BUG accept and deprecate negative integer for max_colwidth

doc/source/whatsnew/v1.0.1.rst

@@ -10,6 +10,14 @@ including other versions of pandas.
 
 .. ---------------------------------------------------------------------------
 
+.. _whatsnew_101.deprecations:
+
+Deprecations
+~~~~~~~~~~~~
+
+- Support for negative integer for :attr:`pd.options.display.max_colwidth` is deprecated in favor of using ``None`` (:issue:`31532`)
+
+.. ---------------------------------------------------------------------------
 
 .. _whatsnew_101.bug_fixes:
 
@@ -129,6 +137,7 @@ ExtensionArray
 Other
 ^^^^^
 - Regression fixed in objTOJSON.c fix return-type warning (:issue:`31463`)
+- Fixed a regression where setting :attr:`pd.options.display.max_colwidth` was not accepting negative integer. In addition, this behavior has been deprecated in favor of using ``None`` (:issue:`31532`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/config_init.py

@@ -9,6 +9,8 @@
 module is imported, register them here rather than in the module.
 
 """
+import warnings
+
 import pandas._config.config as cf
 from pandas._config.config import (
     is_bool,
@@ -341,8 +343,26 @@ def is_terminal() -> bool:
         validator=is_instance_factory([type(None), int]),
     )
     cf.register_option("max_categories", 8, pc_max_categories_doc, validator=is_int)
+
+    def _deprecate_negative_int_max_colwidth(key):
+        value = cf.get_option(key)
+        if value is not None and value < 0:
+            warnings.warn(
+                "Passing a negative integer is deprecated in version 1.0 and "
+                "will not be supported in future version. Instead, use None "
+                "to not limit the column width.",
+                FutureWarning,
+                stacklevel=4,
+            )
+
     cf.register_option(
-        "max_colwidth", 50, max_colwidth_doc, validator=is_nonnegative_int
+        # FIXME: change `validator=is_nonnegative_int`
+        # in version 1.2
+        "max_colwidth",
+        50,
+        max_colwidth_doc,
+        validator=is_instance_factory([type(None), int]),
+        cb=_deprecate_negative_int_max_colwidth,
     )
     if is_terminal():
         max_cols = 0  # automatically determine optimal number of columns

pandas/tests/io/formats/test_format.py

@@ -239,6 +239,15 @@ def test_repr_truncation(self):
         with option_context("display.max_colwidth", max_len + 2):
             assert "..." not in repr(df)
 
+    def test_repr_deprecation_negative_int(self):
+        # FIXME: remove in future version after deprecation cycle
+        # Non-regression test for:
+        # https://github.com/pandas-dev/pandas/issues/31532
+        width = get_option("display.max_colwidth")
+        with tm.assert_produces_warning(FutureWarning):
+            set_option("display.max_colwidth", -1)
+        set_option("display.max_colwidth", width)
+
     def test_repr_chop_threshold(self):
         df = DataFrame([[0.1, 0.5], [0.5, -0.1]])
         pd.reset_option("display.chop_threshold")  # default None