diff --git a/misc/batch.py b/misc/batch.py
index fcd0e5cf..0b3793b2 100644
--- a/misc/batch.py
+++ b/misc/batch.py
@@ -52,7 +52,7 @@ logging.basicConfig(
 
 ocrmypdf.configure_logging(ocrmypdf.Verbosity.default)
 
-for dir_name, subdirs, file_list in os.walk(start_dir):
+for dir_name, _subdirs, file_list in os.walk(start_dir):
     logging.info(dir_name + '\n')
     os.chdir(dir_name)
     for filename in file_list:
diff --git a/misc/synology.py b/misc/synology.py
index 6e294ce1..7e243229 100644
--- a/misc/synology.py
+++ b/misc/synology.py
@@ -46,7 +46,7 @@ if len(sys.argv) > 1:
 else:
     start_dir = '.'
 
-for dir_name, subdirs, file_list in os.walk(start_dir):
+for dir_name, _subdirs, file_list in os.walk(start_dir):
     logging.info(dir_name)
     os.chdir(dir_name)
     for filename in file_list:
diff --git a/setup.cfg b/setup.cfg
index 42e362d5..a23304f7 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -107,3 +107,9 @@ test = pytest
 [check-manifest]
 ignore =
     .github
+
+[flake8]
+ignore = D203,F401,W503,E501,E203,F841
+exclude = .git,__pycache__,docs/conf.py,build,dist,.venv,.venvpp,.eggs,tmp,src/ocrmypdf/lib/
+max-complexity = 10
+max-line-length = 100
\ No newline at end of file
diff --git a/src/ocrmypdf/_exec/tesseract.py b/src/ocrmypdf/_exec/tesseract.py
index 33ead41e..79374670 100644
--- a/src/ocrmypdf/_exec/tesseract.py
+++ b/src/ocrmypdf/_exec/tesseract.py
@@ -20,7 +20,6 @@ from typing import List, Optional
 
 from PIL import Image
 
-from ocrmypdf.api import StrPath
 from ocrmypdf.exceptions import (
     MissingDependencyError,
     SubprocessOutputError,
diff --git a/src/ocrmypdf/_exec/unpaper.py b/src/ocrmypdf/_exec/unpaper.py
index 3c3ae72c..aec365c2 100644
--- a/src/ocrmypdf/_exec/unpaper.py
+++ b/src/ocrmypdf/_exec/unpaper.py
@@ -96,12 +96,12 @@ def run(
         try:
             with Image.open(output_pnm) as imout:
                 imout.save(output_file, dpi=(dpi, dpi))
-        except (FileNotFoundError, OSError):
+        except OSError as e:
             raise SubprocessOutputError(
                 "unpaper: failed to produce the expected output file. "
                 + " Called with: "
                 + str(args_unpaper)
-            ) from None
+            ) from e
 
 
 def validate_custom_args(args: str) -> List[str]:
diff --git a/src/ocrmypdf/_sync.py b/src/ocrmypdf/_sync.py
index 303b0cf6..b6611595 100644
--- a/src/ocrmypdf/_sync.py
+++ b/src/ocrmypdf/_sync.py
@@ -399,7 +399,7 @@ def run_pipeline(options, *, plugin_manager, api=False):
                 return ExitCode.invalid_output_pdf
             report_output_file_size(options, start_input_file, options.output_file)
 
-    except (KeyboardInterrupt if not api else NeverRaise) as e:
+    except (KeyboardInterrupt if not api else NeverRaise):
         if options.verbose >= 1:
             log.exception("KeyboardInterrupt")
         else:
@@ -413,7 +413,7 @@ def run_pipeline(options, *, plugin_manager, api=False):
         else:
             log.error(type(e).__name__)
         return e.exit_code
-    except (Exception if not api else NeverRaise) as e:  # pylint: disable=broad-except
+    except (Exception if not api else NeverRaise):  # pylint: disable=broad-except
         log.exception("An exception occurred while executing the pipeline")
         return ExitCode.other_error
     finally:
diff --git a/src/ocrmypdf/_validation.py b/src/ocrmypdf/_validation.py
index f5ca30e6..47d97680 100644
--- a/src/ocrmypdf/_validation.py
+++ b/src/ocrmypdf/_validation.py
@@ -26,12 +26,7 @@ from ocrmypdf.exceptions import (
     MissingDependencyError,
     OutputFileAccessError,
 )
-from ocrmypdf.helpers import (
-    is_file_writable,
-    is_iterable_notstr,
-    monotonic,
-    safe_symlink,
-)
+from ocrmypdf.helpers import is_file_writable, monotonic, safe_symlink
 from ocrmypdf.hocrtransform import HOCR_OK_LANGS
 from ocrmypdf.subprocess import check_external_program
 
