TYP: enforce annotation on SingleBlockManager.__init__

pandas/core/internals/managers.py

@@ -3,12 +3,12 @@
 import itertools
 import operator
 import re
-from typing import Dict, List, Optional, Sequence, Tuple, Union
+from typing import Dict, List, Optional, Sequence, Tuple, TypeVar, Union
 
 import numpy as np
 
 from pandas._libs import Timedelta, Timestamp, internals as libinternals, lib
-from pandas._typing import DtypeObj, Label
+from pandas._typing import ArrayLike, DtypeObj, Label
 from pandas.util._validators import validate_bool_kwarg
 
 from pandas.core.dtypes.cast import (
@@ -58,6 +58,8 @@
 
 # TODO: flexible with index=None and/or items=None
 
+T = TypeVar("T", bound="BlockManager")
+
 
 class BlockManager(PandasObject):
     """
@@ -149,6 +151,13 @@ def __init__(
         self._blknos = None
         self._blklocs = None
 
+    @classmethod
+    def from_blocks(cls, blocks: List[Block], axes: List[Index]):
+        """
+        Constructor for BlockManager and SingleBlockManager with same signature.
+        """
+        return cls(blocks, axes, do_integrity_check=False)
+
     @property
     def blknos(self):
         """
@@ -176,18 +185,19 @@ def blklocs(self):
 
         return self._blklocs
 
-    def make_empty(self, axes=None) -> "BlockManager":
+    def make_empty(self: T, axes=None) -> T:
         """ return an empty BlockManager with the items axis of len 0 """
         if axes is None:
             axes = [Index([])] + self.axes[1:]
 
         # preserve dtype if possible
         if self.ndim == 1:
             assert isinstance(self, SingleBlockManager)  # for mypy
-            blocks = np.array([], dtype=self.array_dtype)
+            arr = np.array([], dtype=self.array_dtype)
+            blocks = [make_block(arr, placement=slice(0, 0), ndim=1)]
         else:
             blocks = []
-        return type(self)(blocks, axes)
+        return type(self).from_blocks(blocks, axes)
 
     def __nonzero__(self) -> bool:
         return True
@@ -380,7 +390,7 @@ def reduce(self, func, *args, **kwargs):
 
         return res
 
-    def apply(self, f, filter=None, **kwargs) -> "BlockManager":
+    def apply(self: T, f, filter=None, **kwargs) -> T:
         """
         Iterate over the blocks, collect and create a new BlockManager.
 
@@ -458,8 +468,8 @@ def apply(self, f, filter=None, **kwargs) -> "BlockManager":
 
         if len(result_blocks) == 0:
             return self.make_empty(self.axes)
-        bm = type(self)(result_blocks, self.axes, do_integrity_check=False)
-        return bm
+
+        return type(self).from_blocks(result_blocks, self.axes)
 
     def quantile(
         self,
@@ -658,7 +668,7 @@ def comp(s, regex=False):
                 rb = new_rb
             result_blocks.extend(rb)
 
-        bm = type(self)(result_blocks, self.axes)
+        bm = type(self).from_blocks(result_blocks, self.axes)
         bm._consolidate_inplace()
         return bm
 
@@ -747,7 +757,7 @@ def combine(self, blocks: List[Block], copy: bool = True) -> "BlockManager":
         axes = list(self.axes)
         axes[0] = self.items.take(indexer)
 
-        return type(self)(new_blocks, axes, do_integrity_check=False)
+        return type(self).from_blocks(new_blocks, axes)
 
     def get_slice(self, slobj: slice, axis: int = 0) -> "BlockManager":
 
@@ -774,7 +784,7 @@ def __contains__(self, item) -> bool:
     def nblocks(self) -> int:
         return len(self.blocks)
 
-    def copy(self, deep=True) -> "BlockManager":
+    def copy(self: T, deep=True) -> T:
         """
         Make deep or shallow copy of BlockManager
 
@@ -1244,14 +1254,14 @@ def reindex_axis(
         )
 
     def reindex_indexer(
-        self,
+        self: T,
         new_axis,
         indexer,
         axis: int,
         fill_value=None,
-        allow_dups=False,
+        allow_dups: bool = False,
         copy: bool = True,
-    ):
+    ) -> T:
         """
         Parameters
         ----------
@@ -1299,7 +1309,8 @@ def reindex_indexer(
 
         new_axes = list(self.axes)
         new_axes[axis] = new_axis
-        return type(self)(new_blocks, new_axes)
+
+        return type(self).from_blocks(new_blocks, new_axes)
 
