added exercise 20

Author: mwarkentin

ex20.py

@@ -0,0 +1,33 @@
+from sys import argv
+
+script, input_file = argv
+
+def print_all(f):
+    print f.read()
+    
+def rewind(f):
+    f.seek(0)
+    
+def print_a_line(line_count, f):
+    print line_count, f.readline()
+    
+current_file = open(input_file)
+
+print "First let's print the whole file:\n"
+
+print_all(current_file)
+
+print "Now let's rewind, kind of like a tape."
+
+rewind(current_file)
+
+print "Let's print three lines:"
+
+current_line = 1
+print_a_line(current_line, current_file)
+
+current_line = current_line + 1
+print_a_line(current_line, current_file)
+
+current_line = current_line + 1
+print_a_line(current_line, current_file)