378 lines
13 KiB
Python
378 lines
13 KiB
Python
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from __future__ import annotations
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from typing import Any
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import numpy as np
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from pandas._libs.lib import infer_dtype
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from pandas._libs.tslibs import iNaT
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from pandas.errors import NoBufferPresent
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from pandas.util._decorators import cache_readonly
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import pandas as pd
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from pandas.api.types import (
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is_categorical_dtype,
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is_string_dtype,
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)
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from pandas.core.interchange.buffer import PandasBuffer
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from pandas.core.interchange.dataframe_protocol import (
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Column,
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ColumnBuffers,
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ColumnNullType,
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DtypeKind,
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)
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from pandas.core.interchange.utils import (
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ArrowCTypes,
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Endianness,
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dtype_to_arrow_c_fmt,
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)
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_NP_KINDS = {
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"i": DtypeKind.INT,
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"u": DtypeKind.UINT,
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"f": DtypeKind.FLOAT,
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"b": DtypeKind.BOOL,
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"U": DtypeKind.STRING,
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"M": DtypeKind.DATETIME,
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"m": DtypeKind.DATETIME,
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}
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_NULL_DESCRIPTION = {
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DtypeKind.FLOAT: (ColumnNullType.USE_NAN, None),
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DtypeKind.DATETIME: (ColumnNullType.USE_SENTINEL, iNaT),
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DtypeKind.INT: (ColumnNullType.NON_NULLABLE, None),
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DtypeKind.UINT: (ColumnNullType.NON_NULLABLE, None),
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DtypeKind.BOOL: (ColumnNullType.NON_NULLABLE, None),
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# Null values for categoricals are stored as `-1` sentinel values
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# in the category date (e.g., `col.values.codes` is int8 np.ndarray)
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DtypeKind.CATEGORICAL: (ColumnNullType.USE_SENTINEL, -1),
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# follow Arrow in using 1 as valid value and 0 for missing/null value
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DtypeKind.STRING: (ColumnNullType.USE_BYTEMASK, 0),
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}
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_NO_VALIDITY_BUFFER = {
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ColumnNullType.NON_NULLABLE: "This column is non-nullable",
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ColumnNullType.USE_NAN: "This column uses NaN as null",
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ColumnNullType.USE_SENTINEL: "This column uses a sentinel value",
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}
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class PandasColumn(Column):
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"""
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A column object, with only the methods and properties required by the
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interchange protocol defined.
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A column can contain one or more chunks. Each chunk can contain up to three
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buffers - a data buffer, a mask buffer (depending on null representation),
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and an offsets buffer (if variable-size binary; e.g., variable-length
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strings).
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Note: this Column object can only be produced by ``__dataframe__``, so
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doesn't need its own version or ``__column__`` protocol.
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"""
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def __init__(self, column: pd.Series, allow_copy: bool = True) -> None:
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"""
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Note: doesn't deal with extension arrays yet, just assume a regular
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Series/ndarray for now.
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"""
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if not isinstance(column, pd.Series):
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raise NotImplementedError(f"Columns of type {type(column)} not handled yet")
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# Store the column as a private attribute
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self._col = column
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self._allow_copy = allow_copy
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def size(self) -> int:
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"""
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Size of the column, in elements.
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"""
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return self._col.size
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@property
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def offset(self) -> int:
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"""
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Offset of first element. Always zero.
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"""
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# TODO: chunks are implemented now, probably this should return something
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return 0
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@cache_readonly
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def dtype(self) -> tuple[DtypeKind, int, str, str]:
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dtype = self._col.dtype
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if is_categorical_dtype(dtype):
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codes = self._col.values.codes
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(
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_,
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bitwidth,
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c_arrow_dtype_f_str,
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_,
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) = self._dtype_from_pandasdtype(codes.dtype)
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return (
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DtypeKind.CATEGORICAL,
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bitwidth,
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c_arrow_dtype_f_str,
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Endianness.NATIVE,
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)
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elif is_string_dtype(dtype):
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if infer_dtype(self._col) == "string":
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return (
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DtypeKind.STRING,
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8,
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dtype_to_arrow_c_fmt(dtype),
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Endianness.NATIVE,
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)
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raise NotImplementedError("Non-string object dtypes are not supported yet")
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else:
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return self._dtype_from_pandasdtype(dtype)
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def _dtype_from_pandasdtype(self, dtype) -> tuple[DtypeKind, int, str, str]:
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"""
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See `self.dtype` for details.
