Fix dependencies being installed even if not necessary for current system

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Fixed `add` command picking up prereleases by default.
 - Fixed dependendency resolution on Windows when unpacking distributions.
 - Fixed dependency resolution with post releases.
+- Fixed dependencies being installed even if not necessary for current system.
 
 
 ## [0.8.1] - 2018-04-13

poetry/installation/installer.py

@@ -455,6 +455,10 @@ class Installer:
                     op.skip('Not needed for the current python version')
                     continue
 
+            if not package.python_constraint.matches(Constraint('=', python)):
+                op.skip('Not needed for the current python version')
+                continue
+
             if 'platform' in package.requirements:
                 platform_constraint = GenericConstraint.parse(
                     package.requirements['platform']
@@ -466,6 +470,12 @@ class Installer:
                     op.skip('Not needed for the current platform')
                     continue
 
+            if not package.platform_constraint.matches(
+                    GenericConstraint('=', sys.platform)
+            ):
+                op.skip('Not needed for the current platform')
+                continue
+
             if self._update:
                 extras = {}
                 for extra, deps in self._package.extras.items():