REF: move union_categoricals call outside of cython

pandas/_libs/parsers.pyi

@@ -58,7 +58,6 @@ class TextReader:
         true_values=...,
         false_values=...,
         allow_leading_cols: bool = ...,
-        low_memory: bool = ...,
         skiprows=...,
         skipfooter: int = ...,  # int64_t
         verbose: bool = ...,
@@ -75,3 +74,4 @@ class TextReader:
     def close(self) -> None: ...
 
     def read(self, rows: int | None = ...) -> dict[int, ArrayLike]: ...
+    def read_low_memory(self, rows: int | None) -> list[dict[int, ArrayLike]]: ...

pandas/_libs/parsers.pyx

@@ -94,7 +94,6 @@ from pandas._libs.khash cimport (
 )
 
 from pandas.errors import (
-    DtypeWarning,
     EmptyDataError,
     ParserError,
     ParserWarning,
@@ -108,9 +107,7 @@ from pandas.core.dtypes.common import (
     is_float_dtype,
     is_integer_dtype,
     is_object_dtype,
-    pandas_dtype,
 )
-from pandas.core.dtypes.concat import union_categoricals
 
 cdef:
     float64_t INF = <float64_t>np.inf
@@ -317,7 +314,7 @@ cdef class TextReader:
 
     cdef public:
         int64_t leading_cols, table_width, skipfooter, buffer_lines
-        bint allow_leading_cols, mangle_dupe_cols, low_memory
+        bint allow_leading_cols, mangle_dupe_cols
         bint delim_whitespace
         object delimiter  # bytes or str
         object converters
@@ -362,7 +359,6 @@ cdef class TextReader:
                   true_values=None,
                   false_values=None,
                   bint allow_leading_cols=True,
-                  bint low_memory=False,
                   skiprows=None,
                   skipfooter=0,         # int64_t
                   bint verbose=False,
@@ -479,7 +475,6 @@ cdef class TextReader:
         self.na_filter = na_filter
 
         self.verbose = verbose
-        self.low_memory = low_memory
 
         if float_precision == "round_trip":
             # see gh-15140
@@ -492,12 +487,10 @@ cdef class TextReader:
             raise ValueError(f'Unrecognized float_precision option: '
                              f'{float_precision}')
 
-        if isinstance(dtype, dict):
-            dtype = {k: pandas_dtype(dtype[k])
-                     for k in dtype}
-        elif dtype is not None:
-            dtype = pandas_dtype(dtype)
-
+        # Caller is responsible for ensuring we have one of
+        # - None
+        # - DtypeObj
+        # - dict[Any, DtypeObj]
         self.dtype = dtype
 
         # XXX
@@ -773,17 +766,18 @@ cdef class TextReader:
         """
         rows=None --> read all rows
         """
-        if self.low_memory:
-            # Conserve intermediate space
-            columns = self._read_low_memory(rows)
-        else:
-            # Don't care about memory usage
-            columns = self._read_rows(rows, 1)
+        # Don't care about memory usage
+        columns = self._read_rows(rows, 1)
 
         return columns
 
-    #  -> dict[int, "ArrayLike"]
-    cdef _read_low_memory(self, rows):
+    def read_low_memory(self, rows: int | None)-> list[dict[int, "ArrayLike"]]:
+        """
+        rows=None --> read all rows
+        """
+        # Conserve intermediate space
+        # Caller is responsible for concatenating chunks,
+        #  see c_parser_wrapper._concatenatve_chunks
         cdef:
             size_t rows_read = 0
             list chunks = []
@@ -818,8 +812,7 @@ cdef class TextReader:
         if len(chunks) == 0:
             raise StopIteration
 
-        # destructive to chunks
-        return _concatenate_chunks(chunks)
+        return chunks
 
     cdef _tokenize_rows(self, size_t nrows):
         cdef:
@@ -1907,49 +1900,6 @@ cdef raise_parser_error(object base, parser_t *parser):
     raise ParserError(message)
 
