[PATCH-next 30/40] lnst.RecipeCommon.PerfResult: override std_deviation of PerfInterval and add string descrition

From: Ondrej Lichtner <olichtne(a)redhat.com&gt;

The standard deviation of a single interval measurement is 0 since it's
the smallest measurement precision we have.

This also adds a __str__ method to pretty print the PerfInterval
objects.

Signed-off-by: Ondrej Lichtner <olichtne(a)redhat.com&gt;
---
 lnst/RecipeCommon/PerfResult.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lnst/RecipeCommon/PerfResult.py b/lnst/RecipeCommon/PerfResult.py
index 9afac16..f48fd0a 100644
--- a/lnst/RecipeCommon/PerfResult.py
+++ b/lnst/RecipeCommon/PerfResult.py
@@ -28,6 +28,14 @@ class PerfInterval(PerfStatMixin):
     def unit(self):
         return self._unit
 
+    @property
+    def std_deviation(self):
+        return 0
+
+    def __str__(self):
+        return "{} {} in {} seconds".format(
+                self.value, self.unit, self.duration)
+
 class PerfList(list):
     _sub_type = None
 
-- 
2.17.0