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pdb like a pro

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Philip Bauer
Philip Bauer

The Python Debugger pdb is one of the most important tools available to python-developers. Knowing how to apply the many features in your daily routine makes you more productive and boosts the understanding of the code you otherwise bang your head against. This talk explores the features of pdb, some of its alternatives and showcases some advanced tipps and tricks.

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PDB LIKE A PRO PHILIP BAUER @StarzelDe pbauer
pdb like a pro
THE BASICS
PHILIP BAUER: PDB LIKE A PRO PYCON.DE 2016 The Basics ▸ l[ist]
 List source code for the current file ▸ n[ext]
 Continue execution until the next line ▸ s[tep] 
 Execute the current line, stop at the first possible occasion ▸ r[eturn]
 Continue execution until the current function returns ▸ c[ontinue]
 Continue execution, only stop when a breakpoint is encountered
PHILIP BAUER: PDB LIKE A PRO PYCON.DE 2016 The Basics II ▸ w[here] 
 Show stack trace. Recent frame at bottom ▸ u[p] 
 Move to older frame in stack trace ▸ d[own] 
 Move to newer frame in stack trace ▸ b[reakpoint] 
 Set a new breakpoint. tbreak Temporary breakpoint ▸ a[rgs]
 Print the argument list of the current function.
PIMP MY PDB
PIMP MY PDB Some Enhancements ▸ pdb++ ▸ ipdb ▸ bpdb ▸ pudb ▸ and…
PIMP MY PDB pdb in python 3.2+ ▸ ll | longlist
 List all source code for the current function or frame. ▸ source
 Try to get source code for the given object and display it. ▸ …
PIMP MY PDB ipdb ▸ + features ▸ - dependencies ▸ - weird sqllite ▸ - need to use ``import ipdb; ipdb.set_trace()``
PIMP MY PDB pudb ▸ + features ▸ +/- UI ▸ - need to use import pudb; pudb.set_trace()
TEXT
PIMP MY PDB ▸ it’s actually called pdbpp ▸ + features ▸ + no problematic dependencies ▸ + drop-in-replacement ▸ - needs configuration to not break readline pdb++
DEMO PDB++
PIMP MY PDB pdb++ ▸ you’ll need a file ~/.fancycompleterrc.py
pdb++ ▸ Syntax highlighting ▸ ll | longlist
 List all source code for the current function or frame. ▸ track EXPRESSION ▸ display EXPRESSION ▸ source EXPRESSION ▸ obj? ▸ obj.__class__?? ▸ Sticky mode PIMP MY PDB
Give pdb++ a try! The person you trust PIMP MY PDB
INTROSPECTION
INTROSPECTION python stuff you should know ▸ dir(obj) - bäh! ▸ help(obj) ▸ help(obj.__class__) ▸ pp locals() ▸ obj.__dict__ ▸ obj.__class__.__dict__ ▸ pp dict(obj.__class__.__dict__)
INTROSPECTION the inspect module ▸ It’s what is used by other nice tools ▸ import inspect ▸ print inspect.getsource(obj.__class__) ▸ inspect.getmro(obj.__class__)
ADVANCED TIPS
▸ break if some condition applies ▸ very useful in loops ▸ you need it in try/except blocks (e.g. indexing) TIPS AND TRICKS Conditional Breakpoints
TIPS AND TRICKS debug ▸ Call any code and step into it with a pdb in a pdb ▸ You need to step into it after calling it! ▸ (Pdb++) self.context
 <Document at /Plone/startseite>
 (Pdb++) debug self.context.canSetLayout()
 ENTERING RECURSIVE DEBUGGER
 ((Pdb++)) s
 --Call--
 [2] >[…]/Products/CMFDynamicViewFTI/ browserdefault.py(183)canSetLayout()
 ((Pdb++)) ll
 183 -> @security.public
 184 def canSetLayout(self):
 186 mtool = getToolByName(self, 'portal_membership')
DEMO DEBUG
▸ Whenever there is a traceback you end up in a pdb! ▸ try:
 run_your_stuff()
 except:
 import pdb; pdb.set_trace() ▸ Plone: Products.PDBDebugMode ▸ Disable when migrating/reindexing TIPS AND TRICKS Post-Mortem Debugging
▸ Drops you in a pdb whenever a test fails or errors ▸ Zope/Plone: bin/test -D
 pytest & nosetest: pytest / nosetest --pdb
 unittest: need to wrap in try/except ▸ Test-Driven Debug-Driven Development TIPS AND TRICKS Post-Mortem Testing
pdb like a pro
▸ add a config-file ~/.pdbrc ▸ alias inspect import inspect;
 print inspect.getsource(%1) ▸ alias help !print help(%1) TIPS AND TRICKS Customize your pdb
TEXT Sentry ▸ Collect tracebacks from production-environments ▸ The traceback includes all arguments for all stacks ▸ OpenSource / Django ▸ github.com/getsentry/sentry ▸ Available as a service: sentry.io ▸ starzel.de is working on a DebOps/Ansible role for Sentry
SUMMARY
SUMMARY What to take away ▸ learn the basics ▸ try pdb++ ▸ learn introspection ▸ try debug callable() ▸ use post-mortem-debugging ▸ always use pretty-print ▸ use Sentry to debug errors on production
QUESTIONS Philip Bauer @StarzelDe pbauer All Fotos by Philip Bauer. Taken in Boothbay, Maine October 2016