 
-# chunks: list[dict[int, "ArrayLike"]]
-# -> dict[int, "ArrayLike"]
-def _concatenate_chunks(list chunks) -> dict:
-    cdef:
-        list names = list(chunks[0].keys())
-        object name
-        list warning_columns = []
-        object warning_names
-        object common_type
-
-    result = {}
-    for name in names:
-        arrs = [chunk.pop(name) for chunk in chunks]
-        # Check each arr for consistent types.
-        dtypes = {a.dtype for a in arrs}
-        numpy_dtypes = {x for x in dtypes if not is_categorical_dtype(x)}
-        if len(numpy_dtypes) > 1:
-            common_type = np.find_common_type(numpy_dtypes, [])
-            if common_type == object:
-                warning_columns.append(str(name))
-
-        dtype = dtypes.pop()
-        if is_categorical_dtype(dtype):
-            sort_categories = isinstance(dtype, str)
-            result[name] = union_categoricals(arrs,
-                                              sort_categories=sort_categories)
-        else:
-            if is_extension_array_dtype(dtype):
-                array_type = dtype.construct_array_type()
-                result[name] = array_type._concat_same_type(arrs)
-            else:
-                result[name] = np.concatenate(arrs)
-
-    if warning_columns:
-        warning_names = ','.join(warning_columns)
-        warning_message = " ".join([
-            f"Columns ({warning_names}) have mixed types."
-            f"Specify dtype option on import or set low_memory=False."
-          ])
-        warnings.warn(warning_message, DtypeWarning, stacklevel=8)
-    return result
-
-
 # ----------------------------------------------------------------------
 # NA values
 def _compute_na_values():

pandas/io/parsers/c_parser_wrapper.py

@@ -1,5 +1,22 @@
+from __future__ import annotations
+
+import warnings
+
+import numpy as np
+
 import pandas._libs.parsers as parsers
-from pandas._typing import FilePathOrBuffer
+from pandas._typing import (
+    ArrayLike,
+    FilePathOrBuffer,
+)
+from pandas.errors import DtypeWarning
+
+from pandas.core.dtypes.common import (
+    is_categorical_dtype,
+    pandas_dtype,
+)
+from pandas.core.dtypes.concat import union_categoricals
+from pandas.core.dtypes.dtypes import ExtensionDtype
 
 from pandas.core.indexes.api import ensure_index_from_sequences
 
@@ -10,12 +27,16 @@
 
 
 class CParserWrapper(ParserBase):
+    low_memory: bool
+
     def __init__(self, src: FilePathOrBuffer, **kwds):
         self.kwds = kwds
         kwds = kwds.copy()
 
         ParserBase.__init__(self, kwds)
 
+        self.low_memory = kwds.pop("low_memory", False)
+
         # #2442
         kwds["allow_leading_cols"] = self.index_col is not False
 
@@ -47,6 +68,7 @@ def __init__(self, src: FilePathOrBuffer, **kwds):
             # TextIOBase, TextIOWrapper, mmap]" has no attribute "mmap"
             self.handles.handle = self.handles.handle.mmap  # type: ignore[union-attr]
 
+        kwds["dtype"] = ensure_dtype_objs(kwds.get("dtype", None))
         try:
             self._reader = parsers.TextReader(self.handles.handle, **kwds)
         except Exception:
@@ -160,7 +182,13 @@ def set_error_bad_lines(self, status):
 
     def read(self, nrows=None):
         try:
-            data = self._reader.read(nrows)
+            if self.low_memory:
+                chunks = self._reader.read_low_memory(nrows)
+                # destructive to chunks
+                data = _concatenate_chunks(chunks)
+
+            else:
+                data = self._reader.read(nrows)
         except StopIteration:
             if self._first_chunk:
                 self._first_chunk = False
@@ -265,7 +293,76 @@ def _get_index_names(self):
 
