Make the MockTransport example more robust/correct (#1621)

diff --git a/docs/advanced.md b/docs/advanced.md
index 4438cb2d6f280af3001f50dc29401626f3b5988a..5f900cf68776e5f771c044e6cdff77d436bed872 100644 (file)
--- a/docs/advanced.md
+++ b/docs/advanced.md
@@ -1100,7 +1100,7 @@ def handler(request):
 
 
 # Switch to a mock transport, if the TESTING environment variable is set.
-if os.environ['TESTING'].upper() == "TRUE":
+if os.environ.get('TESTING', '').upper() == "TRUE":
     transport = httpx.MockTransport(handler)
 else:
     transport = httpx.HTTPTransport()