Fix issue pypa#1433: parse requirements in markers

* InstallRequirement supports PEP 426 markers
* RequirementSet.add_requirement() ignores an InstallRequirement if
  markers don't match.

pip/req/req_install.py

@@ -45,7 +45,7 @@ class InstallRequirement(object):
 
     def __init__(self, req, comes_from, source_dir=None, editable=False,
                  url=None, as_egg=False, update=True, prereleases=None,
-                 editable_options=None, pycompile=True):
+                 editable_options=None, pycompile=True, markers=None):
         self.extras = ()
         if isinstance(req, six.string_types):
             req = pkg_resources.Requirement.parse(req)
@@ -61,6 +61,7 @@ def __init__(self, req, comes_from, source_dir=None, editable=False,
         self.editable_options = editable_options
         self.url = url
         self.as_egg = as_egg
+        self.markers = markers
         self._egg_info_path = None
         # This holds the pkg_resources.Distribution object if this requirement
         # is already available:
@@ -115,6 +116,13 @@ def from_line(cls, name, comes_from=None, prereleases=None):
         requirement, directory containing 'setup.py', filename, or URL.
         """
         url = None
+        if ';' in name:
+            name, markers = name.split(';', 1)
+            markers = markers.strip()
+            if not markers:
+                markers = None
+        else:
+            markers = None
         name = name.strip()
         req = None
         path = os.path.normpath(os.path.abspath(name))
@@ -165,7 +173,8 @@ def from_line(cls, name, comes_from=None, prereleases=None):
         else:
             req = name
 
-        return cls(req, comes_from, url=url, prereleases=prereleases)
+        return cls(req, comes_from, url=url, prereleases=prereleases,
+                   markers=markers)
 
     def __str__(self):
         if self.req:
@@ -726,6 +735,12 @@ def _clean_zip_name(self, name, prefix):
         name = name.replace(os.path.sep, '/')
         return name
 
+    def match_markers(self):
+        if self.markers is not None:
+            return markers.interpret(self.markers)
+        else:
+            return True
+
     def install(self, install_options, global_options=(), root=None):
         if self.editable:
             self.install_editable(install_options, global_options)

pip/req/req_set.py

@@ -94,6 +94,11 @@ def __str__(self):
         return ' '.join([str(req.req) for req in reqs])
 
     def add_requirement(self, install_req):
+        if not install_req.match_markers():
+            logger.notify("Ignore %s: markers %r don't match",
+                          install_req.name, install_req.markers)
+            return
+
         name = install_req.name
         install_req.as_egg = self.as_egg
         install_req.use_user_site = self.use_user_site