python · skirpichev · May 14, 2025 · May 16, 2025 · May 16, 2025 · Jun 3, 2025
@@ -1,4 +1,10 @@
 #!/bin/sh
+
+# Workaround missing libmpdec-dev on ubuntu 24.04:
+# https://launchpad.net/~ondrej/+archive/ubuntu/php
+# https://deb.sury.org/
+sudo add-apt-repository ppa:ondrej/php
+
 apt-get update
 
 apt-get -yq install \
@@ -13,6 +19,7 @@ apt-get -yq install \
     libgdbm-dev \
     libgdbm-compat-dev \
     liblzma-dev \
+    libmpdec-dev \
     libncurses5-dev \
     libreadline6-dev \
     libsqlite3-dev \

@@ -851,6 +851,13 @@ Libraries options
 
    .. seealso:: :option:`LIBMPDEC_CFLAGS` and :option:`LIBMPDEC_LIBS`.
 
+.. option:: --with-libmpdec
+
+   Whether to build the ``_decimal`` extension module (default is yes).
+   See also the :option:`--with-system-libmpdec` option.
+
+   .. versionadded:: next
+
 .. option:: --with-readline=readline|editline
 
    Designate a backend library for the :mod:`readline` module.

diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
@@ -286,6 +286,13 @@ that may require changes to your code.
 Build changes
 =============
 
+* Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+  Now this should be explicitly enabled with :option:`--with-system-libmpdec`
+  set to ``no`` or with :option:`!--without-system-libmpdec`.
+  The :option:`!--without-libmpdec` should be used to permit building
+  CPython without the ``_decimal`` module.
+  (Contributed by Sergey B Kirpichev in :gh:`115119`.)
+
 
 C API changes
 =============

diff --git a/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst b/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst
@@ -0,0 +1,4 @@
+Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+Now this should be explicitly enabled via :option:`--with-system-libmpdec`
+set to ``no`` or :option:`!--without-system-libmpdec`.  Patch by Sergey
+B Kirpichev.
@@ -4171,33 +4171,42 @@ if test "$ac_cv_ffi_complex_double_supported" = "yes"; then
               [Defined if _Complex C type can be used with libffi.])
 fi
 
+# Check for use of the libmpdec library
+AC_MSG_CHECKING([for --with-libmpdec])
+AC_ARG_WITH(
+  [libmpdec],
+  [AS_HELP_STRING(
+    [--with-libmpdec],
+    [build _decimal module (default is yes)]
+  )])
+AC_MSG_RESULT([$with_libmpdec])
+
 # Check for use of the system libmpdec library
 AC_MSG_CHECKING([for --with-system-libmpdec])
+AC_DEFUN([USE_BUNDLED_LIBMPDEC],
+         [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
+          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
+          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
+          have_mpdec=yes
+          with_system_libmpdec=no])
 AC_ARG_WITH(
   [system_libmpdec],
   [AS_HELP_STRING(
     [--with-system-libmpdec],
     [build _decimal module using an installed mpdecimal library, see Doc/library/decimal.rst (default is yes)]
   )],
-  [],
+  [AS_IF([test "x$with_system_libmpdec" = xno && test "x$with_libmpdec" = xyes],
+         [USE_BUNDLED_LIBMPDEC()])],
   [with_system_libmpdec="yes"])
 AC_MSG_RESULT([$with_system_libmpdec])
 
-AC_DEFUN([USE_BUNDLED_LIBMPDEC],
-         [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
-          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
-          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
-          have_mpdec=yes
-          with_system_libmpdec=no])
-
 AS_VAR_IF(
   [with_system_libmpdec], [yes],
   [PKG_CHECK_MODULES(
     [LIBMPDEC], [libmpdec >= 2.5.0], [],
     [LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""}
      LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"}
-     LIBMPDEC_INTERNAL=])],
-  [USE_BUNDLED_LIBMPDEC()])
+     LIBMPDEC_INTERNAL=])])
 
 AS_VAR_IF([with_system_libmpdec], [yes],
   [WITH_SAVE_ENV([
@@ -4213,16 +4222,17 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       ], [const char *x = mpd_version();])],
       [have_mpdec=yes],
       [have_mpdec=no])
-  ])],
-  [AC_MSG_WARN([m4_normalize([
-     the bundled copy of libmpdecimal is scheduled for removal in Python 3.15;
+  ])])
+AS_IF([test "$with_system_libmpdec" = "no" && test "$with_libmpdec" = "yes"],
+          [AC_MSG_WARN([m4_normalize([
+     the bundled copy of libmpdec is scheduled for removal in Python 3.16;
      consider using a system installed mpdecimal library.])])])
 
-AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
-      [AC_MSG_WARN([m4_normalize([
-         no system libmpdecimal found; falling back to bundled libmpdecimal
-         (deprecated and scheduled for removal in Python 3.15)])])
-       USE_BUNDLED_LIBMPDEC()])
+AS_IF([test "$with_libmpdec" = "yes" && test "$have_mpdec" = "no"],
+      [AC_MSG_ERROR([m4_normalize([
+         no system libmpdec found; use --without-libmpdec to build Python with only pure-Python version for the decimal module])])])
+AS_VAR_IF([with_libmpdec], [no],
+       [AS_VAR_SET([py_cv_module_]_decimal, [n/a])])
 
 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(