`shell invocation` in Python 2

Presentation for Quipper Alumni Meetup held in 2019.

Transcript

1. `shell invocation` in
   Python 2
   @manicmaniac
   

   View Slide

2. Ryosuke Ito
   • iOS software engineer
   • Working for Quipper Ltd. since Apr. 2019
   • Loves Python
   @manicmaniac
   

   View Slide

3. Python 2
   

   View Slide

4. EOL of Python 2
   2020-01-01 00:00:00
   Only 15 days left!
   

   View Slide

5. Shell invocation
   

   View Slide

6. In Ruby
   puts `date`
   

   View Slide

7. In Perl
   print `date`
   

   View Slide

8. In Python 2.7
   import subprocess
   print subprocess.check_output([‘date’]),
   

   View Slide

9. In Python <= 2.6
   import subprocess
   process = subprocess.Popen(['date'], stdout=subprocess.PIPE),
   stdout, _ = process.communicate()
   print stdout,
   

   View Slide

10. How can I `date`?
    

    View Slide

11. Backquotes in
    Python 2
    `date` == repr(date) == date.__repr__()
    

    View Slide

12. Naive implementation
    import subprocess
    class date(object):
    def __repr__(self):
    return subprocess.check_output(‘date’)
    date = date()
    print `date`
    

    View Slide

13. Using meta class
    import subprocess
    def register_command(name):
    def __repr__(self):
    return subprocess.check_output(
    self.__class__.__name__
    )
    globals()[name] = type(
    name,
    (object,),
    dict(__repr__=__repr__)
    )()
    register_command('date')
    print `date`
    

    View Slide

14. But how about other
    commands?
    

    View Slide

15. import backquotes
    print `date`
    

    View Slide

16. View Slide

17. How it works?
    

    View Slide

18. Preprocessing
    import tokenize
    def preprocess(filename, readline):
    for token in tokenize.generate_tokens(readline):
    ...
    if inside_backquotes:
    tokens.extend([
    (tokenize.NAME, 'backquotes'),
    (tokenize.OP, '.'),
    (tokenize.NAME, 'shell'),
    (tokenize.OP, '('),
    ])
    ...
    

    View Slide

19. At Runtime
    frame = inspect.currentframe().f_back
    while frame.f_code.co_filename.startswith('importlib'):
    frame = frame.f_back
    execfile(source, frame.f_globals, frame.f_locals)
    

    View Slide

20. print `date`
    

    View Slide

21. Limitations
    • Doesn’t work on REPL
    • Doesn’t work on Python 3
    • Really buggy
    

    View Slide

22. Backquotes in
    Python 3
    >>> `date`
    File "", line 1
    `date`
    ^
    SyntaxError: invalid syntax
    

    View Slide

23. Don’t use backquotes
    

    View Slide

24. R.I.P Python 2
    Thank you for listening!
    

    View Slide