fix: Prevent a TypeError when additional_checks is a list

docs/changelog.rst

@@ -8,6 +8,10 @@ Changelog
 
 - Support for external examples via the ``externalValue`` keyword. `#884`_
 
+**Fixed**
+
+- Allow to use any iterable type for ``checks`` and ``additional_checks`` arguments to ``Case.validate_response``.
+
 `3.0.4`_ - 2021-01-19
 ---------------------
 

src/schemathesis/models.py

@@ -5,6 +5,7 @@
 from contextlib import contextmanager
 from copy import deepcopy
 from enum import IntEnum
+from itertools import chain
 from logging import LogRecord
 from typing import (
     TYPE_CHECKING,
@@ -355,9 +356,8 @@ def validate_response(
         from .checks import ALL_CHECKS  # pylint: disable=import-outside-toplevel
 
         checks = checks or ALL_CHECKS
-        checks += additional_checks
         errors = []
-        for check in checks:
+        for check in chain(checks, additional_checks):
             try:
                 check(response, self)
             except CheckFailed as exc:

test/conftest.py

@@ -520,6 +520,9 @@ def maker(
         from test.utils import *
         from hypothesis import given, settings, HealthCheck, Phase, assume, strategies as st
         raw_schema = {schema}
+
+        note = print  # An alias to distinguish with leftover prints
+
         schema = schemathesis.from_dict(raw_schema, method={method}, endpoint={endpoint}, tag={tag}, validate_schema={validate_schema})
         """.format(
                 schema=schema,

test/test_pytest.py

@@ -231,3 +231,24 @@ def test(case):
     # Then this test should be skipped with a proper error message
     result.assert_outcomes(skipped=1)
     assert RECURSIVE_REFERENCE_ERROR_MESSAGE in result.stdout.str()
+
+
+def test_checks_as_a_list(testdir, openapi3_base_url):
+    # When the user passes a list of checks instead of a tuple
+    testdir.make_test(
+        f"""
+schema.base_url = "{openapi3_base_url}"
+
+def my_check(response, case):
+    note("CHECKING!")
+
+@schema.parametrize()
+def test(case):
+    response = case.call()
+    case.validate_response(response, checks=(my_check,), additional_checks=[my_check])
+""",
+    )
+    result = testdir.runpytest("-s")
+    # Then it should work
+    result.assert_outcomes(passed=1)
+    assert "CHECKING!" in result.stdout.str()