You can use PyPdf2s PdfMerger class.
File Concatenation
You can simply concatenate files by using the append method.
from PyPDF2 import PdfFileMerger
pdfs = ['file1.pdf', 'file2.pdf', 'file3.pdf', 'file4.pdf']
merger = PdfFileMerger()
for pdf in pdfs:
merger.append(pdf)
merger.write("result.pdf")
merger.close()
You can pass file handles instead file paths if you want.
File Merging
If you want more fine grained control of merging there is a merge method of the PdfMerger, which allows you to specify an insertion point in the output file, meaning you can insert the pages anywhere in the file. The append method can be thought of as a merge where the insertion point is the end of the file.
e.g.
merger.merge(2, pdf)
Here we insert the whole pdf into the output but at page 2.
Page Ranges
If you wish to control which pages are appended from a particular file, you can use the pages keyword argument of append and merge, passing a tuple in the form (start, stop[, step]) (like the regular range function).
e.g.
merger.append(pdf, pages=(0, 3)) # first 3 pages
merger.append(pdf, pages=(0, 6, 2)) # pages 1,3, 5
If you specify an invalid range you will get an IndexError.
Note: also that to avoid files being left open, the PdfFileMergers close method should be called when the merged file has been written. This ensures all files are closed (input and output) in a timely manner. It's a shame that PdfFileMerger isn't implemented as a context manager, so we can use the with keyword, avoid the explicit close call and get some easy exception safety.
You might also want to look at the pdfcat script provided as part of pypdf2. You can potentially avoid the need to write code altogether.
The PyPdf2 github also includes some example code demonstrating merging.