     def _slice_take_blocks_ax0(self, slice_or_indexer, fill_tuple=None):
         """
@@ -1500,6 +1511,8 @@ def __init__(
         do_integrity_check: bool = False,
         fastpath: bool = False,
     ):
+        assert isinstance(block, Block), type(block)
+
         if isinstance(axis, list):
             if len(axis) != 1:
                 raise ValueError(
@@ -1510,38 +1523,29 @@ def __init__(
         # passed from constructor, single block, single axis
         if fastpath:
             self.axes = [axis]
-            if isinstance(block, list):
-
-                # empty block
-                if len(block) == 0:
-                    block = [np.array([])]
-                elif len(block) != 1:
-                    raise ValueError(
-                        "Cannot create SingleBlockManager with more than 1 block"
-                    )
-                block = block[0]
         else:
             self.axes = [ensure_index(axis)]
 
-            # create the block here
-            if isinstance(block, list):
-
-                # provide consolidation to the interleaved_dtype
-                if len(block) > 1:
-                    dtype = _interleaved_dtype(block)
-                    block = [b.astype(dtype) for b in block]
-                    block = _consolidate(block)
-
-                if len(block) != 1:
-                    raise ValueError(
-                        "Cannot create SingleBlockManager with more than 1 block"
-                    )
-                block = block[0]
+        self.blocks = tuple([block])
 
-        if not isinstance(block, Block):
-            block = make_block(block, placement=slice(0, len(axis)), ndim=1)
+    @classmethod
+    def from_blocks(
+        cls, blocks: List[Block], axes: List[Index]
+    ) -> "SingleBlockManager":
+        """
+        Constructor for BlockManager and SingleBlockManager with same signature.
+        """
+        assert len(blocks) == 1
+        assert len(axes) == 1
+        return cls(blocks[0], axes[0], do_integrity_check=False, fastpath=True)
 
-        self.blocks = tuple([block])
+    @classmethod
+    def from_array(cls, array: ArrayLike, index: Index) -> "SingleBlockManager":
+        """
+        Constructor for if we have an array that is not yet a Block.
+        """
+        block = make_block(array, placement=slice(0, len(index)), ndim=1)
+        return cls(block, index, fastpath=True)
 
     def _post_setstate(self):
         pass
@@ -1568,7 +1572,10 @@ def get_slice(self, slobj: slice, axis: int = 0) -> "SingleBlockManager":
         if axis >= self.ndim:
             raise IndexError("Requested axis not found in manager")
 
-        return type(self)(self._block._slice(slobj), self.index[slobj], fastpath=True)
+        blk = self._block
+        array = blk._slice(slobj)
+        block = blk.make_block_same_class(array, placement=range(len(array)))
+        return type(self)(block, self.index[slobj], fastpath=True)
 
     @property
     def index(self) -> Index:
@@ -1630,7 +1637,7 @@ def fast_xs(self, loc):
         """
         raise NotImplementedError("Use series._values[loc] instead")
 
-    def concat(self, to_concat, new_axis) -> "SingleBlockManager":
+    def concat(self, to_concat, new_axis: Index) -> "SingleBlockManager":
         """
         Concatenate a list of SingleBlockManagers into a single
         SingleBlockManager.

pandas/core/series.py

@@ -205,7 +205,7 @@ def __init__(
 
             # data is an ndarray, index is defined
             if not isinstance(data, SingleBlockManager):
-                data = SingleBlockManager(data, index, fastpath=True)
+                data = SingleBlockManager.from_array(data, index)
             if copy:
                 data = data.copy()
             if index is None:
@@ -317,7 +317,7 @@ def __init__(
             else:
                 data = sanitize_array(data, index, dtype, copy, raise_cast_failure=True)
 
-                data = SingleBlockManager(data, index, fastpath=True)
+                data = SingleBlockManager.from_array(data, index)
 
         generic.NDFrame.__init__(self, data)
         self.name = name

pandas/tests/extension/test_external_block.py

@@ -2,7 +2,7 @@
 import pytest
 
 import pandas as pd
-from pandas.core.internals import BlockManager
+from pandas.core.internals import BlockManager, SingleBlockManager
 from pandas.core.internals.blocks import Block, NonConsolidatableMixIn
 
 
@@ -36,7 +36,8 @@ def test_concat_series():
     # GH17728
     values = np.arange(3, dtype="int64")
     block = CustomBlock(values, placement=slice(0, 3))
-    s = pd.Series(block, pd.RangeIndex(3), fastpath=True)
+    mgr = SingleBlockManager(block, pd.RangeIndex(3))
+    s = pd.Series(mgr, pd.RangeIndex(3), fastpath=True)
 
     res = pd.concat([s, s])
     assert isinstance(res._data.blocks[0], CustomBlock)