diff --git a/tests/constants.py b/tests/constants.py
new file mode 100644
index 0000000..3d2b3b7
--- /dev/null
+++ b/tests/constants.py
@@ -0,0 +1,4 @@
+# Just a arbitrary large number.
+# It is used when calling `MplexStream.read(MAX_READ_LEN)`,
+#   to avoid `MplexStream.read()`, which blocking reads until EOF.
+MAX_READ_LEN = 2 ** 32 - 1