old_contents: import PyPDF2
  from formlayout import fedit
  
  def main():
  	paths = [('Input', ''), ('Output', '')]
  
  	pathsRead = fedit(paths, 
  	                  title="Interleave pdf",
  	                  comment="Enter the full path to the source pdf and a path to output the result."
  	                  )
  	# Full path to files should be specified eg C:\Users\Sam\Documents\Input.pdf and C:\Users\Sam\Documents\Input.pdf
  
  	document = PyPDF2.PdfFileReader(pathsRead[0])
  	writer = PyPDF2.PdfFileWriter()
  
  	for page in document.pages:
  		writer.addPage(page)
  		writer.addBlankPage()
  
  	outputStream = open(pathsRead[1], 'wb')
  	writer.write(outputStream)
  	outputStream.close()
  
  if __name__ == "__main__":
  	main()
  
new_contents: import PyPDF2
  from tkinter import *
  from tkinter.filedialog import askopenfilename
  from tkinter.filedialog import asksaveasfilename
  
  class Application(Frame):
  	def __init__(self):
  		self.input_path = None;
  		self.output_path = None;
  
  		Frame.__init__(self)
  
  		self.master.resizable(False, False)
  		self.master.title('Interleave PDF')
  		self.grid()
  
  		self.button = Button(self, text="Select input", command=self.load_file, width=12)
  		self.button.grid(row=1, column=0, sticky=W)
  
  		self.button = Button(self, text="Select output", command=self.save_file, width=12)
  		self.button.grid(row=1, column=2, sticky=W)
  
  		self.button = Button(self, text="Interleave", command=self.interleave, width=12)
  		self.button.grid(row=1, column=3, sticky=W)
  
  	def load_file(self):
  		self.input_path = askopenfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  
  	def save_file(self):
  		self.output_path = asksaveasfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  
  	def interleave(self):
  		if self.input_path and self.output_path:
  			document = PyPDF2.PdfFileReader(self.input_path)
  			writer = PyPDF2.PdfFileWriter()
  
  			for page in document.pages:
  				writer.addPage(page)
  				writer.addBlankPage()
  
  			outputStream = open(self.output_path, 'wb')
  			writer.write(outputStream)
  			outputStream.close()		
  
  if __name__ == "__main__":
  	Application().mainloop()
  
subject: Replace formlayout GUI with tkinter