Ensure user_properties is a list

AUTHORS

@@ -10,6 +10,7 @@ Ahn Ki-Wook
 Alan Velasco
 Alexander Johnson
 Alexei Kozlenok
+Allan Feldman
 Anatoly Bubenkoff
 Anders Hovmöller
 Andras Tim
@@ -179,6 +180,7 @@ Raphael Pierzina
 Raquel Alegre
 Ravi Chandra
 Roberto Polli
+Roland Puntaier
 Romain Dorgueil
 Roman Bolshakov
 Ronny Pfannschmidt
@@ -223,6 +225,5 @@ Wim Glenn
 Wouter van Ackooy
 Xuan Luong
 Xuecong Liao
+Zac Hatfield-Dodds
 Zoltán Máté
-Roland Puntaier
-Allan Feldman

changelog/4034.bugfix.rst

@@ -0,0 +1,3 @@
+The ``.user_properties`` attribute of ``TestReport`` objects is a list
+of (name, value) tuples, but could sometimes be instantiated as a tuple
+of tuples.  It is now always a list.

src/_pytest/reports.py

@@ -110,7 +110,7 @@ class TestReport(BaseReport):
         when,
         sections=(),
         duration=0,
-        user_properties=(),
+        user_properties=None,
         **extra
     ):
         #: normalized collection node id
@@ -136,6 +136,11 @@ class TestReport(BaseReport):
 
         #: user properties is a list of tuples (name, value) that holds user
         #: defined properties of the test
+        if user_properties is None:
+            user_properties = []
+        elif not isinstance(user_properties, list):
+            # This used to be a tuple; so cast for backwards-compatibility
+            user_properties = list(user_properties)
         self.user_properties = user_properties
 
         #: list of pairs ``(str, str)`` of extra information which needs to