Python debugging

Description

Using Python command-line debugger (pdb) to debug Plone and Python applications.

Introduction

Python debugger (pdb) is interactive command-line debugger.

It is very limited in functionality, but it will work in every environment and console. Plone also has through-the-web-browser PBD debugging add-on products.

Note

PDB is different from Python interactive shell. PDB let's you to step through the code, whilst Python shell allows you just to inspect and manipulate objects.

If you wish to play around with Zope in interactive Python shell or run scripts instead of debugging (exceptions), please read Command line documentation.

See also

• http://plone.org/documentation/kb/using-pdb
• http://plone.org/documentation/kb/best-practices/debugging-plone

Using pdb

Warning

In following text word set_trace has been replaced with xxx_trace due to commit problems.

# Go to your code and insert statement import pdb ; pdb.xxx_trace. When the code is run next time

the execution will stop there and you can examine the current context variables from Python command prompt

# After you have put import pdb ; pdb.xxx_trace() to your code, stop Zope and start it

on foreground using bin/instance fg command.

# TextMate support for pdb can be found at http://pypi.python.org/pypi/PdbTextMateSupport/0.3.

# mr.freeze allows traces to be added without restarting: http://pypi.python.org/pypi/mr.freeze.

Example:

class AREditForm(crud.EditForm):
    """ Present edit     le containing rows per each item added and delete controls """
    editsubform_factory = AREditSubForm

    template = viewpagetemplatefile.ViewPageTemplateFile('ar-crud-    le.pt')

    @property
    def fields(self):

        #
        # Execution will stop here and interactive Python prompt is opened
        #

        import pdb ; xxx.set_trace()
        constructor = ARFormConstructor(self.context, self.context.context, self.request)
        return constructor.getFields()

Pretty printing objects

Example:

>>> pp folder.__dict__
{
  '_Access_contents_information_Permission': ['Anonymous',
                                              'Manager',
                                              'Reviewer'],
  '_List_folder_contents_Permission': ('Manager', 'Owner', 'Member'),
  '_Modify_portal_content_Permission': ('Manager', 'Owner'),
  '_View_Permission': ['Anonymous', 'Manager', 'Reviewer'],
  '__ac_local_roles__': {'gregweb': ['Owner']},
  '_objects': ({'meta_type': 'Document', 'id': 'doc1'},
               {'meta_type': 'Document', 'id': 'doc2'}),
  'contributors': (),
  'creation_date': DateTime('2005/02/14 20:03:37.171 GMT+1'),
  'description': 'Dies ist der Mitglieder-Ordner.',
  'doc1': <Document at doc1>,
  'doc2': <Document at doc2>,
  'effective_date': None,
  'expiration_date': None,
  'format': 'text/html',
  'id': 'folder',
  'language': '',
  'modification_date': DateTime('2005/02/14 20:03:37.203 GMT+1'),
  'portal_type': 'Folder',
  'rights': '',
  'subject': (),
  'title': "Documents",
  'workflow_history': {'folder_workflow': ({'action': None,
    'review_state': 'visible', 'comments': '', 'actor': 'gregweb',
    'time': DateTime('2005/02/14 20:03:37.187 GMT+1')},)}
}

Useful pdb commands

Just type the command and hit enter

• s = step into, go into the function in the cursor
• n = step over, execute the function under the cursor without stepping into it
• c = continue, resume program
• w = where am I? displays current location in stack trace
• b = set breakpoint
• cl = clear breakpoint
• bt = print stack trace
• up = go to the scope of the caller function
• pp = pretty print object
• until = Continue execution until the line with the line number greater than the current one is reached or when returning from current frame

Note

The until command (or unt) is available only on Plone 4.x or superior as it is a new feature provided by the pdb module under Python 2.6.

Useful pdb snippets

Output object's class:

(Pdb) print obj.__class__

Output object attributes and methods:

(Pdb) for i in dir(obj): print i

Print local variables in the current function:

(Pdb) print locals()

Dumping incoming HTTP GET or HTTP POST:

(Pdb) print "Got request:"
(Pdb) for i in self.request.form.items(): print i

Executing code on the context of the current stack frame:

(Pdb) pp my_tags
['bar', 'barbar']

(Pdb) !my_tags = ['foo', 'foobar']
(Pdb) pp my_tags
['foo', 'foobar']

Note

The example above will modify the previous value of the variable my_tags in the current stack frame.

Automatically start debugger when exception is raised (browser)

You can start interactive through-the-browser Python debugger when your site throws an exception.

• http://plone.org/products/pdbdebugmode
• http://pypi.python.org/pypi/Products.PDBDebugMode

Automatically start debugger when exception is raised (command line)

Note

This cannot be directly applied to a web server, but works with command line scripts

Note

This does not work with Zope web server launch as it forks a process.

Example:

python -m pdb myscript.py

Hit c and enter to start the application. It keeps running, until uncaught exception is risen when it falls back to pdb debug prompt.

For more information see

• http://docs.python.org/library/pdb.html