Initial script, ugly but functional

foliator.py

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+#!/usr/bin/env python3
+from pypdf import PaperSize, PdfReader, PdfWriter, Transformation
+import math
+
+input_pdf = PdfReader("to_foliate.pdf")
+num_A5 = math.ceil(len(input_pdf.pages)/4.0) * 2
+
+# First pretend to be printing on A5 paper
+writer = PdfWriter()
+
+for i in range(num_A5):
+    dpage = writer.add_blank_page(width=PaperSize.A5.height,
+                                  height=PaperSize.A5.width)
+    ipage = input_pdf.pages[i]
+    ipage.mediabox = dpage.mediabox
+
+    if i % 2 == 0: # Even pages are filled right to left
+        ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
+                                                                            0))
+    dpage.merge_page(ipage)
+
+
+# Now for the fun part: work backwards to fill the remaining pages
+for j in range(len(input_pdf.pages) - num_A5):
+    dpage = writer.pages[num_A5 - 1 - j]
+    ipage = input_pdf.pages[num_A5+j]
+    ipage.mediabox = dpage.mediabox
+    if (num_A5 - 1 - j) % 2 == 1:
+        ipage.add_transformation(Transformation().translate(PaperSize.A6.width,
+                                                                            0))
+    dpage.merge_page(ipage)
+
+writer.write("output_A5.pdf")
+
+w2 = PdfWriter()
+if(num_A5 % 4 == 2):
+    writer.add_blank_page(width=PaperSize.A5.height,
+                          height=PaperSize.A5.width)
+    writer.add_blank_page(width=PaperSize.A5.height,
+                          height=PaperSize.A5.width)
+    num_A5 += 2
+
+for i in range(0, num_A5-3, 4):
+    dpage_r = w2.add_blank_page(width=PaperSize.A4.width,
+                               height=PaperSize.A4.height)
+
+    dpage_v = w2.add_blank_page(width=PaperSize.A4.width,
+                                height=PaperSize.A4.height)
+    print(i)
+    top_pr = writer.pages[i]
+    top_pv = writer.pages[i+1]
+
+    bot_pr = writer.pages[i+2]
+    bot_pv = writer.pages[i+3]
+
+    top_pr.mediabox = dpage_r.mediabox
+    bot_pr.mediabox = dpage_r.mediabox
+
+    top_pv.mediabox = dpage_v.mediabox
+    bot_pv.mediabox = dpage_v.mediabox
+
+    bot_pr.add_transformation(Transformation().translate(0, PaperSize.A5.width))
+    bot_pv.add_transformation(Transformation().translate(0, PaperSize.A5.width))
+
+    dpage_r.merge_page(top_pr)
+    dpage_r.merge_page(bot_pr)
+    dpage_v.merge_page(top_pv)
+    dpage_v.merge_page(bot_pv)
+
+w2.write("output.pdf")

shell.nix

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+let
+	pkgs = import <nixpkgs> {};
+	python3Packages = pkgs.python3Packages;
+in
+pkgs.mkShell {
+  packages = [
+  (pkgs.python3.withPackages (pythonPackages: with python3Packages; [
+    pypdf
+  ]))
+  ];
+}