@@ -68,7 +63,7 @@ def check_options_languages(options, ocr_engine_languages):
     missing_languages = options.languages - ocr_engine_languages
     if missing_languages:
         msg = (
-            f"OCR engine does not have language data for the following "
+            "OCR engine does not have language data for the following "
             "requested languages: \n"
         )
         msg += '\n'.join(lang for lang in missing_languages)
diff --git a/src/ocrmypdf/api.py b/src/ocrmypdf/api.py
index de3561e4..8f731fa3 100644
--- a/src/ocrmypdf/api.py
+++ b/src/ocrmypdf/api.py
@@ -15,10 +15,7 @@ from pathlib import Path
 from typing import AnyStr, BinaryIO, Iterable, Optional, Union
 from warnings import warn
 
-from ocrmypdf._logging import (  # pylint: disable=unused-import
-    PageNumberFilter,
-    TqdmConsole,
-)
+from ocrmypdf._logging import PageNumberFilter, TqdmConsole
 from ocrmypdf._plugin_manager import get_plugin_manager
 from ocrmypdf._sync import run_pipeline
 from ocrmypdf._validation import check_options
@@ -338,3 +335,17 @@ def ocr(  # pylint: disable=unused-argument
         options = create_options(**create_options_kwargs)
         check_options(options, plugin_manager)
         return run_pipeline(options=options, plugin_manager=plugin_manager, api=True)
+
+
+__all__ = [
+    'PageNumberFilter',
+    'TqdmConsole',
+    'Verbosity',
+    'check_options',
+    'configure_logging',
+    'create_options',
+    'get_parser',
+    'get_plugin_manager',
+    'ocr',
+    'run_pipeline',
+]
diff --git a/src/ocrmypdf/builtin_plugins/concurrency.py b/src/ocrmypdf/builtin_plugins/concurrency.py
index 1fe34d8c..74e6b504 100644
--- a/src/ocrmypdf/builtin_plugins/concurrency.py
+++ b/src/ocrmypdf/builtin_plugins/concurrency.py
@@ -21,7 +21,7 @@ import sys
 import threading
 from contextlib import suppress
 from multiprocessing.pool import Pool, ThreadPool
-from typing import Callable, Iterable, Optional, Tuple, Type, Union
+from typing import Callable, Iterable, Type, Union
 
 from tqdm import tqdm
 
diff --git a/src/ocrmypdf/builtin_plugins/tesseract_ocr.py b/src/ocrmypdf/builtin_plugins/tesseract_ocr.py
index a6973b01..f4f45639 100644
--- a/src/ocrmypdf/builtin_plugins/tesseract_ocr.py
+++ b/src/ocrmypdf/builtin_plugins/tesseract_ocr.py
@@ -11,7 +11,6 @@ import os
 from ocrmypdf import hookimpl
 from ocrmypdf._exec import tesseract
 from ocrmypdf.cli import numeric
-from ocrmypdf.exceptions import MissingDependencyError
 from ocrmypdf.helpers import clamp
 from ocrmypdf.pluginspec import OcrEngine
 from ocrmypdf.subprocess import check_external_program
diff --git a/src/ocrmypdf/helpers.py b/src/ocrmypdf/helpers.py
index 3f012ef1..7ff2ea71 100644
--- a/src/ocrmypdf/helpers.py
+++ b/src/ocrmypdf/helpers.py
@@ -226,7 +226,7 @@ def check_pdf(input_file: Path) -> bool:
             except (
                 # Workaround for a problematic pikepdf version
                 # pragma: no cover
-                getattr(pikepdf, 'ForeignObjectError')
+                pikepdf.ForeignObjectError
                 if pikepdf.__version__ == '2.1.0'
                 else NeverRaise
             ):
diff --git a/src/ocrmypdf/optimize.py b/src/ocrmypdf/optimize.py
index bec39f6e..55a8841a 100644
--- a/src/ocrmypdf/optimize.py
+++ b/src/ocrmypdf/optimize.py
@@ -533,12 +533,15 @@ def transcode_pngs(
         _transcode_png(pike, filename, xref)
 
