pdb is an interactive tool that comes with Python, which allows you to break your program at an arbitrary point, examine values, and step through code. It’s often much more useful than print statements or logging statements to examine program state. You can place a pdb.set_trace() statement in your Pyramid application at a place where you’d like to examine program state. When you issue a request to the application, and that point in your code is reached, you will be dropped into the pdb debugging console within the terminal that you used to start your application.
There are lots of great resources that can help you learn PDB.
Below is a debugging scenario using PDB to debug 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.
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 exposes a number of standard interactive debugging commands, including:
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
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:
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.
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"])