Python debugging

Description

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

Introduction

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

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

Note

pdb is not the same as the Python interactive shell. pdb allows you to step through the code, whilst the 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

Using pdb

# Go to your code and insert the statement import pdb; pdb.set_trace() at
the point where you want have a closer look. Next time the code is run, the execution will stop there and you can examine the current context variables from a Python command prompt.
# After you have added import pdb; pdb.set_trace() to your code, stop
Zope and start it in the foreground using the bin/instance fg command.
# TextMate support for pdb can be found at
https://pypi.python.org/pypi/PdbTextMateSupport/0.3.
# mr.freeze allows traces to be added without restarting:
https://pypi.python.org/pypi/mr.freeze.

Example:

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

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

    @property
    def fields(self):

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

        import pdb ; pdb.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.

Instead of getting "We're sorry there seems to be an error..." page you get a pdb prompt which allows you to debug the exception. This is also known as post-mortem debugging.

This can be achieved with ` Products.PDBDebugMode` add-on.

Note

PDBDebugMode is not safe to install on the production server due to sandbox security escape.

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 an uncaught exception is raised. At this point, it falls back to the pdb debug prompt.

For more information see