 
+DEFAULT_EXECUTOR = SerialExecutor()
+
+
 def optimize(
     input_file: Path,
     output_file: Path,
     context,
     save_settings,
-    executor: Executor = SerialExecutor(),
+    executor: Executor = DEFAULT_EXECUTOR,
 ) -> None:
     options = context.options
     if options.optimize == 0:
diff --git a/src/ocrmypdf/pdfinfo/info.py b/src/ocrmypdf/pdfinfo/info.py
index c977ce73..b656e107 100644
--- a/src/ocrmypdf/pdfinfo/info.py
+++ b/src/ocrmypdf/pdfinfo/info.py
@@ -9,7 +9,7 @@
 import atexit
 import logging
 import re
-from collections import defaultdict, namedtuple
+from collections import defaultdict
 from contextlib import ExitStack
 from decimal import Decimal
 from enum import Enum
@@ -449,7 +449,7 @@ def _image_xobjects(container) -> Iterator[Tuple[Object, str]]:
     xobjs = resources['/XObject'].as_dict()
     for xobj in xobjs:
         candidate: Object = xobjs[xobj]
-        if not '/Subtype' in candidate:
+        if '/Subtype' not in candidate:
             continue
         if candidate['/Subtype'] == '/Image':
             pdfimage = candidate
@@ -877,6 +877,9 @@ class PageInfo:
         )
 
 
+DEFAULT_EXECUTOR = SerialExecutor()
+
+
 class PdfInfo:
     """Get summary information about a PDF"""
 
@@ -888,7 +891,7 @@ class PdfInfo:
         progbar: bool = False,
         max_workers: int = None,
         check_pages=None,
-        executor: Executor = SerialExecutor(),
+        executor: Executor = DEFAULT_EXECUTOR,
     ):
         self._infile = infile
         if check_pages is None:
diff --git a/src/ocrmypdf/subprocess/__init__.py b/src/ocrmypdf/subprocess/__init__.py
index 99f052c6..28604648 100644
--- a/src/ocrmypdf/subprocess/__init__.py
+++ b/src/ocrmypdf/subprocess/__init__.py
@@ -15,7 +15,6 @@ from collections.abc import Mapping
 from contextlib import suppress
 from distutils.version import LooseVersion, Version
 from functools import lru_cache
-from pathlib import Path
 from subprocess import PIPE, STDOUT, CalledProcessError, CompletedProcess, Popen
 from subprocess import run as subprocess_run
 from typing import Callable, Optional, Type, Union
diff --git a/src/ocrmypdf/subprocess/_windows.py b/src/ocrmypdf/subprocess/_windows.py
index aec082b7..d1cc31e0 100644
--- a/src/ocrmypdf/subprocess/_windows.py
+++ b/src/ocrmypdf/subprocess/_windows.py
@@ -9,9 +9,9 @@ import os
 import shutil
 import sys
 from distutils.version import LooseVersion
-from itertools import chain, filterfalse
+from itertools import chain
 from pathlib import Path
-from typing import Any, Callable, Iterator, Optional, Tuple, TypeVar, cast
+from typing import Any, Callable, Iterable, Iterator, Set, Tuple, TypeVar
 
 try:
     import winreg
@@ -137,14 +137,12 @@ def fix_windows_args(program, args, env):
     return args
 
 
-def unique_everseen(iterable, key=None):
-    "List unique elements, preserving order. Remember all elements ever seen."
+def unique_everseen(iterable: Iterable[T], key: Callable[[T], T]) -> Iterator[T]:
+    "List unique elements, preserving order."
     # unique_everseen('AAAABBBCCDAABBB') --> A B C D
     # unique_everseen('ABBCcAD', str.lower) --> A B C D
-    seen = set()
+    seen: Set[T] = set()
     seen_add = seen.add
-    if key is None:
-        key = lambda x: x
     for element in iterable:
         k = key(element)
         if k not in seen:
diff --git a/tests/test_metadata.py b/tests/test_metadata.py
index ebcbf031..cea73dd7 100644
--- a/tests/test_metadata.py
+++ b/tests/test_metadata.py
@@ -287,7 +287,7 @@ def test_srgb_in_unicode_path(tmp_path):
 
 
 def test_kodak_toc(resources, outpdf):
-    _output = check_ocrmypdf(
+    check_ocrmypdf(
         resources / 'kcs.pdf',
         outpdf,
         '--output-type',