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Debugging Pyramid

This tutorial provides a brief introduction to using the python debugger (pdb) for debugging pyramid applications.

This scenario assume you’ve created a Pyramid project already. The scenario assumes you’ve created a Pyramid project named buggy using the alchemy scaffold.

Introducing PDB

  • This single line of python is your new friend:

    import pdb;  pdb.set_trace()
    
  • As valid python, that can be inserted practically anywhere in a Python source file. When the python interpreter hits it - execution will be suspended providing you with interactive control from the parent TTY.

PDB Commands

  • pdb exposes a number of standard interactive debugging commands, including:

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    Documented commands (type help <topic>):
    ========================================
    EOF    bt         cont      enable  jump  pp       run      unt
    a      c          continue  exit    l     q        s        until
    alias  cl         d         h       list  quit     step     up
    args   clear      debug     help    n     r        tbreak   w
    b      commands   disable   ignore  next  restart  u        whatis
    break  condition  down      j       p     return   unalias  where
    
    Miscellaneous help topics:
    ==========================
    exec  pdb
    
    Undocumented commands:
    ======================
    retval  rv
    

Debugging Our buggy App

  • Back to our demo buggy application we generated from the alchemy scaffold, lets see if we can learn anything debugging it.
  • The traversal documentation describes how pyramid first acquires a root object, and then descends the resource tree using the __getitem__ for each respective resource.

Huh?

  • Let’s drop a pdb statement into our root factory object’s __getitem__ method and have a look. Edit the project’s models.py and add the aforementioned pdb line in MyModel.__getitem__:

    def __getitem__(self, key):
        import pdb; pdb.set_trace()
        session = DBSession()
        # ...
    
  • Restart the Pyramid application, and request a page. Note the request requires a path to hit our break-point:

    http://localhost:6543/   <- misses the break-point, no traversal
    http://localhost:6543/1  <- should find an object
    http://localhost:6543/2  <- does not
    
  • For a very simple case, attempt to insert a missing key by default. Set item to a valid new MyModel in MyRoot.__getitem__ if a match isn’t found in the database:

    item = session.query(MyModel).get(id)
    if item is None:
        item = MyModel(name='test %d'%id, value=str(id))  # naive insertion
    
  • Move the break-point within the if clause to avoid the false positive hits:

    if item is None:
        import pdb; pdb.set_trace()
        item = MyModel(name='test %d'%id, value=str(id))  # naive insertion
    
  • Run again, note multiple request to the same id continue to create new MyModel instances. That’s not right!

  • Ah, of course, we forgot to add the new item to the session. Another line added to our __getitem__ method:

    if item is None:
        import pdb; pdb.set_trace()
        item = MyModel(name='test %d'%id, value=str(id))
        session.add(item)
    
  • Restart and test. Observe the stack; debug again. Examine the item returning from MyModel:

    (pdb) session.query(MyModel).get(id)
    
  • Finally, we realize the item.id needs to be set as well before adding:

    if item is None:
        item = MyModel(name='test %d'%id, value=str(id))
        item.id = id
        session.add(item)
    
  • Many great resources can be found describing the details of using pdb. Try the interactive help (hit ‘h’) or a search engine near you.

Note

There is a well known bug in PDB in UNIX, when user cannot see what he is typing in terminal window after any interruption during PDB session (it can be caused by CTRL-C or when the server restarts automatically). This can be fixed by launching any of this commands in broken terminal: reset, stty sane. Also one can add one of this commands into ~/.pdbrc file, so they will be launched before PDB session:

from subprocess import Popen
Popen(["stty", "sane"])

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