Example 20.1

scratch_20.py

from sys import argv  # from module sys import object named argv

script, input_file = argv  # set the variables from command line


def print_all(f):  # defines variable that is a file and a function object ???
    print(f.read())  # what do the function, prints all that is written in (f)ile


def rewind(f: object) -> object:  # defines function
    f.seek(0)  # rewind to the start of f (input_file)


def print_a_line(line_count, f):  # defines  another function with two variables
    print(line_count, f.readline())  # read the line where is positioned python and prints it in the screen


current_file = open(input_file)  # set the variable current_file to object open, and opens the file that I put in command line running the script

print("First let's print the whole file:\n")  # prints and add empty line after

print_all(current_file)  # calls, run or use the function print_all with prints the variable current_file that has open(input_file)

print("Now let's rewind, kind of like a tape.")

rewind(current_file)

print("Let's print three lines:")  # prints out first three lines after writing 1, 2 and 3 into each line.

current_line = 1
print_a_line(current_line, current_file)

current_line += 1
print_a_line(current_line, current_file)

current_line += 1
print_a_line(current_line, current_file)