         return names, idx_names
 
-    def _maybe_parse_dates(self, values, index, try_parse_dates=True):
+    def _maybe_parse_dates(self, values, index: int, try_parse_dates=True):
         if try_parse_dates and self._should_parse_dates(index):
             values = self._date_conv(values)
         return values
+
+
+def _concatenate_chunks(chunks: list[dict[int, ArrayLike]]) -> dict:
+    """
+    Concatenate chunks of data read with low_memory=True.
+
+    The tricky part is handling Categoricals, where different chunks
+    may have different inferred categories.
+    """
+    names = list(chunks[0].keys())
+    warning_columns = []
+
+    result = {}
+    for name in names:
+        arrs = [chunk.pop(name) for chunk in chunks]
+        # Check each arr for consistent types.
+        dtypes = {a.dtype for a in arrs}
+        # TODO: shouldn't we exclude all EA dtypes here?
+        numpy_dtypes = {x for x in dtypes if not is_categorical_dtype(x)}
+        if len(numpy_dtypes) > 1:
+            # error: Argument 1 to "find_common_type" has incompatible type
+            # "Set[Any]"; expected "Sequence[Union[dtype[Any], None, type,
+            # _SupportsDType, str, Union[Tuple[Any, int], Tuple[Any,
+            # Union[int, Sequence[int]]], List[Any], _DTypeDict, Tuple[Any, Any]]]]"
+            common_type = np.find_common_type(
+                numpy_dtypes,  # type: ignore[arg-type]
+                [],
+            )
+            if common_type == object:
+                warning_columns.append(str(name))
+
+        dtype = dtypes.pop()
+        if is_categorical_dtype(dtype):
+            result[name] = union_categoricals(arrs, sort_categories=False)
+        else:
+            if isinstance(dtype, ExtensionDtype):
+                # TODO: concat_compat?
+                array_type = dtype.construct_array_type()
+                # error: Argument 1 to "_concat_same_type" of "ExtensionArray"
+                # has incompatible type "List[Union[ExtensionArray, ndarray]]";
+                # expected "Sequence[ExtensionArray]"
+                result[name] = array_type._concat_same_type(
+                    arrs  # type: ignore[arg-type]
+                )
+            else:
+                result[name] = np.concatenate(arrs)
+
+    if warning_columns:
+        warning_names = ",".join(warning_columns)
+        warning_message = " ".join(
+            [
+                f"Columns ({warning_names}) have mixed types."
+                f"Specify dtype option on import or set low_memory=False."
+            ]
+        )
+        warnings.warn(warning_message, DtypeWarning, stacklevel=8)
+    return result
+
+
+def ensure_dtype_objs(dtype):
+    """
+    Ensure we have either None, a dtype object, or a dictionary mapping to
+    dtype objects.
+    """
+    if isinstance(dtype, dict):
+        dtype = {k: pandas_dtype(dtype[k]) for k in dtype}
+    elif dtype is not None:
+        dtype = pandas_dtype(dtype)
+    return dtype

pandas/tests/io/parser/test_textreader.py

@@ -21,6 +21,7 @@
     TextFileReader,
     read_csv,
 )
+from pandas.io.parsers.c_parser_wrapper import ensure_dtype_objs
 
 
 class TestTextReader:
@@ -206,6 +207,8 @@ def test_numpy_string_dtype(self):
 aaaaa,5"""
 
         def _make_reader(**kwds):
+            if "dtype" in kwds:
+                kwds["dtype"] = ensure_dtype_objs(kwds["dtype"])
             return TextReader(StringIO(data), delimiter=",", header=None, **kwds)
 
         reader = _make_reader(dtype="S5,i4")
@@ -233,6 +236,8 @@ def test_pass_dtype(self):
 4,d"""
 
         def _make_reader(**kwds):
+            if "dtype" in kwds:
+                kwds["dtype"] = ensure_dtype_objs(kwds["dtype"])
             return TextReader(StringIO(data), delimiter=",", **kwds)
 
         reader = _make_reader(dtype={"one": "u1", 1: "S1"})