Core Python Programming (2nd Edition)

28 while True:
29 while True:
30 try:
31 choice = raw_input(pr).strip()[0].lower()
32 except (EOFError,KeyboardInterrupt,IndexError):
33 choice = 'q'
34
35 print '\nYou picked: [%s]' % choice
36 if choice not in 'uovq':
37 print 'Invalid option, try again'
38 else:
39 break
40
41 if choice == 'q':
42 break
43 CMDs[choice]()
44
45 if __name__ == '__main__':
46 showmenu()
Line-by-Line Explanation
Lines 13
In addition to the Unix startup line, we take this opportunity to clear the stack (a list).
Lines 56
The pushit() function adds an element (a string prompted from the user) to the stack.
Lines 812
The popit() function removes an element from the stack (the more recent one). An error occurs when
trying to remove an element from an empty stack. In this case, a warning is sent back to the user.
When an object is popped from the stack, the user sees which element was removed. We use single
backquotes or backticks ( ' ) to symbolize the repr() command, showing the string complete with
quotes, not just the contents of the string.
Lines 1415
The viewstack() function displays the current contents of the stack.
Line 17
Although we cover dictionaries formally in the next chapter, we wanted to give you a really small
example of one here, a command vector (CMDs). The contents of the dictionary are the three "action"
functions defined above, and they are accessed through the letter that the user must type to execute
that command. For example, to push a string onto the stack, the user must enter 'u', so 'u' is how
access the pushit() from the dictionary. The chosen function is then executed on line 43.