Video: PyCon US 2016: Alex Martelli: Exception and error handling in Python 2 and Python 3

By Vasudev Ram

I started watching this recent PyCon US 2016 video. I found it informative and interesting:

Alex Martelli: Exception and error handling in Python 2 and Python 3

Alex Martelli is the author or co-author of some well-known Python books like Python in a Nutshell and the Python Cookbook (for at least one of the versions); I've bought and read some of them, and they are pretty good.

[ Interestingly, the Wikipedia article about Alex, linked above, says that he is also noted for coining the phrase duck typing. ]

The video is also embedded below:



- Enjoy.

- Vasudev Ram - Online Python training and consulting

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Trapping KeyboardInterrupt and EOFError for program cleanup

By Vasudev Ram

Ctrl-C and Ctrl-Z handling

I had written this small Python utility for my own use, to show the ASCII code for any input character typed at the keyboard. Since it was a quick utility, I was initially just using Ctrl-C to exit the program. But that leaves behind a messy traceback, so I thought of trapping the exceptions KeyboardInterrupt (raised by Ctrl-C) and EOFError (raised by Ctrl-Z). With that, the program now exits cleanly on typing either of those keys.

Here is the resulting utility, char_to_ascii_code.py:

from __future__ import print_function
"""
char_to_ascii_code.py
Purpose: Show ASCII code for a given character, interactively, 
in a loop. Show trapping of KeyboardInterrupt and EOFError exceptions.
Author: Vasudev Ram
Web site: https://vasudevram.github.io
Blog: https://jugad2.blogspot.com
Product store: https://gumroad.com/vasudevram
"""

print("This program shows the ASCII code for any given ASCII character.")
print("Exit the program by pressing Ctrl-C or Ctrl-Z.")
print()

while True:
    try:
        c = raw_input( \
        "Enter an ASCII character to see its ASCII code: ")
        if len(c) != 1:
            print("Error: need a string of length 1; retry.")
            continue
        print("Character:", c)
        print("Code:", ord(c))
    except KeyboardInterrupt as ki:
        print("Caught:", repr(ki))
        print("Exiting.")
        break
    except EOFError as eofe:
        print("Caught:", repr(eofe))
        print("Exiting.")
        break

Here is a sample run, that shows the ASCII codes for the comma, tab and pipe characters, which are commonly used as field delimiters in Delimiter-Separated Value (DSV) files.

$ python char_to_ascii_code.py
This program shows the ASCII code for any given ASCII character.
Exit the program by pressing Ctrl-C or Ctrl-Z.

Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: ,
Character: ,
Code: 44
Enter an ASCII character to see its ASCII code, or Ctrl-C to exit:
Character:
Code: 9
Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: |
Character: |
Code: 124
Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: 
Caught: KeyboardInterrupt()
Exiting.
$

I pressed the Ctrl-C key combination to exit the program. Ctrl-C does not show on the screen, but the exception handler for it is activated, and prints the last message above.

Another run shows a few more codes and the trapping of the Ctrl-Z key combination.

$ python char_to_ascii_code.py
This program shows the ASCII code for any given ASCII character.
Exit the program by pressing Ctrl-C or Ctrl-Z.

Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: !
Character: !
Code: 33
Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: ~
Character: ~
Code: 126
Enter an ASCII character to see its ASCII code, or Ctrl-C to exit: ^Z
Caught: EOFError()
Exiting.

- Vasudev Ram - Online Python training and consulting

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