How to randomly choose a line from a TXT file in Python 3

You don’t say how big the file is.

If it is relatively small - read the whole file in (using readlines(), and then use random.choice()

1. import random 
2.  
3. def choose_line(file_name): 
4. """Choose a line at random from the text file""" 
5. with open(file_name, 'r') as file: 
6. lines = file.readlines() 
7. random_line = random.choice(lines) 
8. return random_line 

If the file is too big to read the entire thing into memory - then I would suggest a three step process - 1st identify the number of lines in the files and record their positions in the file, 2nd choose a random number corresponding to the number of lines - 3rd - pick out that line

1. def choose_line_big_file(file_name): 
2. """Choose a line at random from the a big file"""  
3.  
4. line_pos = {} 
5.  
6. with open(file_name, 'r') as file: 
7. # Count the number of lines & record their position 
8. for line_count in itertools.count(): 
9. file_pos = file.tell() 
10. line = file.readline() 
11. if not line: 
12. break 
13. line_pos[line_count] = file_pos 
14.  
15. # Choose a line number 
16. chosen_line = random.randint(0, line_count-1) 
17.  
18. # Go to the start of the chosen line. 
19. file.seek(line_pos[chosen_line]) 
20. line = file.readline() 
21. return line 

If this is something that needs to be done regularly with an single program then opening, reading and close the file repeatedly might be wasteful - so some method of caching some of the results might be considered.

Disclaimer : Neither solution has been fully tested