ENH: Add defaultdict support for dtype in read_csv

doc/source/user_guide/io.rst

@@ -186,6 +186,11 @@ dtype : Type name or dict of column -> type, default ``None``
   (unsupported with ``engine='python'``). Use ``str`` or ``object`` together
   with suitable ``na_values`` settings to preserve and
   not interpret dtype.
+  .. versionadded:: 1.5.0
+
+    Support for defaultdict was added. Specify a defaultdict as input where
+    the default determines the dtype of the columns which are not explicitly
+    listed.
 engine : {``'c'``, ``'python'``, ``'pyarrow'``}
   Parser engine to use. The C and pyarrow engines are faster, while the python engine
   is currently more feature-complete. Multithreading is currently only supported by

doc/source/whatsnew/v1.5.0.rst

@@ -39,6 +39,7 @@ Other enhancements
 - :meth:`to_numeric` now preserves float64 arrays when downcasting would generate values not representable in float32 (:issue:`43693`)
 - :meth:`Series.reset_index` and :meth:`DataFrame.reset_index` now support the argument ``allow_duplicates`` (:issue:`44410`)
 - :meth:`.GroupBy.min` and :meth:`.GroupBy.max` now supports `Numba <https://numba.pydata.org/>`_ execution with the ``engine`` keyword (:issue:`45428`)
+- :func:`read_csv` now supports ``defaultdict`` as a ``dtype`` parameter (:issue:`41574`)
 - :meth:`DataFrame.rolling` and :meth:`Series.rolling` now support a ``step`` parameter with fixed-length windows (:issue:`15354`)
 - Implemented a ``bool``-dtype :class:`Index`, passing a bool-dtype array-like to ``pd.Index`` will now retain ``bool`` dtype instead of casting to ``object`` (:issue:`45061`)
 - Implemented a complex-dtype :class:`Index`, passing a complex-dtype array-like to ``pd.Index`` will now retain complex dtype instead of casting to ``object`` (:issue:`45845`)

pandas/_libs/parsers.pyx

@@ -1,6 +1,7 @@
 # Copyright (c) 2012, Lambda Foundry, Inc.
 # See LICENSE for the license
 from base64 import decode
+from collections import defaultdict
 from csv import (
     QUOTE_MINIMAL,
     QUOTE_NONE,
@@ -964,6 +965,8 @@ cdef class TextReader:
 
         results = {}
         nused = 0
+        is_default_dict_dtype = isinstance(self.dtype, defaultdict)
+
         for i in range(self.table_width):
             if i < self.leading_cols:
                 # Pass through leading columns always
@@ -994,6 +997,8 @@ cdef class TextReader:
                         col_dtype = self.dtype[name]
                     elif i in self.dtype:
                         col_dtype = self.dtype[i]
+                    elif is_default_dict_dtype:
+                        col_dtype = self.dtype[name]
                 else:
                     if self.dtype.names:
                         # structured array

pandas/io/parsers/base_parser.py

@@ -466,12 +466,16 @@ def _clean_mapping(self, mapping):
         if not isinstance(mapping, dict):
             return mapping
         clean = {}
+        # for mypy
+        assert self.orig_names is not None
+
         for col, v in mapping.items():
-            # for mypy
-            assert self.orig_names is not None
             if isinstance(col, int) and col not in self.orig_names:
                 col = self.orig_names[col]
             clean[col] = v
+        if isinstance(mapping, defaultdict):
+            remaining_cols = set(self.orig_names) - set(clean.keys())
+            clean.update({col: mapping[col] for col in remaining_cols})
         return clean
 
     @final

pandas/io/parsers/c_parser_wrapper.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from collections import defaultdict
 from typing import (
     Hashable,
     Mapping,
@@ -415,7 +416,14 @@ def ensure_dtype_objs(
     Ensure we have either None, a dtype object, or a dictionary mapping to
     dtype objects.
     """
-    if isinstance(dtype, dict):
+    if isinstance(dtype, defaultdict):
+        # "None" not callable  [misc]
+        default_dtype = pandas_dtype(dtype.default_factory())  # type: ignore[misc]
+        dtype_converted: defaultdict = defaultdict(lambda: default_dtype)
+        for key in dtype.keys():
+            dtype_converted[key] = pandas_dtype(dtype[key])
+        return dtype_converted
+    elif isinstance(dtype, dict):
         return {k: pandas_dtype(dtype[k]) for k in dtype}
     elif dtype is not None:
         return pandas_dtype(dtype)

pandas/io/parsers/readers.py

@@ -168,6 +168,12 @@
     to preserve and not interpret dtype.
     If converters are specified, they will be applied INSTEAD
     of dtype conversion.
+
+    .. versionadded:: 1.5.0
+
+        Support for defaultdict was added. Specify a defaultdict as input where
+        the default determines the dtype of the columns which are not explicitly
+        listed.
 engine : {{'c', 'python', 'pyarrow'}}, optional
     Parser engine to use. The C and pyarrow engines are faster, while the python engine
     is currently more feature-complete. Multithreading is currently only supported by

pandas/tests/io/parser/dtypes/test_dtypes_basic.py

@@ -2,6 +2,7 @@
 Tests dtype specification during parsing
 for all of the parsers defined in parsers.py
 """
+from collections import defaultdict
 from io import StringIO
 
 import numpy as np
@@ -343,3 +344,40 @@ def test_nullable_int_dtype(all_parsers, any_int_ea_dtype):
     )
     actual = parser.read_csv(StringIO(data), dtype=dtype)
     tm.assert_frame_equal(actual, expected)
+
+
+@pytest.mark.parametrize("default", ["float", "float64"])
+def test_dtypes_defaultdict(all_parsers, default):
+    # GH#41574
+    data = """a,b
+1,2
+"""
+    dtype = defaultdict(lambda: default, a="int64")
+    parser = all_parsers
+    result = parser.read_csv(StringIO(data), dtype=dtype)
+    expected = DataFrame({"a": [1], "b": 2.0})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_dtypes_defaultdict_mangle_dup_cols(all_parsers):
+    # GH#41574
+    data = """a,b,a,b,b.1
+1,2,3,4,5
+"""
+    dtype = defaultdict(lambda: "float64", a="int64")
+    dtype["b.1"] = "int64"
+    parser = all_parsers
+    result = parser.read_csv(StringIO(data), dtype=dtype)
+    expected = DataFrame({"a": [1], "b": [2.0], "a.1": [3], "b.2": [4.0], "b.1": [5]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_dtypes_defaultdict_invalid(all_parsers):
+    # GH#41574
+    data = """a,b
+1,2
+"""
+    dtype = defaultdict(lambda: "invalid_dtype", a="int64")
+    parser = all_parsers
+    with pytest.raises(TypeError, match="not understood"):
+        parser.read_csv(StringIO(data), dtype=dtype)