnion]
+    ],
     axis=0,
     join: str = "outer",
     ignore_index: bool = False,
@@ -37,7 +58,24 @@ def concat(
     verify_integrity: bool = False,
     sort: bool = False,
     copy: bool = True,
-):
+) -> FrameOrSeriesUnion:
+    ...
+
+
+def concat(
+    objs: Union[
+        Sequence[FrameOrSeriesUnion], Mapping[Optional[Hashable], FrameOrSeriesUnion]
+    ],
+    axis=0,
+    join="outer",
+    ignore_index: bool = False,
+    keys=None,
+    levels=None,
+    names=None,
+    verify_integrity: bool = False,
+    sort: bool = False,
+    copy: bool = True,
+) -> FrameOrSeriesUnion:
     """
     Concatenate pandas objects along a particular axis with optional set logic
     along the other axes.

pandas/core/reshape/melt.py

@@ -192,7 +192,9 @@ def lreshape(data: DataFrame, groups, dropna: bool = True, label=None) -> DataFr
     return data._constructor(mdata, columns=id_cols + pivot_cols)
 
 
-def wide_to_long(df: DataFrame, stubnames, i, j, sep: str = "", suffix: str = r"\d+"):
+def wide_to_long(
+    df: DataFrame, stubnames, i, j, sep: str = "", suffix: str = r"\d+"
+) -> DataFrame:
     r"""
     Wide panel to long format. Less flexible but more user-friendly than melt.
 

pandas/core/reshape/pivot.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Callable, Dict, Tuple, Union
+from typing import TYPE_CHECKING, Callable, Dict, List, Tuple, Union
 
 import numpy as np
 
@@ -40,7 +40,7 @@ def pivot_table(
     columns = _convert_by(columns)
 
     if isinstance(aggfunc, list):
-        pieces = []
+        pieces: List[DataFrame] = []
         keys = []
         for func in aggfunc:
             table = pivot_table(
@@ -459,7 +459,7 @@ def crosstab(
     margins_name: str = "All",
     dropna: bool = True,
     normalize=False,
-):
+) -> "DataFrame":
     """
     Compute a simple cross tabulation of two (or more) factors. By default
     computes a frequency table of the factors unless an array of values and an

pandas/core/reshape/reshape.py

@@ -1,5 +1,6 @@
 from functools import partial
 import itertools
+from typing import List
 
 import numpy as np
 
@@ -755,7 +756,7 @@ def get_dummies(
     sparse=False,
     drop_first=False,
     dtype=None,
-):
+) -> "DataFrame":
     """
     Convert categorical variable into dummy/indicator variables.
 
@@ -899,7 +900,7 @@ def check_len(item, name):
 
         if data_to_encode.shape == data.shape:
             # Encoding the entire df, do not prepend any dropped columns
-            with_dummies = []
+            with_dummies: List[DataFrame] = []
         elif columns is not None:
             # Encoding only cols specified in columns. Get all cols not in
             # columns to prepend to result.

pandas/io/formats/format.py

@@ -281,7 +281,9 @@ def _chk_truncate(self) -> None:
                 series = series.iloc[:max_rows]
             else:
                 row_num = max_rows // 2
-                series = concat((series.iloc[:row_num], series.iloc[-row_num:]))
+                series = series._ensure_type(
+                    concat((series.iloc[:row_num], series.iloc[-row_num:]))
+                )
             self.tr_row_num = row_num
         else:
             self.tr_row_num = None