1 files changed, 2 insertions, 0 deletions
diff --git a/tox.ini b/tox.ini
index bbd2c6d..b34117b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,8 @@
 envlist = py27, py35, py36, py37, pypy
 
 [testenv]
+# For performance, but also for using "source" with coveragepy (https://github.com/nedbat/coveragepy/issues/268).
+usedevelop = True
 deps =
     pytest
     # 4.5.2a1 for py37 fix