1. PDB PDX
DEBUGGING FOR HUMANS
CreatedbyAleckLandgrafandfriends / @aleck_landgraf/ aleck.me/pdbpdx

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

3. WHY DEBUGGING? WTF?
forjinjs:
mn=j.get('model','unknown')
js2[mn]=js2.get(mn,[])+[j]

4. STEPS TO DEBUGGING
1. linter
2. print debug info
3. read the traceback
4. jump into the shell
5. debugger
6. add tests

5. LINTER
your first line of defense

6. HERE'S WHAT MINE LOOKS LIKE

7. PYTHON DEBUG 101
PRINT!
forjinjs:
mn=j.get('model','unknown')
js2[mn]=js2.get(mn,[])+[j]
forjinjs:
mn=j.get('model','unknown')
js2[mn]=js2.get(mn,[])+[j]
print'WTF',js2.get(mn,[])+[j]
better yet
forjinjs:
mn=j.get('model','unknown')
js2[mn]=js2.get(mn,[])+[j]
print'nnline72oftime_utils.pyWTF',js2.get(mn,[])+[j],'nn'

8. AND OF COURSE, YOU MIGHT WANT TO USE LOGGER
importlogging
logger=logging.getLogger(__name__)
logging.basicConfig(format='%(filename)s:%(lineno)s-%(message)s')
forjinjs:
mn=j.get('model','unknown')
js2[mn]=js2.get(mn,[])+[j]
logger.debug(js2.get(mn,[])+[j])

9. WHAT TO PRINT?

10. TRACEBACKS
get comfortable reading these
File"django/core/handlers/base.py",line112,inget_response
response=wrapped_callback(request,*callback_args,**callback_kwargs)
File"seed/utils/api.py",line91,in_wrapped
returnfn(request,*args,**kwargs)
File"annoying/decorators.py",line164,inwrapper
response=func(request,*args,**kwargs)
File"superperms/orgs/decorators.py",line197,in_wrapped
returnfn(request,*args,**kwargs)
File"django/contrib/auth/decorators.py",line22,in_wrapped_view
returnview_func(request,*args,**kwargs)
File"ecm/views.py",line150,inupdate_ecm
ecm.mapped_data.update(ecm_data.get('mapped_data'),{})
TypeError:updateexpectedatmost1arguments,got2

11. USE AN ERROR LOGGING SERVICE
like getsentry.com

12. or newrelic.com

13. ENTERING THE SHELL

14. some regular python Kung Fu
_
obj.__dict__
dir(obj)
In[1]:requests.get('http://api.openweathermap.org/data/2.5/weather?id=5746545')
Out[1]:Response[200]
In[2]:response=_

15. DIR AND __DICT__
In[1]:classGreeter(object):
def__init__(self,name='Aleck'):
super(Greeter,self).__init__()
self.name=name
defgreet(self):
"""printshello``self.name``"""
print'hello{}'.format(self.name)
In[2]:d=Greeter()
dir
In[3]:dir(d)
Out[3]:
['__class__','__delattr__','__dict__','__doc__','__format__','__getattribute__','__h
__dict__
In[4]:d.__dict__
Out[4]:{'name':'Aleck'}

16. IPYTHON
with ipython's rich feature set
tab completion
? and ??
%history
... so much more

17. TAB COMPLETION
In[1]:importtime
In[2]:time.
TAB
In[2]:time.
time.accept2dyear time.clock time.gmtime time.sleep time.struct_ti
time.altzone time.ctime time.localtime time.strftime time.time
time.asctime time.daylight time.mktime time.strptime time.timezone

18. ? AND ??
In[1]:classGreeter(object):
...
defgreet(self):
"""printshello``self.name``"""
print'hello{}'.format(self.name)
In[2]:d=Greeter()
?
In[3]:d.greet?
Type: instancemethod
Stringform:>
File: /code/pdbpdx/
Definition: d.greet(self)
Docstring: printshello``self.name``
??
In[4]:d.greet??
Type: instancemethod
...
Source:
defgreet(self):
"""printshello``self.name``"""
print'hello{}'.format(self.name)

19. PICKING A DEBUGGER

20. PDB
“batteries included”
frompdbimportset_trace
set_trace()

21. IPDB
all the wonder of ipython plus some neat tricks:
pip install ipdb
90% use case:
with launch_ipdb_on_exception():
ipython --pdb main.py
fromipdbimportset_trace
set_trace()

22. PUDB

23. PEANUT BUTTER DEMO TIME

24. COMMON DEBUGGING PATTERNS

25. HANDLE ERRORS OVER LONG ITERATIONS
foruinusers:
u.set_home_location()
BLAMO!!! OK, try this...
foruinusers:
try:
u.set_home_location()
exceptKeyError:
importpudb;pudb.set_trace()
Can't run in the shell. OK, try this...
foruinusers:
try:
u.set_home_location()
exceptKeyError:
fromtracebackimportprint_exc
print_exc() #orlog

26. WHAT TO DO WITH PDB NAMESPACE COLLISIONS?
n=len(cols)
#assumingipythonshellinpudb
my_n=locals()['n']
#oruse!
!n

27. SLOW CODE? PROFILE IT
count and time things!

28. REDUCE YOUR FOOTPRINT
map vs. imap
items vs. iteritems
stream files and chuck large DB queries

29. 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

30. A QUICK NOTE ON COVERAGE

31. HOW MUCH IS ENOUGH?
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%
----------------------------------------------------------------------
Ran32testsin0.255s
pythonsetup.pynosetests--cover-min-percentage=100

32. STELLAR RESOURCES
your friends
Stack Overflow
ipython
pudb
nose + pudb
sentry
newrelic
python memory profiler
y u no measure it
pycallgraph
ipdb post_mortem debug

33. THE END
 COMMENTS
aleck.landgraf@gmail.com