diff --git a/test/py/tests/test_ut.py b/test/py/tests/test_ut.py
index 05e1583..39aa103 100644
--- a/test/py/tests/test_ut.py
+++ b/test/py/tests/test_ut.py
@@ -1,6 +1,12 @@
 # SPDX-License-Identifier: GPL-2.0
-# Copyright (c) 2016, NVIDIA CORPORATION. All rights reserved.
+"""
+Unit-test runner
 
+Provides a test_ut() function which is used by conftest.py to run each unit
+test one at a time, as well setting up some files needed by the tests.
+
+# Copyright (c) 2016, NVIDIA CORPORATION. All rights reserved.
+"""
 import collections
 import getpass
 import gzip
@@ -44,8 +50,8 @@
     if second_part:
         spec += '\ntype=c'
 
-    u_boot_utils.run_and_log(cons, 'qemu-img create %s 20M' % fname)
-    u_boot_utils.run_and_log(cons, 'sudo sfdisk %s' % fname,
+    u_boot_utils.run_and_log(cons, f'qemu-img create {fname} 20M')
+    u_boot_utils.run_and_log(cons, f'sudo sfdisk {fname}',
                              stdin=spec.encode('utf-8'))
     return fname, mnt
 
@@ -61,13 +67,13 @@
     Returns:
         str: Name of loop device used
     """
-    out = u_boot_utils.run_and_log(cons, 'sudo losetup --show -f -P %s' % fname)
+    out = u_boot_utils.run_and_log(cons, f'sudo losetup --show -f -P {fname}')
     loop = out.strip()
     part = f'{loop}p1'
     u_boot_utils.run_and_log(cons, f'sudo mkfs.{fstype} {part}')
     opts = ''
     if fstype == 'vfat':
-         opts += f' -o uid={os.getuid()},gid={os.getgid()}'
+        opts += f' -o uid={os.getuid()},gid={os.getgid()}'
     u_boot_utils.run_and_log(cons, f'sudo mount -o loop {part} {mnt}{opts}')
     u_boot_utils.run_and_log(cons, f'sudo chown {getpass.getuser()} {mnt}')
     return loop
@@ -82,9 +88,7 @@
     """
     infname = os.path.join(cons.config.source_dir,
                            f'test/py/tests/bootstd/mmc{mmc_dev}.img.xz')
-    u_boot_utils.run_and_log(
-        cons,
-        ['sh', '-c', 'xz -dc %s >%s' % (infname, fname)])
+    u_boot_utils.run_and_log(cons, ['sh', '-c', f'xz -dc {infname} >{fname}'])
 
 def setup_bootmenu_image(cons):
     """Create a 20MB disk image with a single ext4 partition
@@ -101,9 +105,6 @@
         loop = mount_image(cons, fname, mnt, 'ext4')
         mounted = True
 
-        vmlinux = 'Image'
-        initrd = 'uInitrd'
-        dtbdir = 'dtb'
         script = '''# DO NOT EDIT THIS FILE
 #
 # Please edit /boot/armbianEnv.txt to set supported parameters
@@ -177,12 +178,12 @@
 
 # Recompile with:
 # mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr
-''' % (mmc_dev)
+'''
         bootdir = os.path.join(mnt, 'boot')
         mkdir_cond(bootdir)
         cmd_fname = os.path.join(bootdir, 'boot.cmd')
         scr_fname = os.path.join(bootdir, 'boot.scr')
-        with open(cmd_fname, 'w') as outf:
+        with open(cmd_fname, 'w', encoding='ascii') as outf:
             print(script, file=outf)
 
         infname = os.path.join(cons.config.source_dir,
@@ -212,13 +213,12 @@
         complete = True
 
     except ValueError as exc:
-        print('Falled to create image, failing back to prepared copy: %s',
-              str(exc))
+        print(f'Falled to create image, failing back to prepared copy: {exc}')
     finally:
         if mounted:
-            u_boot_utils.run_and_log(cons, 'sudo umount --lazy %s' % mnt)
+            u_boot_utils.run_and_log(cons, f'sudo umount --lazy {mnt}')
         if loop:
-            u_boot_utils.run_and_log(cons, 'sudo losetup -d %s' % loop)
+            u_boot_utils.run_and_log(cons, f'sudo losetup -d {loop}')
 
     if not complete:
         copy_prepared_image(cons, mmc_dev, fname)
@@ -254,32 +254,32 @@
         ext = os.path.join(mnt, 'extlinux')
         mkdir_cond(ext)
 
-        with open(os.path.join(ext, 'extlinux.conf'), 'w') as fd:
+        conf = os.path.join(ext, 'extlinux.conf')
+        with open(conf, 'w', encoding='ascii') as fd:
             print(script, file=fd)
 
         inf = os.path.join(cons.config.persistent_data_dir, 'inf')
         with open(inf, 'wb') as fd:
             fd.write(gzip.compress(b'vmlinux'))
-        u_boot_utils.run_and_log(cons, 'mkimage -f auto -d %s %s' %
-                                 (inf, os.path.join(mnt, vmlinux)))
+        u_boot_utils.run_and_log(
+            cons, f'mkimage -f auto -d {inf} {os.path.join(mnt, vmlinux)}')
 
