BUG: read_csv(on_bad_lines='warn') did not raise a Python warning

doc/source/whatsnew/v2.2.0.rst

@@ -227,6 +227,7 @@ MultiIndex
 
 I/O
 ^^^
+- Bug in :func:`read_csv` where ``on_bad_lines="warn"`` would write to ``stderr`` instead of raise a Python warning. This now yields a :class:`.errors.ParserWarning` (:issue:`54296`)
 - Bug in :func:`read_excel`, with ``engine="xlrd"`` (``xls`` files) erroring when file contains NaNs/Infs (:issue:`54564`)
 
 Period

pandas/_libs/parsers.pyx

@@ -6,7 +6,6 @@ from csv import (
     QUOTE_NONE,
     QUOTE_NONNUMERIC,
 )
-import sys
 import time
 import warnings
 
@@ -880,9 +879,15 @@ cdef class TextReader:
 
     cdef _check_tokenize_status(self, int status):
         if self.parser.warn_msg != NULL:
-            print(PyUnicode_DecodeUTF8(
-                self.parser.warn_msg, strlen(self.parser.warn_msg),
-                self.encoding_errors), file=sys.stderr)
+            warnings.warn(
+                PyUnicode_DecodeUTF8(
+                    self.parser.warn_msg,
+                    strlen(self.parser.warn_msg),
+                    self.encoding_errors
+                ),
+                ParserWarning,
+                stacklevel=find_stack_level()
+            )
             free(self.parser.warn_msg)
             self.parser.warn_msg = NULL
 

pandas/io/parsers/python_parser.py

@@ -13,23 +13,25 @@
 import csv
 from io import StringIO
 import re
-import sys
 from typing import (
     IO,
     TYPE_CHECKING,
     DefaultDict,
     Literal,
     cast,
 )
+import warnings
 
 import numpy as np
 
 from pandas._libs import lib
 from pandas.errors import (
     EmptyDataError,
     ParserError,
+    ParserWarning,
 )
 from pandas.util._decorators import cache_readonly
+from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.common import (
     is_bool_dtype,
@@ -778,8 +780,11 @@ def _alert_malformed(self, msg: str, row_num: int) -> None:
         if self.on_bad_lines == self.BadLineHandleMethod.ERROR:
             raise ParserError(msg)
         if self.on_bad_lines == self.BadLineHandleMethod.WARN:
-            base = f"Skipping line {row_num}: "
-            sys.stderr.write(base + msg + "\n")
+            warnings.warn(
+                f"Skipping line {row_num}: {msg}\n",
+                ParserWarning,
+                stacklevel=find_stack_level(),
+            )
 
     def _next_iter_line(self, row_num: int) -> list[Scalar] | None:
         """

pandas/tests/io/parser/common/test_read_errors.py

@@ -15,6 +15,7 @@
 from pandas.errors import (
     EmptyDataError,
     ParserError,
+    ParserWarning,
 )
 
 from pandas import DataFrame
@@ -129,18 +130,16 @@ def test_unexpected_keyword_parameter_exception(all_parsers):
         parser.read_table("foo.tsv", foo=1)
 
 
-def test_suppress_error_output(all_parsers, capsys):
+def test_suppress_error_output(all_parsers):
     # see gh-15925
     parser = all_parsers
     data = "a\n1\n1,2,3\n4\n5,6,7"
     expected = DataFrame({"a": [1, 4]})
 
-    result = parser.read_csv(StringIO(data), on_bad_lines="skip")
+    with tm.assert_produces_warning(None):
+        result = parser.read_csv(StringIO(data), on_bad_lines="skip")
     tm.assert_frame_equal(result, expected)
 
-    captured = capsys.readouterr()
-    assert captured.err == ""
-
 
 def test_error_bad_lines(all_parsers):
     # see gh-15925
@@ -152,19 +151,18 @@ def test_error_bad_lines(all_parsers):
         parser.read_csv(StringIO(data), on_bad_lines="error")
 
 
-def test_warn_bad_lines(all_parsers, capsys):
+def test_warn_bad_lines(all_parsers):
     # see gh-15925
     parser = all_parsers
     data = "a\n1\n1,2,3\n4\n5,6,7"
     expected = DataFrame({"a": [1, 4]})
 
-    result = parser.read_csv(StringIO(data), on_bad_lines="warn")
+    with tm.assert_produces_warning(
+        ParserWarning, match="Skipping line", check_stacklevel=False
+    ):
+        result = parser.read_csv(StringIO(data), on_bad_lines="warn")
     tm.assert_frame_equal(result, expected)
 
-    captured = capsys.readouterr()
-    assert "Skipping line 3" in captured.err
-    assert "Skipping line 5" in captured.err
-
 
 def test_read_csv_wrong_num_columns(all_parsers):
     # Too few columns.
@@ -245,7 +243,7 @@ def test_bad_header_uniform_error(all_parsers):
         parser.read_csv(StringIO(data), index_col=0, on_bad_lines="error")
 
 
-def test_on_bad_lines_warn_correct_formatting(all_parsers, capsys):
+def test_on_bad_lines_warn_correct_formatting(all_parsers):
     # see gh-15925
     parser = all_parsers
     data = """1,2
@@ -256,17 +254,8 @@ def test_on_bad_lines_warn_correct_formatting(all_parsers, capsys):
 """
     expected = DataFrame({"1": "a", "2": ["b"] * 2})
 
