Allow `file:` for `requires` statements in setup.cfg

Refs #1951

4 files changed, 51 insertions, 13 deletions

diff --git a/changelog.d/3253.change.rst b/changelog.d/3253.change.rst
new file mode 100644
index 00000000..0c29e2a6
--- /dev/null
+++ b/changelog.d/3253.change.rst
@@ -0,0 +1 @@
+Enabled using ``file:`` for requirements in setup.cfg -- by :user:`akx`
diff --git a/docs/userguide/declarative_config.rst b/docs/userguide/declarative_config.rst
index aa8bc7ea..a57bb607 100644
--- a/docs/userguide/declarative_config.rst
+++ b/docs/userguide/declarative_config.rst
@@ -211,13 +211,13 @@ obsoletes                                          list-comma
 Options
 -------
 
-=======================  ===================================  =============== =========
+=======================  ===================================  =============== ====================
 Key                      Type                                 Minimum Version Notes
-=======================  ===================================  =============== =========
+=======================  ===================================  =============== ====================
 zip_safe                 bool
 setup_requires           list-semi                            36.7.0
-install_requires         list-semi
-extras_require           section                                              [#opt-2]_
+install_requires         file:, list-semi                                     [#opt-6]_
+extras_require           file:, section                                       [#opt-2]_, [#opt-6]_
 python_requires          str                                  34.4.0
 entry_points             file:, section                       51.0.0
 scripts                  list-comma
@@ -232,7 +232,7 @@ exclude_package_data     section
 namespace_packages       list-comma                                           [#opt-5]_
 py_modules               list-comma                            34.4.0
 data_files               section                              40.6.0          [#opt-4]_
-=======================  ===================================  =============== =========
+=======================  ===================================  =============== ====================
 
 **Notes**:
 
@@ -266,6 +266,11 @@ data_files               section                              40.6.0          [#
    namespaces (:pep:`420`). Check :doc:`the Python Packaging User Guide
    <PyPUG:guides/packaging-namespace-packages>` for more information.
 
+.. [#opt-6] ``file:`` directives for reading requirements are supported since version 63.0.
+   The format for the file is basically the same as for a ``requirements.txt`` file.
+   Library developers should avoid tightly pinning their dependencies to a specific
+   version (e.g. via a "locked" requirements file).
+
 
 Compatibility with other tools
 ==============================
diff --git a/setuptools/config/setupcfg.py b/setuptools/config/setupcfg.py
index b2d5c346..caf7694a 100644
--- a/setuptools/config/setupcfg.py
+++ b/setuptools/config/setupcfg.py
@@ -568,15 +568,27 @@ class ConfigOptionsHandler(ConfigHandler["Distribution"]):
         self.root_dir = target_obj.src_root
         self.package_dir: Dict[str, str] = {}  # To be filled by `find_packages`
 
+    @classmethod
+    def _parse_list_semicolon(cls, value):
+        return cls._parse_list(value, separator=';')
+
+    def _parse_file_in_root(self, value):
+        return self._parse_file(value, root_dir=self.root_dir)
+
+    def _parse_requirements_list(self, value):
+        # Parse a requirements list, either by reading in a `file:`, or a list.
+        parsed = self._parse_list_semicolon(self._parse_file_in_root(value))
+        # Filter it to only include lines that are not comments. `parse_list`
+        # will have stripped each line and filtered out empties.
+        return [line for line in parsed if not line.startswith("#")]
+
     @property
     def parsers(self):
         """Metadata item name to parser function mapping."""
         parse_list = self._parse_list
-        parse_list_semicolon = partial(self._parse_list, separator=';')
         parse_bool = self._parse_bool
         parse_dict = self._parse_dict
         parse_cmdclass = self._parse_cmdclass
-        parse_file = partial(self._parse_file, root_dir=self.root_dir)
 
         return {
             'zip_safe': parse_bool,
@@ -591,11 +603,11 @@ class ConfigOptionsHandler(ConfigHandler["Distribution"]):
                 "consider using implicit namespaces instead (PEP 420).",
                 SetuptoolsDeprecationWarning,
             ),
-            'install_requires': parse_list_semicolon,
-            'setup_requires': parse_list_semicolon,
-            'tests_require': parse_list_semicolon,
+            'install_requires': self._parse_requirements_list,
+            'setup_requires': self._parse_list_semicolon,
+            'tests_require': self._parse_list_semicolon,
             'packages': self._parse_packages,
-            'entry_points': parse_file,
+            'entry_points': self._parse_file_in_root,
             'py_modules': parse_list,
             'python_requires': SpecifierSet,
             'cmdclass': parse_cmdclass,
@@ -682,8 +694,10 @@ class ConfigOptionsHandler(ConfigHandler["Distribution"]):
 
         :param dict section_options:
         """
-        parse_list = partial(self._parse_list, separator=';')
-        parsed = self._parse_section_to_dict(section_options, parse_list)
+        parsed = self._parse_section_to_dict(
+            section_options,
+            self._parse_requirements_list,
+        )
         self['extras_require'] = parsed
 
     def parse_section_data_files(self, section_options):
diff --git a/setuptools/tests/config/test_setupcfg.py b/setuptools/tests/config/test_setupcfg.py
index 904b1ef8..b2563a10 100644
--- a/setuptools/tests/config/test_setupcfg.py
+++ b/setuptools/tests/config/test_setupcfg.py
@@ -884,6 +884,24 @@ class TestOptions:
             assert cmdclass.__module__ == "custom_build"
             assert module_path.samefile(inspect.getfile(cmdclass))
 
+    def test_requirements_file(self, tmpdir):
+        fake_env(
+            tmpdir,
+            DALS("""
+            [options]
+            install_requires = file:requirements.txt
+            [options.extras_require]
+            colors = file:requirements-extra.txt
+            """)
+        )
+
+        tmpdir.join('requirements.txt').write('\ndocutils>=0.3\n\n')
+        tmpdir.join('requirements-extra.txt').write('colorama')
+
+        with get_dist(tmpdir) as dist:
+            assert dist.install_requires == ['docutils>=0.3']
+            assert dist.extras_require == {'colors': ['colorama']}
+
 
 saved_dist_init = _Distribution.__init__