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"""
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# Note: 'c' (complex) not handled yet (not in array spec v1).
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# 'b', 'B' (bytes), 'S', 'a', (old-style string) 'V' (void) not handled
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# datetime and timedelta both map to datetime (is timedelta handled?)
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kind = _NP_KINDS.get(dtype.kind, None)
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if kind is None:
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# Not a NumPy dtype. Check if it's a categorical maybe
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raise ValueError(f"Data type {dtype} not supported by interchange protocol")
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return kind, dtype.itemsize * 8, dtype_to_arrow_c_fmt(dtype), dtype.byteorder
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@property
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def describe_categorical(self):
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"""
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If the dtype is categorical, there are two options:
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- There are only values in the data buffer.
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- There is a separate non-categorical Column encoding for categorical values.
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Raises TypeError if the dtype is not categorical
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Content of returned dict:
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- "is_ordered" : bool, whether the ordering of dictionary indices is
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semantically meaningful.
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- "is_dictionary" : bool, whether a dictionary-style mapping of
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categorical values to other objects exists
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- "categories" : Column representing the (implicit) mapping of indices to
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category values (e.g. an array of cat1, cat2, ...).
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None if not a dictionary-style categorical.
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"""
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if not self.dtype[0] == DtypeKind.CATEGORICAL:
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raise TypeError(
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"describe_categorical only works on a column with categorical dtype!"
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)
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return {
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"is_ordered": self._col.cat.ordered,
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"is_dictionary": True,
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"categories": PandasColumn(pd.Series(self._col.cat.categories)),
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}
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@property
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def describe_null(self):
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kind = self.dtype[0]
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try:
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null, value = _NULL_DESCRIPTION[kind]
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except KeyError:
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raise NotImplementedError(f"Data type {kind} not yet supported")
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return null, value
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@cache_readonly
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def null_count(self) -> int:
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"""
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Number of null elements. Should always be known.
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"""
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return self._col.isna().sum().item()
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@property
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def metadata(self) -> dict[str, pd.Index]:
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"""
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Store specific metadata of the column.
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"""
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return {"pandas.index": self._col.index}
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def num_chunks(self) -> int:
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"""
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Return the number of chunks the column consists of.
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"""
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return 1
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def get_chunks(self, n_chunks: int | None = None):
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"""
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Return an iterator yielding the chunks.
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See `DataFrame.get_chunks` for details on ``n_chunks``.
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"""
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if n_chunks and n_chunks > 1:
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size = len(self._col)
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step = size // n_chunks
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if size % n_chunks != 0:
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step += 1
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for start in range(0, step * n_chunks, step):
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yield PandasColumn(
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self._col.iloc[start : start + step], self._allow_copy
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)
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else:
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yield self
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def get_buffers(self) -> ColumnBuffers:
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"""
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Return a dictionary containing the underlying buffers.
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The returned dictionary has the following contents:
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- "data": a two-element tuple whose first element is a buffer
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containing the data and whose second element is the data
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buffer's associated dtype.
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- "validity": a two-element tuple whose first element is a buffer
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containing mask values indicating missing data and
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whose second element is the mask value buffer's
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associated dtype. None if the null representation is
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not a bit or byte mask.
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- "offsets": a two-element tuple whose first element is a buffer
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containing the offset values for variable-size binary
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data (e.g., variable-length strings) and whose second
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element is the offsets buffer's associated dtype. None
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if the data buffer does not have an associated offsets
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buffer.
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"""
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buffers: ColumnBuffers = {
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"data": self._get_data_buffer(),
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"validity": None,
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"offsets": None,
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}
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try:
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buffers["validity"] = self._get_validity_buffer()
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except NoBufferPresent:
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pass
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try:
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buffers["offsets"] = self._get_offsets_buffer()
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except NoBufferPresent:
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pass
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return buffers
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def _get_data_buffer(
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self,
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) -> tuple[PandasBuffer, Any]: # Any is for self.dtype tuple
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"""
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Return the buffer containing the data and the buffer's associated dtype.
