Add support for dynamic dependencies

The dynamic dependencies are part of pep621.
For now only the setuptools supports dynamic dependencies.
Because of that the implementation is somehow dependent on setuptools.
When other tool adds support for dynamic dependencies
only the part for gathering of the dependencies has to be changed.

src/flake8_requirements/checker.py

@@ -560,6 +560,27 @@ def get_pyproject_toml_pep621(cls):
         cfg_pep518 = cls.get_pyproject_toml()
         return cfg_pep518.get('project', {})
 
+    @classmethod
+    def get_setuptools_dynamic_requirements(cls):
+        """Retrieve dynamic requirements defined in setuptools configuration."""
+        cfg = cls.get_pyproject_toml()
+        dynamic_keys = cfg.get('project', {}).get('dynamic', [])
+        dynamic_config = cfg.get('tool', {}).get('setuptools', {}).get('dynamic', {})
+        requirements = []
+        files_to_parse = []
+        if 'dependencies' in dynamic_keys:
+            files_to_parse.extend(dynamic_config.get('dependencies', {}).get('file', []))
+        if 'optional-dependencies' in dynamic_keys:
+            for element in dynamic_config.get('optional-dependencies', {}).values():
+                files_to_parse.extend(element.get('file', []))
+        for file_path in files_to_parse:
+            try:
+                with open(file_path, 'r') as file:
+                    requirements.extend(parse_requirements(file))
+            except (IOError) as e:
+                LOG.debug("Couldn't open requirements file: %s", e)
+        return requirements
+
     @classmethod
     def get_pyproject_toml_pep621_requirements(cls):
         """Try to get PEP 621 metadata requirements."""
@@ -569,6 +590,8 @@ def get_pyproject_toml_pep621_requirements(cls):
             pep621.get("dependencies", ())))
         for r in pep621.get("optional-dependencies", {}).values():
             requirements.extend(parse_requirements(r))
+        if len(requirements) == 0:
+            requirements = cls.get_setuptools_dynamic_requirements()
         return requirements
 
     @classmethod

test/test_pep621.py

@@ -1,6 +1,10 @@
 import unittest
 from unittest import mock
 from unittest.mock import mock_open
+from unittest.mock import patch
+
+from pkg_resources import Requirement
+from pkg_resources import parse_requirements
 
 from flake8_requirements.checker import Flake8Checker
 from flake8_requirements.checker import ModuleSet
@@ -80,3 +84,57 @@ def test_3rd_party(self):
             checker = Flake8Checker(None, None)
             mods = checker.get_mods_3rd_party(False)
             self.assertEqual(mods, ModuleSet({"tools": {}, "dev_tools": {}}))
+
+    def test_valid_requirements(self):
+        requirements_content = "package1\npackage2>=2.0"
+        data = {
+            "project": {"dynamic": ["dependencies"]},
+            "tool": {"setuptools": {"dynamic": {"dependencies": {"file": ["requirements.txt"]}}}}
+        }
+        with patch('flake8_requirements.checker.Flake8Checker.get_pyproject_toml', return_value=data):
+            with patch('builtins.open', mock_open(read_data=requirements_content)) as mocked_open:
+                result = Flake8Checker.get_setuptools_dynamic_requirements()
+                expected_results = ['package1', 'package2>=2.0']
+                parsed_results = [str(req) for req in result]
+                self.assertEqual(parsed_results, expected_results)
+
+    def test_optional_dependencies(self):
+        data = {
+            "project": {"dynamic": ["dependencies", "optional-dependencies"]},
+            "tool": {
+                "setuptools": {
+                    "dynamic": {
+                        "dependencies": {"file": ["requirements.txt"]},
+                        "optional-dependencies": {"test": {"file": ["optional-requirements.txt"]}}
+                    }
+                }
+            }
+        }
+        requirements_content = """
+        package1
+        package2>=2.0
+        """
+        optional_requirements_content = "package3[extra] >= 3.0"
+        with mock.patch('flake8_requirements.checker.Flake8Checker.get_pyproject_toml', return_value=data):
+            with mock.patch('builtins.open', mock.mock_open()) as mocked_file:
+                mocked_file.side_effect = [
+                    mock.mock_open(read_data=requirements_content).return_value,
+                    mock.mock_open(read_data=optional_requirements_content).return_value
+                ]
+                result = Flake8Checker.get_setuptools_dynamic_requirements()
+                expected = list(parse_requirements(requirements_content))
+                expected += list(parse_requirements(optional_requirements_content))
+
+                self.assertEqual(len(result), len(expected))
+                for i in range(len(result)):
+                    self.assertEqual(result[i], expected[i])
+                #assert(sorted(result), sorted(expected))
+
+    def test_missing_requirements_file(self):
+        data = {
+            "project": {"dynamic": ["dependencies"]},
+            "tool": {"setuptools": {"dynamic": {"dependencies": {"file": ["nonexistent-requirements.txt"]}}}}
+        }
+        with mock.patch('flake8_requirements.checker.Flake8Checker.get_pyproject_toml', return_value=data):
+            result = Flake8Checker.get_setuptools_dynamic_requirements()
+            self.assertEqual(result, [])