gh-58857: Add error messages to bare assertions in wsgiref.validate

Lib/test/test_wsgiref.py

@@ -189,9 +189,52 @@ def bad_app(e,s):
             b"A server error occurred.  Please contact the administrator."
         )
         self.assertEqual(
-            err.splitlines()[-2], "AssertionError"
+            err.splitlines()[-2],
+            "AssertionError: wsgi.input.read() takes exactly one argument"
         )
 
+    def test_wsgi_input_readline_type(self):
+        def bad_app(e, s):
+            # Monkey-patch the underlying input to return wrong type
+            class BadInput:
+                def read(self, size): return b""
+                def readline(self, *args): return "not bytes"
+                def readlines(self, *args): return []
+                def __iter__(self): return self
+            e["wsgi.input"].input = BadInput()
+            e["wsgi.input"].readline()
+            s("200 OK", [("Content-Type", "text/plain; charset=utf-8")])
+            return [b"data"]
+        out, err = run_amock(validator(bad_app))
+        self.assertIn("wsgi.input.readline() must return bytes",
+            err.splitlines()[-2])
+
+    def test_wsgi_errors_write_type(self):
+        def bad_app(e, s):
+            e["wsgi.errors"].write(b"not a string")
+            s("200 OK", [("Content-Type", "text/plain; charset=utf-8")])
+            return [b"data"]
+        out, err = run_amock(validator(bad_app))
+        self.assertIn("wsgi.errors.write() requires a str argument",
+            err.splitlines()[-2])
+
+    def test_wsgi_write_wrapper_type(self):
+        def bad_app(e, s):
+            write = s("200 OK", [("Content-Type", "text/plain; charset=utf-8")])
+            write("not bytes")
+            return [b"data"]
+        out, err = run_amock(validator(bad_app))
+        self.assertIn("write() argument must be a bytes instance",
+            err.splitlines()[-2])
+
+    def test_headers_tuple_length(self):
+        def bad_app(e, s):
+            s("200 OK", [("Content-Type",)])
+            return [b"data"]
+        out, err = run_amock(validator(bad_app))
+        self.assertIn("Individual headers must be 2-item tuples",
+            err.splitlines()[-2])
+
     @force_not_colorized
     def test_bytes_validation(self):
         def app(e, s):

Lib/wsgiref/validate.py

@@ -194,23 +194,32 @@ def __init__(self, wsgi_input):
         self.input = wsgi_input
 
     def read(self, *args):
-        assert_(len(args) == 1)
+        assert_(len(args) == 1,
+            "wsgi.input.read() takes exactly one argument")
         v = self.input.read(*args)
-        assert_(type(v) is bytes)
+        assert_(type(v) is bytes,
+            "wsgi.input.read() must return bytes, got %s" % type(v))
         return v
 
     def readline(self, *args):
-        assert_(len(args) <= 1)
+        assert_(len(args) <= 1,
+            "wsgi.input.readline() takes at most one argument")
         v = self.input.readline(*args)
-        assert_(type(v) is bytes)
+        assert_(type(v) is bytes,
+            "wsgi.input.readline() must return bytes, got %s" % type(v))
         return v
 
     def readlines(self, *args):
-        assert_(len(args) <= 1)
+        assert_(len(args) <= 1,
+            "wsgi.input.readlines() takes at most one argument")
         lines = self.input.readlines(*args)
-        assert_(type(lines) is list)
+        assert_(type(lines) is list,
+            "wsgi.input.readlines() must return a list, got %s"
+            % type(lines))
         for line in lines:
-            assert_(type(line) is bytes)
+            assert_(type(line) is bytes,
+                "wsgi.input.readlines() must yield bytes, got %s"
+                % type(line))
         return lines
 
     def __iter__(self):
@@ -226,7 +235,9 @@ def __init__(self, wsgi_errors):
         self.errors = wsgi_errors
 
     def write(self, s):
-        assert_(type(s) is str)
+        assert_(type(s) is str,
+            "wsgi.errors.write() requires a str argument, got %s"
+            % type(s))
         self.errors.write(s)
 
     def flush(self):
@@ -245,7 +256,9 @@ def __init__(self, wsgi_writer):
         self.writer = wsgi_writer
 
     def __call__(self, s):
-        assert_(type(s) is bytes)
+        assert_(type(s) is bytes,
+            "write() argument must be a bytes instance, got %s"
+            % type(s))
         self.writer(s)
 
 class PartialIteratorWrapper:
@@ -391,7 +404,8 @@ def check_headers(headers):
         assert_(type(item) is tuple,
             "Individual headers (%r) must be of type tuple: %r"
             % (item, type(item)))
-        assert_(len(item) == 2)
+        assert_(len(item) == 2,
+            "Individual headers must be 2-item tuples, got %r" % (item,))
         name, value = item
         name = check_string_type(name, "Header name")
         value = check_string_type(value, "Header value")

Misc/NEWS.d/next/Library/2026-04-09-12-56-34.gh-issue-58857.Kw9mPx.rst

@@ -0,0 +1,3 @@
+Add descriptive error messages to all bare assertions in
+:mod:`wsgiref.validate` middleware, making it easier to diagnose WSGI
+compliance issues.