Write a function to insert a sentence in a text file, assuming that text file is very big and can not fit in

Q. Write a function to insert a sentence in a text file, assuming that text file is very big and can't fit in computer's memory.

Answer = 

def add(sen) :
	f.write("\n"+ sen)

f = open("pathwala.txt",'a')
sen = input("Enter a sentance :-")
add(sen)

f.close()