Explain things.

3exercises/ex20.py

@@ -1,33 +1,36 @@
+# Import the argv tool from system tools.
 from sys import argv
 
+# Grab some command line arguments with argv.
 script, input_file = argv
 
-def print_all(f):
-    print(f.read())
 
-def rewind(f):
-    f.seek(0)
+def print_all(f): # Define a function with one argument.
+    print(f.read()) # Read an a file passed to this function.
 
-def print_a_line(line_count, f):
-    print(line_count, f.readline())
+def rewind(f): # Define a function with one argument.
+    f.seek(0) # Start from the first line.
 
-current_file = open(input_file)
+def print_a_line(line_count, f): # Define a function with two arguments.
+    print(line_count, f.readline()) # Print the line.
 
-print("First let's print the whole file:\n")
+current_file = open(input_file) # Open the file input_file as "current_file"
 
-print_all(current_file)
+print("First let's print the whole file:\n") # Tell the user we're printing the whole file.
 
-print("Now let's rewind, kind of like a tape.")
+print_all(current_file) # Print the whole file.
 
-rewind(current_file)
+print("Now let's rewind, kind of like a tape.") # Tell the user we're starting over.
 
-print("Let's print three lines:")
+rewind(current_file) # Print the first line.
 
-current_line = 1
-print_a_line(current_line, current_file)
+print("Let's print three lines:") # Tell the user we're printing three lines.
 
-current_line = current_line + 1
-print_a_line(current_line, current_file)
+current_line = 1 # Set the current line to the second line.
+print_a_line(current_line, current_file) # Print the current line.
 
-current_line = current_line + 1
-print_a_line(current_line, current_file)
+current_line = current_line + 1 # Increment the current line by one, making it the third line.
+print_a_line(current_line, current_file) # Print the current line.
+
+current_line = current_line + 1 # Increment the current line by one, making it the fourth line.
+print_a_line(current_line, current_file) # Print the current line.