How to add bookmarks to pdf using python pypdf2

pypdf_add_bookmarks_pdf.py

from PyPDF2 import PdfFileReader, PdfFileMerger

pdf_merger = PdfFileMerger()
f = open('book.pdf', 'rb')
pdf_merger.append(f)

offset = 25
addbm = pdf_merger.addBookmark

addbm('Contents', 5)
ch1_bm = addbm('1 Computations with Structured Matrices', offset + 1)

# Children
for title, page in ch1_sub_boomarks_contents:
    addbm(title, offset+page, parent=ch1_bm)

addbm('2 Toeplitz/Hankel Matrix, Polynomial Computations', offset + 23)

with open('out.pdf', 'wb') as f:
    pdf_merger.write(f)


# To check
f = open('out.pdf', 'rb')
pdf_reader = PdfFileReader(f)
print(pdf_reader.outlines)




# Edit bookmarks

# pdf_merger.bookmarks.extend(pdf_reader.outlines)
f = open('book.pdf', 'rb')
# May need to hack _trim_outline method...
pdf_merger.append(f, import_bookmarks=True)

def reduce_top(bookmarks):
    for bm in bookmarks:
        if isinstance(bm, dict):
            bm[NameObject('/Top')] = NumberObject(bm['/Top']+500)
        elif isinstance(bm, list):
            reduce_top(bm)
            
reduce_top(pdf_merger.bookmarks)

PyPDF2 is B.S. For example, it mutates pdf_merger.bookmarks when it writes.