python module: implicitly add python dep to extensions

If there isn't a preexisting dependency on python, append one. It's
almost assuredly needed, so just do the right thing out of the box.

docs/markdown/Python-module.md

@@ -99,9 +99,9 @@ Additionally, the following diverge from [[shared_module]]'s default behavior:
   of Python that support this (the python headers define `PyMODINIT_FUNC` has
   default visibility).
 
-`extension_module` does not add any dependencies to the library so
-user may need to add `dependencies : py_installation.dependency()`,
-see [[dependency]].
+*since 0.63.0* `extension_module` automatically adds a dependency to the library
+if one is not explicitly provided. To support older versions, the user may need to
+add `dependencies : py_installation.dependency()`, see [[dependency]].
 
 **Returns**: a [[@build_tgt]] object
 

docs/markdown/snippets/python-extension-module-implicit-dependency.md

@@ -0,0 +1,13 @@
+## Python extension modules now depend on the python library by default
+
+Python extension modules are usually expected to link to the python library
+and/or its headers in order to build correctly (via the default `embed: false`,
+which may not actually link to the library itself). This means that every
+single use of `.extension_module()` needed to include the `dependencies:
+py_installation.dependency()` kwarg explicitly.
+
+In the interest of doing the right thing out of the box, this is now the
+default for extension modules that don't already include a dependency on
+python. This is not expected to break anything, because it should always be
+needed. Nevertheless, `py_installation.dependency().partial_dependency()` will
+be detected as already included while providing no compile/link args.

mesonbuild/modules/python.py

@@ -516,16 +516,26 @@ class PythonInstallation(ExternalProgramHolder):
 
             kwargs['install_dir'] = os.path.join(self.platlib_install_path, subdir)
 
-        # On macOS and some Linux distros (Debian) distutils doesn't link
-        # extensions against libpython. We call into distutils and mirror its
-        # behavior. See https://github.com/mesonbuild/meson/issues/4117
-        if not self.link_libpython:
-            new_deps = []
-            for dep in mesonlib.extract_as_list(kwargs, 'dependencies'):
-                if isinstance(dep, _PythonDependencyBase):
+        new_deps = []
+        has_pydep = False
+        for dep in mesonlib.extract_as_list(kwargs, 'dependencies'):
+            if isinstance(dep, _PythonDependencyBase):
+                has_pydep = True
+                # On macOS and some Linux distros (Debian) distutils doesn't link
+                # extensions against libpython. We call into distutils and mirror its
+                # behavior. See https://github.com/mesonbuild/meson/issues/4117
+                if not self.link_libpython:
                     dep = dep.get_partial_dependency(compile_args=True)
-                new_deps.append(dep)
-            kwargs['dependencies'] = new_deps
+            new_deps.append(dep)
+        if not has_pydep:
+            pydep = self._dependency_method_impl({})
+            if not pydep.found():
+                raise mesonlib.MesonException('Python dependency not found')
+            new_deps.append(pydep)
+            FeatureNew.single_use('python_installation.extension_module with implicit dependency on python',
+                                  '0.63.0', self.subproject, 'use python_installation.dependency()',
+                                  self.current_node)
+        kwargs['dependencies'] = new_deps
 
         # msys2's python3 has "-cpython-36m.dll", we have to be clever
         # FIXME: explain what the specific cleverness is here

test cases/python/2 extmodule/ext/meson.build

@@ -1,6 +1,5 @@
 pylib = py.extension_module('tachyon',
   'tachyon_module.c',
-  dependencies : py_dep,
   c_args: '-DMESON_MODULENAME="tachyon"',
   install: true,
 )

test cases/python/2 extmodule/ext/nested/meson.build

@@ -1,6 +1,5 @@
 py.extension_module('tachyon',
   '../tachyon_module.c',
-  dependencies : py_dep,
   c_args: '-DMESON_MODULENAME="nested.tachyon"',
   install: true,
   subdir: 'nested'

test cases/python/2 extmodule/ext/wrongdir/meson.build

@@ -1,6 +1,5 @@
 py.extension_module('tachyon',
   '../tachyon_module.c',
-  dependencies : py_dep,
   c_args: '-DMESON_MODULENAME="tachyon"',
   install: true,
   install_dir: get_option('libdir')