diff --git a/cbz2ebook.py b/cbz2ebook.py
new file mode 100755
index 0000000..efc1bc2
--- /dev/null
+++ b/cbz2ebook.py
@@ -0,0 +1,97 @@
+#!/usr/bin/env cached-nix-shell
+#!nix-shell -i python3 -p python3 python3Packages.pillow python3Packages.tqdm
+from PIL import Image
+from concurrent.futures import ThreadPoolExecutor
+from tqdm import tqdm
+from zipfile import ZipFile
+import argparse
+import sys
+import zlib
+
+
+def should_scale(in_width, in_height, out_width, out_height):
+    return not (
+        (in_width == out_width and in_height <= out_height)
+        or (in_height == out_height and in_width <= out_width)
+    )
+
+
+def scaled_size(in_width, in_height, out_width, out_height):
+    in_aspect_ratio = float(in_width) / float(in_height)
+    out_aspect_ratio = float(out_width) / float(out_height)
+
+    if in_aspect_ratio > out_aspect_ratio:
+        height = out_width / in_aspect_ratio
+        width = out_width
+    else:
+        width = out_height * in_aspect_ratio
+        height = out_height
+
+    return int(round(width)), round(int(height))
+
+
+def process_image(im):
+    # double pages (wider than high) should be rotated
+    if im.width > im.height:
+        im = im.transpose(Image.ROTATE_90)
+
+    # convert to greyscale
+    im = im.convert("L")
+
+    # resize if necessary
+    if should_scale(im.width, im.height, args.width, args.height):
+        im = im.resize(
+            scaled_size(im.width, im.height, args.width, args.height),
+            resample=Image.LANCZOS,
+        )
+
+    return im
+
+
+def process_archive(srczip, dstzip, multithread=True):
+    executor = ThreadPoolExecutor()
+
+    if multithread:
+        mapper = executor.map
+    else:
+        mapper = map
+
+    srcfiles = sorted(
+        map(
+            lambda zi: zi.filename,
+            filter(lambda zi: not zi.is_dir(), srczip.infolist()),
+        )
+    )
+
+    for idx, im in enumerate(
+        tqdm(
+            mapper(process_image, mapper(Image.open, mapper(srczip.open, srcfiles))),
+            total=len(srcfiles),
+        )
+    ):
+        with dstzip.open(f"{idx+1:04d}.jpg", "w") as outpagefile:
+            im.save(outpagefile, format="JPEG", quality=92)
+
+
+parser = argparse.ArgumentParser()
+parser.add_argument("infile")
+parser.add_argument("outfile")
+parser.add_argument("-W", "--width", default=1440)
+parser.add_argument("-H", "--height", default=1920)
+parser.add_argument("-s", "--singlethreaded", action="store_true")
+args = parser.parse_args()
+
+srczip = ZipFile(args.infile)
+dstzip = ZipFile(args.outfile, "x")
+
+if args.singlethreaded:
+    process_archive(srczip, dstzip, multithread=False)
+else:
+    try:
+        process_archive(srczip, dstzip, multithread=True)
+    except zlib.error as e:
+        print(f"zlib choked ({e}), retrying in single threaded mode", file=sys.stderr)
+        # replace partial archive with new empty one
+        dstzip.close()
+        dstzip = ZipFile(args.outfile, "w")
+        process_archive(srczip, dstzip, multithread=False)
diff --git a/cbz2ebook.sh b/cbz2ebook.sh
deleted file mode 100755
index e36f398..0000000
--- a/cbz2ebook.sh
+++ /dev/null
@@ -1,45 +0,0 @@
-#!/usr/bin/env cached-nix-shell
-#!nix-shell -i zsh -p imagemagick unzip zip zsh
-set -e
-
-size="1440x1920" # Kobo Forma
-#size="768x1024" # Amazon Kindle Paperwhite
-
-vertical_size="$(cut -dx -f2 <<< $size)x$(cut -dx -f1 <<< $size)"
-
-infile="$(realpath $1)"
-outfile="$(realpath $2)"
-tmpdir=$(mktemp -d)
-
-function cleanup {
-  rm -rf "$tmpdir"
-}
-
-trap cleanup EXIT INT SIGTERM
-
-cd "$tmpdir"
-unzip "$infile"
-
-# rename files to have a plain 4 digit filename
-i=1
-for file in **/*.???; do
-    mv -n "$file" "$(dirname $file)/$(printf %04d $i).${file##*.}"
-    i=$((i+1))
-done
-
-length=$(ls -1 **/*.??? | wc -l)
-position=0
-for image in **/*.???; do
-    width=$(identify -format "%W" "$image")
-    height=$(identify -format "%H" "$image")
-    if (($width > $height)); then
-        mogrify -resize "$vertical_size" -rotate 270 "$image"
-    else
-        mogrify -resize "$size" "$image"
-    fi
-    position=$(($position + 1))
-    echo -ne "$(printf '%3s' $((100 * $position / $length))) %   "$(printf "%0*d" "$((72 * $position / $length))" 0 | tr '0' '#')'\r'
-done
-echo
-zip "$outfile" **/*.???
-cd -