old_contents: import PyPDF2
  import tkinter as tk
  import tkinter.filedialog as fd
  
  class Application(tk.Frame):
  	def __init__(self, master=None):
  		self.input_path = None;
  		self.output_path = None;
  
  		tk.Frame.__init__(self, master)
  
  		self.master.resizable(False, False)
  		self.master.title('Interleave PDF')
  		self.grid()
  
  		self.label = tk.Label(self, text="Input", width=12)
  		self.label.grid(row=0, column=0)
  
  		self.entry_in = tk.Entry(self, width=50, textvariable=self.input_path)
  		self.entry_in.grid(row=0, column=1)
  		self.entry_in.insert("end","Path...")
  
  		self.button = tk.Button(self, text="Browse", command=self.load_file, width=12)
  		self.button.grid(row=0, column=2)
  
  		self.label = tk.Label(self, text="Output", width=12)
  		self.label.grid(row=1, column=0)
  
  		self.entry_out = tk.Entry(self, width=50, textvariable=self.output_path)
  		self.entry_out.grid(row=1, column=1)
  		self.entry_out.insert("end","Path...")
  		
  		self.button = tk.Button(self, text="Browse", command=self.save_file, width=12)
  		self.button.grid(row=1, column=2)
  
  		self.button = tk.Button(self, text="Interleave", command=self.interleave, width=12)
  		self.button.grid(row=2, column=2)
  
  	def load_file(self):
  		self.input_path = fd.askopenfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  		self.entry_in.delete(0, "end")
  		self.entry_in.insert("end",self.input_path)
  		
  	def save_file(self):
  		self.output_path = fd.asksaveasfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  		self.entry_out.delete(0, "end")
  		self.entry_out.insert("end",self.output_path)
  
  	def interleave(self):
  		self.input_path = self.entry_in.get()
  		self.output_path = self.entry_out.get()
  
  		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()
  
new_contents: import PyPDF2
  import tkinter as tk
  import tkinter.filedialog as fd
  
  class Application(tk.Frame):
  	def __init__(self, master=None):
  		tk.Tk()
  		self.input_path = tk.StringVar();
  		self.output_path = tk.StringVar();
  
  		tk.Frame.__init__(self, master)
  
  		self.master.resizable(False, False)
  		self.master.title('Interleave PDF')
  		self.grid()
  
  		self.label = tk.Label(self, text="Input", width=12)
  		self.label.grid(row=0, column=0)
  
  		self.entry_in = tk.Entry(self, width=50, textvariable=self.input_path)
  		self.entry_in.grid(row=0, column=1)
  
  		self.button = tk.Button(self, text="Browse", command=self.load_file, width=12)
  		self.button.grid(row=0, column=2)
  
  		self.label = tk.Label(self, text="Output", width=12)
  		self.label.grid(row=1, column=0)
  
  		self.entry_out = tk.Entry(self, width=50, textvariable=self.output_path)
  		self.entry_out.grid(row=1, column=1)
  
  		self.button = tk.Button(self, text="Browse", command=self.save_file, width=12)
  		self.button.grid(row=1, column=2)
  
  		self.button = tk.Button(self, text="Interleave", command=self.interleave, width=12)
  		self.button.grid(row=2, column=2)
  
  	def load_file(self):
  		path = fd.askopenfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  		self.input_path.set(path)
  
  	def save_file(self):
  		path = fd.asksaveasfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))
  		self.output_path.set(path)
  
  	def interleave(self):
  		self.input_path = self.entry_in.get()
  		self.output_path = self.entry_out.get()
  
  		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: Change path variables to tk.StringVar() objects