fix issue 3396

CHANGES.rst

@@ -21,7 +21,8 @@ Unreleased
     :issue:`4096`
 -   The CLI loader handles ``**kwargs`` in a ``create_app`` function.
     :issue:`4170`
-
+-   Fix ``Unable to extend FlaskClient with follow_redirects`` issue.
+    :issue:`3396`
 
 Version 2.0.1
 -------------

src/flask/testing.py

@@ -108,6 +108,7 @@ class FlaskClient(Client):
 
     application: "Flask"
     preserve_context = False
+    ex_builder = None
 
     def __init__(self, *args: t.Any, **kwargs: t.Any) -> None:
         super().__init__(*args, **kwargs)
@@ -189,9 +190,9 @@ class FlaskClient(Client):
                 "flask._preserve_context": self.preserve_context,
             }
 
-        if not kwargs and len(args) == 1:
-            arg = args[0]
+        arg = args[0]
 
+        if not kwargs and len(args) == 1:
             if isinstance(arg, werkzeug.test.EnvironBuilder):
                 builder = copy(arg)
                 builder.environ_base = copy_environ(builder.environ_base or {})
@@ -204,9 +205,18 @@ class FlaskClient(Client):
                 request = copy(arg)
                 request.environ = copy_environ(request.environ)
 
+        if isinstance(arg, werkzeug.test.EnvironBuilder):
+            builder = copy(arg)
+            builder.environ_base = copy_environ(builder.environ_base or {})
+            request = builder.get_request()
+
         if request is None:
-            kwargs["environ_base"] = copy_environ(kwargs.get("environ_base", {}))
-            builder = EnvironBuilder(self.application, *args, **kwargs)
+            if self.ex_builder is None:
+                kwargs["environ_base"] = copy_environ(kwargs.get("environ_base", {}))
+                builder = EnvironBuilder(self.application, *args, **kwargs)
+                self.ex_builder = builder
+            else:
+                builder = self.ex_builder
 
             try:
                 request = builder.get_request()