diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index fe66b1d41bf..70079106689 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -472,7 +472,9 @@ class FakeArgs:
 
 class RustBuild(object):
     """Provide all the methods required to build Rust"""
-    def __init__(self, config_toml="", args=FakeArgs()):
+    def __init__(self, config_toml="", args=None):
+        if args is None:
+            args = FakeArgs()
         self.git_version = None
         self.nix_deps_dir = None
         self._should_fix_bins_and_dylibs = None
diff --git a/src/bootstrap/bootstrap_test.py b/src/bootstrap/bootstrap_test.py
index 3c91e403df3..dc06a4c9734 100644
--- a/src/bootstrap/bootstrap_test.py
+++ b/src/bootstrap/bootstrap_test.py
@@ -4,7 +4,6 @@ Run these with `x test bootstrap`, or `python -m unittest src/bootstrap/bootstra
 
 from __future__ import absolute_import, division, print_function
 import os
-import doctest
 import unittest
 import tempfile
 import hashlib
@@ -16,12 +15,15 @@ from shutil import rmtree
 bootstrap_dir = os.path.dirname(os.path.abspath(__file__))
 # For the import below, have Python search in src/bootstrap first.
 sys.path.insert(0, bootstrap_dir)
-import bootstrap
-import configure
+import bootstrap # noqa: E402
+import configure # noqa: E402
 
-def serialize_and_parse(configure_args, bootstrap_args=bootstrap.FakeArgs()):
+def serialize_and_parse(configure_args, bootstrap_args=None):
     from io import StringIO
 
+    if bootstrap_args is None:
+        bootstrap_args = bootstrap.FakeArgs()
+
     section_order, sections, targets = configure.parse_args(configure_args)
     buffer = StringIO()
     configure.write_config_toml(buffer, section_order, targets, sections)
@@ -129,7 +131,14 @@ class GenerateAndParseConfig(unittest.TestCase):
 class BuildBootstrap(unittest.TestCase):
     """Test that we generate the appropriate arguments when building bootstrap"""
 
-    def build_args(self, configure_args=[], args=[], env={}):
+    def build_args(self, configure_args=None, args=None, env=None):
+        if configure_args is None:
+            configure_args = []
+        if args is None:
+            args = []
+        if env is None:
+            env = {}
+
         env = env.copy()
         env["PATH"] = os.environ["PATH"]
 
diff --git a/src/ci/docker/scripts/fuchsia-test-runner.py b/src/ci/docker/scripts/fuchsia-test-runner.py
index af01f9ccbbc..f78d446c8b7 100755
--- a/src/ci/docker/scripts/fuchsia-test-runner.py
+++ b/src/ci/docker/scripts/fuchsia-test-runner.py
@@ -15,12 +15,10 @@ import hashlib
 import json
 import os
 import platform
-import re
 import shutil
-import signal
 import subprocess
 import sys
-from typing import ClassVar, List, Optional
+from typing import ClassVar, List
 
 
 @dataclass
@@ -523,7 +521,7 @@ class TestEnvironment:
                     env_vars += '\n            "RUST_BACKTRACE=0",'
 
                 # Use /tmp as the test temporary directory
-                env_vars += f'\n            "RUST_TEST_TMPDIR=/tmp",'
+                env_vars += '\n            "RUST_TEST_TMPDIR=/tmp",'
 
                 cml.write(
                     self.CML_TEMPLATE.format(env_vars=env_vars, exe_name=exe_name)
diff --git a/x.py b/x.py
index ba959a3047e..6c4c106817e 100755
--- a/x.py
+++ b/x.py
@@ -40,7 +40,7 @@ if __name__ == '__main__':
 
             This message can be suppressed by setting `RUST_IGNORE_OLD_PYTHON=1`
         """.format(major, minor))
-        warnings.warn(msg)
+        warnings.warn(msg, stacklevel=1)
 
     rust_dir = os.path.dirname(os.path.abspath(__file__))
     # For the import below, have Python search in src/bootstrap first.