PDB PDX

debugging for humans

Created by Aleck Landgraf and friends  /  @aleck_landgraf / aleck.me/pdbpdx

about me

• VP Software Engineering at Building Energy
• startups for the last 4+ years
• mostly developing with python, Django, and JS
• ping ping advocate

linter

your first line of defense

here's what mine looks like

python debug 101

print!

for j in js:
    mn = j.get('model','unknown')
    js2[mn]=js2.get(mn,[])+[j]
                    

for j in js:
    mn = j.get('model','unknown')
    js2[mn]=js2.get(mn,[])+[j]
    print 'WTF', js2.get(mn,[])+[j]
                    

better yet

for j in js:
    mn = j.get('model','unknown')
    js2[mn]=js2.get(mn,[])+[j]
    print '\n\nline 72 of time_utils.py WTF', js2.get(mn,[])+[j], '\n\n'
                    

logger does it all

import logging
logger = logging.getLogger(__name__)
logging.basicConfig(format='%(filename)s:%(lineno)s - %(message)s')
for j in js:
    mn = j.get('model','unknown')
    js2[mn]=js2.get(mn,[])+[j]
    logger.debug(js2.get(mn,[])+[j])
                    

what to print?

tracebacks

get comfortable reading these

  File "django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "seed/utils/api.py", line 91, in _wrapped
    return fn(request, *args, **kwargs)
  File "annoying/decorators.py", line 164, in wrapper
    response = func(request, *args, **kwargs)
  File "superperms/orgs/decorators.py", line 197, in _wrapped
    return fn(request, *args, **kwargs)
  File "django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "ecm/views.py", line 150, in update_ecm
    ecm.mapped_data.update(ecm_data.get('mapped_data'), {})
TypeError: update expected at most 1 arguments, got 2
                    

use an error logging service

like getsentry.com

or newrelic.com

entering the shell

some regular python Kung Fu

• _
• obj.__dict__
• dir(obj)
• vars(obj)

```pycon In [1]: requests.get('http://api.openweathermap.org/data/2.5/weather?id=5746545') Out[1]: Response [200] In [2]: response = _ ```

dir and __dict__

```pycon In [1]: class Greeter(object): def __init__(self, name='Aleck'): super(Greeter, self).__init__() self.name = name def greet(self): """prints hello ``self.name``""" print 'hello {}'.format(self.name) In [2]: d = Greeter() ```

dir

```pycon In [3]: dir(d) Out[3]: ['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'greet', 'name'] ```

__dict__

```pycon In [4]: d.__dict__ Out[4]: {'name': 'Aleck'} ```

ipython

with ipython's rich feature set

• tab completion
• ? and ??
• %history
• ... so much more

tab completion

```pycon In [1]: import time In [2]: time. ```

TAB

```pycon In [2]: time. time.accept2dyear time.clock time.gmtime time.sleep time.struct_time time.tzname time.altzone time.ctime time.localtime time.strftime time.time time.tzset time.asctime time.daylight time.mktime time.strptime time.timezone ```

? and ??

```pycon In [1]: class Greeter(object): ... def greet(self): """prints hello ``self.name``""" print 'hello {}'.format(self.name) In [2]: d = Greeter() ```

?

```pycon In [3]: d.greet? Type: instancemethod String form: > File: /code/pdbpdx/ Definition: d.greet(self) Docstring: prints hello ``self.name`` ```

??

```pycon In [4]: d.greet?? Type: instancemethod ... Source: def greet(self): """prints hello ``self.name``""" print 'hello {}'.format(self.name) ```

picking a debugger

pdb

“batteries included”

```python from pdb import set_trace set_trace() ```

ipdb

all the wonder of ipython plus some neat tricks:

• pip install ipdb
• 90% use case:

  ```python from ipdb import set_trace set_trace() ```

• `with launch_ipdb_on_exception():`
• ipython --pdb main.py

pudb

common debugging patterns

handle errors over long iterations

```python for u in users: u.set_home_location() ```

BLAMO!!! OK, try this...

```python for u in users: try: u.set_home_location() except KeyError: import pudb; pudb.set_trace() ```

Can't run in the shell. OK, try this...

```python for u in users: try: u.set_home_location() except KeyError: from traceback import print_exc print_exc() # or log ```

What to do with pdb namespace collisions?

```python n = len(cols) ```

```python # assuming ipython shell in pudb my_n = locals()['n'] # or use ! !n ```

slow code? profile it

count and time things!

reduce your footprint

• map vs. imap
• items vs. iteritems
• stream files and chuck large DB queries

steps to prevent debugging

1. you have a linter, right?
2. access to all errors and tracebacks?
3. flake8 should pass for tests to pass
4. draw a line in the sand: enforce code coverage
5. add tests to prevent regressions

a quick note on coverage

How much is enough?

```console Name Stmts Miss Cover Missing ------------------------------------------- espm 8 0 100% espm.espm 97 3 97% 109-112 espm.models 276 0 100% espm.util 19 0 100% ------------------------------------------- TOTAL 400 3 99% ---------------------------------------------------------------------- Ran 32 tests in 0.255s ```

```console python setup.py nosetests --cover-min-percentage=100 ```