Python Functions and Files

This chapter is from the book

Learn Python 3 the Hard Way: A Very Simple Introduction to the Terrifyingly Beautiful World of Computers and Code, 4th Edition

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This chapter is from the book

This chapter is from the book 

Learn Python 3 the Hard Way: A Very Simple Introduction to the Terrifyingly Beautiful World of Computers and Code, 4th Edition

Learn More Buy

Remember your checklist for functions, then do this exercise paying close attention to how functions and files can work together to make useful stuff.

ex20.py

 1    from sys import argv
 2
 3    script, input_file = argv
 4
 5    def print_all(f):
 6        print(f.read())
 7
 8    def rewind(f):
 9        f.seek(0)
10
11    def print_a_line(line_count, f):
12        print(line_count, f.readline())
13
14    current_file = open(input_file)
15
16    print("First let's print the whole file:\n")
17
18    print_all(current_file)
19
20    print("Now let's rewind, kind of like a tape.")
21
22    rewind(current_file)
23
24    print("Let's print three lines:")
25
26    current_line = 1
27    print_a_line(current_line, current_file)
28
29    current_line = current_line + 1
30    print_a_line(current_line, current_file)
31
32    current_line = current_line + 1
33    print_a_line(current_line, current_file)

Pay close attention to how we pass in the current line number each time we run print_a_line.

What You Should See

Exercise 20 Session

$ python3.6 ex20.py test.txt
First let's print the whole file:

This is line 1
This is line 2
This is line 3

Now let's rewind, kind of like a tape.
Let's print three lines:
1 This is line 1

2 This is line 2

3 This is line 3