Add support for distro requirement markers

README.md

@@ -947,26 +947,33 @@ using os specific configurations.
 
 ### Defining Distros
 
-The `distos` key in distro and config definitions is used to define the distro version
+The `distos` key in distro and config definitions are used to define the distro version
 requirements. When a config is processed the distro requirements are evaluated recursively
 to include the requirements of the latest DistroVersion matching the specifier.
-This uses the python packaging module to resolve version specifiers so you can use the
-same configuration syntax you would use in a pip requirements file.
+This uses the python [packaging module](https://packaging.pypa.io/en/stable/requirements.html)
+to resolve version specifiers so you can use the same configuration syntax you
+would use in a pip requirements file.
 
-```json
+```json5
     "distros": [
         "maya2020",
         "maya2022",
         "houdini18.5",
         "hsite",
         "animBot<=1.4",
-        "studiolibrary==2.5.7.post1"
+        "studiolibrary==2.5.7.post1",
+        // Use markers to only include 3ds max if hab is currently running on
+        // windows. It can't be run on linux.
+        "3dsmax2019;platform_system=='Windows'"
     ]
-
 ```
 
 The resolved versions matching the requested distros are shown in the `versions` property.
 
+It also supports [markers](https://packaging.pypa.io/en/stable/markers.html). Hab
+should support all of the [officially supported markers](https://peps.python.org/pep-0508/),
+but the most common marker likely to be used with hab is `platform_system`.
+
 ### Platform specific code
 
 Hab works on windows, linux and osx(needs tested). To make it easier to handle

hab/solvers.py

@@ -100,6 +100,19 @@ def _resolve(
 
         for req in reqs:
             name = req.name
+
+            marker = req.marker
+            if marker and not marker.evaluate():
+                # If a marker is specified and its not valid for the current
+                # system, skip this requirement.
+                # https://packaging.pypa.io/en/stable/markers.html
+                msg = f"Requirement ignored due to marker: {req}"
+                if name in self.forced:
+                    logger.critical(f"Forced {msg}")  # pragma: no cover
+                else:
+                    logger.warning(f"{msg}")
+                continue
+
             if name in self.forced:
                 if name in reported:
                     # Once we have processed this requirement, there is no need to

tests/test_resolver.py

@@ -467,6 +467,45 @@ def test_resolve_requirements_recalculate(resolver):
     )
 
 
+@pytest.mark.parametrize(
+    "platform,marker",
+    (
+        ("windows", "Windows"),
+        ("linux", "Linux"),
+        ("osx", "Darwin"),
+    ),
+)
+def test_resolve_requirements_markers(resolver, platform, marker):
+    """Test that platform_system for current host is included or excluded correctly.
+
+    The packaging marker library isn't setup to allow for testing on other
+    platforms, so these tests need to pass on all platforms, if running the test
+    on this platform, the dependency is included, otherwise it should be ignored.
+    """
+    check = [
+        "the_dcc",
+        "the_dcc_plugin_a",
+        "the_dcc_plugin_b",
+        "the_dcc_plugin_d",
+        "the_dcc_plugin_e",
+    ]
+    # This requirement is only included if running on the target platform
+    if utils.Platform.name() == platform:
+        check.append("the_dcc_plugin_c")
+
+    # Build requirements utilizing the platform marker.
+    requirements = {
+        "the_dcc": Requirement("the_dcc"),
+        # the_dcc_plugin_c is only included if the current platform matches that.
+        "the_dcc_plugin_c": Requirement(
+            f"the_dcc_plugin_c;platform_system=='{marker}'"
+        ),
+    }
+
+    ret = resolver.resolve_requirements(requirements)
+    assert set(ret.keys()) == set(check)
+
+
 def test_resolve_requirements_errors(resolver):
     # This requirement is not possible because the_dcc_plugin_b requires the_dcc<1.2
     requirements = {