-    result = parser.read_csv(StringIO(data), on_bad_lines="warn")
+    with tm.assert_produces_warning(
+        ParserWarning, match="Skipping line", check_stacklevel=False
+    ):
+        result = parser.read_csv(StringIO(data), on_bad_lines="warn")
     tm.assert_frame_equal(result, expected)
-
-    captured = capsys.readouterr()
-    if parser.engine == "c":
-        warn = """Skipping line 3: expected 2 fields, saw 3
-Skipping line 4: expected 2 fields, saw 3
-
-"""
-    else:
-        warn = """Skipping line 3: Expected 2 fields in line 3, saw 3
-Skipping line 4: Expected 2 fields in line 4, saw 3
-"""
-    assert captured.err == warn

pandas/tests/io/parser/test_c_parser_only.py

@@ -19,7 +19,10 @@
 
 from pandas.compat import is_ci_environment
 from pandas.compat.numpy import np_version_gte1p24
-from pandas.errors import ParserError
+from pandas.errors import (
+    ParserError,
+    ParserWarning,
+)
 import pandas.util._test_decorators as td
 
 from pandas import (
@@ -461,7 +464,7 @@ def test_data_after_quote(c_parser_only):
     tm.assert_frame_equal(result, expected)
 
 
-def test_comment_whitespace_delimited(c_parser_only, capsys):
+def test_comment_whitespace_delimited(c_parser_only):
     parser = c_parser_only
     test_input = """\
 1 2
@@ -474,18 +477,17 @@ def test_comment_whitespace_delimited(c_parser_only, capsys):
 8# 1 field, NaN
 9 2 3 # skipped line
 # comment"""
-    df = parser.read_csv(
-        StringIO(test_input),
-        comment="#",
-        header=None,
-        delimiter="\\s+",
-        skiprows=0,
-        on_bad_lines="warn",
-    )
-    captured = capsys.readouterr()
-    # skipped lines 2, 3, 4, 9
-    for line_num in (2, 3, 4, 9):
-        assert f"Skipping line {line_num}" in captured.err
+    with tm.assert_produces_warning(
+        ParserWarning, match="Skipping line", check_stacklevel=False
+    ):
+        df = parser.read_csv(
+            StringIO(test_input),
+            comment="#",
+            header=None,
+            delimiter="\\s+",
+            skiprows=0,
+            on_bad_lines="warn",
+        )
     expected = DataFrame([[1, 2], [5, 2], [6, 2], [7, np.nan], [8, np.nan]])
     tm.assert_frame_equal(df, expected)
 

pandas/tests/io/parser/test_python_parser_only.py

@@ -274,7 +274,7 @@ def test_multi_char_sep_quotes(python_parser_only, quoting):
             parser.read_csv(StringIO(data), quoting=quoting, **kwargs)
 
 
-def test_none_delimiter(python_parser_only, capsys):
+def test_none_delimiter(python_parser_only):
     # see gh-13374 and gh-17465
     parser = python_parser_only
     data = "a,b,c\n0,1,2\n3,4,5,6\n7,8,9"
@@ -283,12 +283,14 @@ def test_none_delimiter(python_parser_only, capsys):
     # We expect the third line in the data to be
     # skipped because it is malformed, but we do
     # not expect any errors to occur.
-    result = parser.read_csv(StringIO(data), header=0, sep=None, on_bad_lines="warn")
+    with tm.assert_produces_warning(
+        ParserWarning, match="Skipping line 3", check_stacklevel=False
+    ):
+        result = parser.read_csv(
+            StringIO(data), header=0, sep=None, on_bad_lines="warn"
+        )
     tm.assert_frame_equal(result, expected)
 
-    captured = capsys.readouterr()
-    assert "Skipping line 3" in captured.err
-
 
 @pytest.mark.parametrize("data", ['a\n1\n"b"a', 'a,b,c\ncat,foo,bar\ndog,foo,"baz'])
 @pytest.mark.parametrize("skipfooter", [0, 1])

pandas/tests/io/parser/test_textreader.py

@@ -12,6 +12,7 @@
 
 import pandas._libs.parsers as parser
 from pandas._libs.parsers import TextReader
+from pandas.errors import ParserWarning
 
 from pandas import DataFrame
 import pandas._testing as tm
@@ -125,7 +126,7 @@ def test_integer_thousands_alt(self):
         expected = DataFrame([123456, 12500])
         tm.assert_frame_equal(result, expected)
 
-    def test_skip_bad_lines(self, capsys):
+    def test_skip_bad_lines(self):
         # too many lines, see #2430 for why
         data = "a:b:c\nd:e:f\ng:h:i\nj:k:l:m\nl:m:n\no:p:q:r"
 
@@ -145,14 +146,11 @@ def test_skip_bad_lines(self, capsys):
         }
         assert_array_dicts_equal(result, expected)
 
-        reader = TextReader(
-            StringIO(data), delimiter=":", header=None, on_bad_lines=1  # Warn
-        )
-        reader.read()
-        captured = capsys.readouterr()
-
-        assert "Skipping line 4" in captured.err
-        assert "Skipping line 6" in captured.err
+        with tm.assert_produces_warning(ParserWarning, match="Skipping line"):
+            reader = TextReader(
+                StringIO(data), delimiter=":", header=None, on_bad_lines=1  # Warn
+            )
+            reader.read()
 
     def test_header_not_enough_lines(self):
         data = "skip this\nskip this\na,b,c\n1,2,3\n4,5,6"