Add a test to check the Twisted version in pinned environments

tests/test_dependencies.py

@@ -1,8 +1,14 @@
+import os
+import re
+from configparser import ConfigParser
 from importlib import import_module
+
+from twisted import version as twisted_version
 from twisted.trial import unittest
 
 
 class ScrapyUtilsTest(unittest.TestCase):
+
     def test_required_openssl_version(self):
         try:
             module = import_module('OpenSSL')
@@ -13,6 +19,32 @@ class ScrapyUtilsTest(unittest.TestCase):
             installed_version = [int(x) for x in module.__version__.split('.')[:2]]
             assert installed_version >= [0, 6], "OpenSSL >= 0.6 required"
 
+    def test_pinned_twisted_version(self):
+        """When running tests within a Tox environment with pinned
+        dependencies, make sure that the version of Twisted is the pinned
+        version.
+
+        See https://github.com/scrapy/scrapy/pull/4814#issuecomment-706230011
+        """
+        if not os.environ.get('SCRAPY_PINNED', None):
+            self.skipTest('Not in a pinned environment')
+
+        tox_config_file_path = os.path.join(
+            os.path.dirname(__file__),
+            '..',
+            'tox.ini',
+        )
+        config_parser = ConfigParser()
+        config_parser.read(tox_config_file_path)
+        pattern = r'Twisted==([\d.]+)'
+        match = re.search(pattern, config_parser['pinned']['deps'])
+        pinned_twisted_version_string = match[1]
+
+        self.assertEqual(
+            twisted_version.short(),
+            pinned_twisted_version_string
+        )
+
 
 if __name__ == "__main__":
     unittest.main()

tox.ini

@@ -92,6 +92,8 @@ install_command =
     # --use-feature=2020-resolver is required, otherwise the latest verion of
     # Twisted gets installed.
     pip install --use-feature=2020-resolver {opts} {packages}
+setenv =
+    SCRAPY_PINNED=true
 
 [testenv:pinned]
 deps =
@@ -100,6 +102,8 @@ deps =
     PyDispatcher==2.0.5
 install_command =
     {[pinned]install_command}
+setenv =
+    {[pinned]setenv}
     
 [testenv:windows-pinned]
 basepython = python3
@@ -111,6 +115,8 @@ deps =
     PyDispatcher==2.0.5
 install_command =
     {[pinned]install_command}
+setenv =
+    {[pinned]setenv}
 
 [testenv:extra-deps]
 deps =
@@ -131,6 +137,8 @@ deps = {[testenv:pinned]deps}
 install_command =
     {[pinned]install_command}
 commands = {[testenv:asyncio]commands}
+setenv =
+    {[pinned]setenv}
 
 [testenv:pypy3]
 basepython = pypy3
@@ -146,6 +154,8 @@ deps =
 install_command =
     {[pinned]install_command}
 commands = {[testenv:pypy3]commands}
+setenv =
+    {[pinned]setenv}
 
 [docs]
 changedir = docs