adding the once rule to uranium.

setup.py

@@ -14,7 +14,7 @@
 ]
 
 setup(name='uranium',
-      version='0.2.29',
+      version='0.2.31b',
       description='a build system for python',
       long_description=open('README.rst').read(),
       author='Yusuke Tsutsumi',

tests/rules/test_once.py

@@ -0,0 +1,18 @@
+from uranium.rules import rule, Once
+
+
+def test_only_runs_once(tmpdir, build):
+
+    g = []
+
+    tmpdir.join("foo.txt").write("foo")
+
+    @build.task
+    @rule(Once())
+    def main(build):
+        g.append("ran")
+
+    build.run_task("main")
+    assert "ran" in g
+    build.run_task("main")
+    assert len(g) == 1

uranium/rules/__init__.py

@@ -2,8 +2,9 @@
 from ..experimental import experimental
 from .base import RuleBase
 from .was_changed import WasChanged
+from .once import Once
 
-__all__ = ["RuleBase", "WasChanged", "rule"]
+__all__ = ["Once", "RuleBase", "WasChanged", "rule"]
 
 
 @experimental

uranium/rules/once.py

@@ -0,0 +1,31 @@
+from .base import RuleBase
+KEY = "_uranium.rules.once"
+
+
+class Once(RuleBase):
+    """
+    Once is a rule that activates if the task has never run.
+    It will not run again, unless the uranium history is cleaned.
+
+    .. code:: python
+
+        import subprocess
+        from uranium import rule
+        from uranium.rules import WasChanged
+
+        # only run tests if the code changed.
+        @rule(Once())
+        def test(build):
+            build.packages.install("pytest")
+            return subprocess.call(["py.test", build.root])
+    """
+
+    def before(self, build):
+        return build.history.get(self.key)
+
+    def after(self, build):
+        build.history[self.key] = True
+
+    @property
+    def key(self):
+        return "{0}.{1}".format(KEY, self.func.__name__)