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"""
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if self.dtype[0] in (
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DtypeKind.INT,
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DtypeKind.UINT,
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DtypeKind.FLOAT,
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DtypeKind.BOOL,
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DtypeKind.DATETIME,
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):
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buffer = PandasBuffer(self._col.to_numpy(), allow_copy=self._allow_copy)
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dtype = self.dtype
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elif self.dtype[0] == DtypeKind.CATEGORICAL:
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codes = self._col.values._codes
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buffer = PandasBuffer(codes, allow_copy=self._allow_copy)
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dtype = self._dtype_from_pandasdtype(codes.dtype)
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elif self.dtype[0] == DtypeKind.STRING:
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# Marshal the strings from a NumPy object array into a byte array
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buf = self._col.to_numpy()
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b = bytearray()
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# TODO: this for-loop is slow; can be implemented in Cython/C/C++ later
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for obj in buf:
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if isinstance(obj, str):
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b.extend(obj.encode(encoding="utf-8"))
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# Convert the byte array to a Pandas "buffer" using
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# a NumPy array as the backing store
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buffer = PandasBuffer(np.frombuffer(b, dtype="uint8"))
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# Define the dtype for the returned buffer
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dtype = (
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DtypeKind.STRING,
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8,
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ArrowCTypes.STRING,
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Endianness.NATIVE,
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) # note: currently only support native endianness
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else:
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raise NotImplementedError(f"Data type {self._col.dtype} not handled yet")
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return buffer, dtype
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def _get_validity_buffer(self) -> tuple[PandasBuffer, Any]:
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"""
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Return the buffer containing the mask values indicating missing data and
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the buffer's associated dtype.
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Raises NoBufferPresent if null representation is not a bit or byte mask.
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"""
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null, invalid = self.describe_null
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if self.dtype[0] == DtypeKind.STRING:
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# For now, use byte array as the mask.
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# TODO: maybe store as bit array to save space?..
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buf = self._col.to_numpy()
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# Determine the encoding for valid values
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valid = invalid == 0
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invalid = not valid
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mask = np.zeros(shape=(len(buf),), dtype=np.bool_)
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for i, obj in enumerate(buf):
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mask[i] = valid if isinstance(obj, str) else invalid
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# Convert the mask array to a Pandas "buffer" using
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# a NumPy array as the backing store
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buffer = PandasBuffer(mask)
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# Define the dtype of the returned buffer
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dtype = (DtypeKind.BOOL, 8, ArrowCTypes.BOOL, Endianness.NATIVE)
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return buffer, dtype
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try:
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msg = f"{_NO_VALIDITY_BUFFER[null]} so does not have a separate mask"
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except KeyError:
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# TODO: implement for other bit/byte masks?
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raise NotImplementedError("See self.describe_null")
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raise NoBufferPresent(msg)
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def _get_offsets_buffer(self) -> tuple[PandasBuffer, Any]:
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"""
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Return the buffer containing the offset values for variable-size binary
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data (e.g., variable-length strings) and the buffer's associated dtype.
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Raises NoBufferPresent if the data buffer does not have an associated
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offsets buffer.
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"""
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if self.dtype[0] == DtypeKind.STRING:
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# For each string, we need to manually determine the next offset
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values = self._col.to_numpy()
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ptr = 0
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offsets = np.zeros(shape=(len(values) + 1,), dtype=np.int64)
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for i, v in enumerate(values):
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# For missing values (in this case, `np.nan` values)
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# we don't increment the pointer
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if isinstance(v, str):
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b = v.encode(encoding="utf-8")
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ptr += len(b)
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offsets[i + 1] = ptr
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# Convert the offsets to a Pandas "buffer" using
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# the NumPy array as the backing store
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buffer = PandasBuffer(offsets)
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# Assemble the buffer dtype info
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dtype = (
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DtypeKind.INT,
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64,
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ArrowCTypes.INT64,
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Endianness.NATIVE,
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) # note: currently only support native endianness
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else:
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raise NoBufferPresent(
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"This column has a fixed-length dtype so "
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"it does not have an offsets buffer"
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)
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return buffer, dtype
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