-        with open(os.path.join(mnt, initrd), 'w') as fd:
+        with open(os.path.join(mnt, initrd), 'w', encoding='ascii') as fd:
             print('initrd', file=fd)
 
         mkdir_cond(os.path.join(mnt, dtbdir))
 
-        dtb_file = os.path.join(mnt, '%s/sandbox.dtb' % dtbdir)
+        dtb_file = os.path.join(mnt, f'{dtbdir}/sandbox.dtb')
         u_boot_utils.run_and_log(
-            cons, 'dtc -o %s' % dtb_file, stdin=b'/dts-v1/; / {};')
+            cons, f'dtc -o {dtb_file}', stdin=b'/dts-v1/; / {};')
         complete = True
     except ValueError as exc:
-        print('Falled to create image, failing back to prepared copy: %s',
-              str(exc))
+        print(f'Falled to create image, failing back to prepared copy: {exc}')
     finally:
         if mounted:
-            u_boot_utils.run_and_log(cons, 'sudo umount --lazy %s' % mnt)
+            u_boot_utils.run_and_log(cons, f'sudo umount --lazy {mnt}')
         if loop:
-            u_boot_utils.run_and_log(cons, 'sudo losetup -d %s' % loop)
+            u_boot_utils.run_and_log(cons, f'sudo losetup -d {loop}')
 
     if not complete:
         copy_prepared_image(cons, mmc_dev, fname)
@@ -303,7 +303,8 @@
         Return:
             bytes: Packed-kernel data
         """
-        kern_part = os.path.join(cons.config.result_dir, 'kern-part-{arch}.bin')
+        kern_part = os.path.join(cons.config.result_dir,
+                                 f'kern-part-{arch}.bin')
         u_boot_utils.run_and_log(
             cons,
             f'futility vbutil_kernel --pack {kern_part} '
@@ -332,7 +333,7 @@
 
     mmc_dev = 5
     fname = os.path.join(cons.config.source_dir, f'mmc{mmc_dev}.img')
-    u_boot_utils.run_and_log(cons, 'qemu-img create %s 20M' % fname)
+    u_boot_utils.run_and_log(cons, f'qemu-img create {fname} 20M')
     #mnt = os.path.join(cons.config.persistent_data_dir, 'mnt')
     #mkdir_cond(mnt)
     u_boot_utils.run_and_log(cons, f'cgpt create {fname}')
@@ -381,20 +382,20 @@
 
     u_boot_utils.run_and_log(cons, f'cgpt boot -p {fname}')
     out = u_boot_utils.run_and_log(cons, f'cgpt show -q {fname}')
-    '''We expect something like this:
-        8239        2048       1  Basic data
-          45        2048       2  ChromeOS kernel
-        8238           1       3  ChromeOS rootfs
-        2093        2048       4  ChromeOS kernel
-        8237           1       5  ChromeOS rootfs
-          41           1       6  ChromeOS kernel
-          42           1       7  ChromeOS rootfs
-        4141        2048       8  Basic data
-          43           1       9  ChromeOS reserved
-          44           1      10  ChromeOS reserved
-          40           1      11  ChromeOS firmware
-        6189        2048      12  EFI System Partition
-    '''
+
+    # We expect something like this:
+    #   8239        2048       1  Basic data
+    #     45        2048       2  ChromeOS kernel
+    #   8238           1       3  ChromeOS rootfs
+    #   2093        2048       4  ChromeOS kernel
+    #   8237           1       5  ChromeOS rootfs
+    #     41           1       6  ChromeOS kernel
+    #     42           1       7  ChromeOS rootfs
+    #   4141        2048       8  Basic data
+    #     43           1       9  ChromeOS reserved
+    #     44           1      10  ChromeOS reserved
+    #     40           1      11  ChromeOS firmware
+    #   6189        2048      12  EFI System Partition
 
     # Create a dict (indexed by partition number) containing the above info
     for line in out.splitlines():
@@ -446,7 +447,7 @@
 
     mmc_dev = 7
     fname = os.path.join(cons.config.source_dir, f'mmc{mmc_dev}.img')
-    u_boot_utils.run_and_log(cons, 'qemu-img create %s 20M' % fname)
+    u_boot_utils.run_and_log(cons, f'qemu-img create {fname} 20M')
     u_boot_utils.run_and_log(cons, f'cgpt create {fname}')
 
     ptr = 40
@@ -498,11 +499,12 @@
     with open(fname, 'wb') as outf:
         outf.write(disk_data)
 
-    print('wrote to {}'.format(fname))
+    print(f'wrote to {fname}')
 
     return fname
 
 def setup_cedit_file(cons):
+    """Set up a .dtb file for use with testing expo and configuration editor"""
     infname = os.path.join(cons.config.source_dir,
                            'test/boot/files/expo_layout.dts')
     inhname = os.path.join(cons.config.source_dir,
@@ -584,7 +586,7 @@
         # ut hush hush_test_simple_dollar prints "Unknown command" on purpose.
         with u_boot_console.disable_check('unknown_command'):
             output = u_boot_console.run_command('ut ' + ut_subtest)
-        assert('Unknown command \'quux\' - try \'help\'' in output)
+        assert 'Unknown command \'quux\' - try \'help\'' in output
     else:
         output = u_boot_console.run_command('ut ' + ut_subtest)
     assert output.endswith('Failures: 0')