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pdb like a pro

  • 1. PDB LIKE A PRO PHILIP BAUER @StarzelDe pbauer
  • 4. PHILIP BAUER: PDB LIKE A PRO PYCON.DE 2016 The Basics ▸ l[ist]
 List source code for the current file ▸ n[ext]
 Continue execution until the next line ▸ s[tep] 
 Execute the current line, stop at the first possible occasion ▸ r[eturn]
 Continue execution until the current function returns ▸ c[ontinue]
 Continue execution, only stop when a breakpoint is encountered
  • 5. PHILIP BAUER: PDB LIKE A PRO PYCON.DE 2016 The Basics II ▸ w[here] 
 Show stack trace. Recent frame at bottom ▸ u[p] 
 Move to older frame in stack trace ▸ d[own] 
 Move to newer frame in stack trace ▸ b[reakpoint] 
 Set a new breakpoint. tbreak Temporary breakpoint ▸ a[rgs]
 Print the argument list of the current function.
  • 7. PIMP MY PDB Some Enhancements ▸ pdb++ ▸ ipdb ▸ bpdb ▸ pudb ▸ and…
  • 8. PIMP MY PDB pdb in python 3.2+ ▸ ll | longlist
 List all source code for the current function or frame. ▸ source
 Try to get source code for the given object and display it. ▸ …
  • 9. PIMP MY PDB ipdb ▸ + features ▸ - dependencies ▸ - weird sqllite ▸ - need to use ``import ipdb; ipdb.set_trace()``
  • 10. PIMP MY PDB pudb ▸ + features ▸ +/- UI ▸ - need to use import pudb; pudb.set_trace()
  • 11. TEXT
  • 12. PIMP MY PDB ▸ it’s actually called pdbpp ▸ + features ▸ + no problematic dependencies ▸ + drop-in-replacement ▸ - needs configuration to not break readline pdb++
  • 14. PIMP MY PDB pdb++ ▸ you’ll need a file ~/.fancycompleterrc.py
  • 15. pdb++ ▸ Syntax highlighting ▸ ll | longlist
 List all source code for the current function or frame. ▸ track EXPRESSION ▸ display EXPRESSION ▸ source EXPRESSION ▸ obj? ▸ obj.__class__?? ▸ Sticky mode PIMP MY PDB
  • 16. Give pdb++ a try! The person you trust PIMP MY PDB
  • 18. INTROSPECTION python stuff you should know ▸ dir(obj) - bäh! ▸ help(obj) ▸ help(obj.__class__) ▸ pp locals() ▸ obj.__dict__ ▸ obj.__class__.__dict__ ▸ pp dict(obj.__class__.__dict__)
  • 19. INTROSPECTION the inspect module ▸ It’s what is used by other nice tools ▸ import inspect ▸ print inspect.getsource(obj.__class__) ▸ inspect.getmro(obj.__class__)
  • 21. ▸ break if some condition applies ▸ very useful in loops ▸ you need it in try/except blocks (e.g. indexing) TIPS AND TRICKS Conditional Breakpoints
  • 22. TIPS AND TRICKS debug ▸ Call any code and step into it with a pdb in a pdb ▸ You need to step into it after calling it! ▸ (Pdb++) self.context
 <Document at /Plone/startseite>
 (Pdb++) debug self.context.canSetLayout()
 ENTERING RECURSIVE DEBUGGER
 ((Pdb++)) s
 --Call--
 [2] >[…]/Products/CMFDynamicViewFTI/ browserdefault.py(183)canSetLayout()
 ((Pdb++)) ll
 183 -> @security.public
 184 def canSetLayout(self):
 186 mtool = getToolByName(self, 'portal_membership')
  • 24. ▸ Whenever there is a traceback you end up in a pdb! ▸ try:
 run_your_stuff()
 except:
 import pdb; pdb.set_trace() ▸ Plone: Products.PDBDebugMode ▸ Disable when migrating/reindexing TIPS AND TRICKS Post-Mortem Debugging
  • 25. ▸ Drops you in a pdb whenever a test fails or errors ▸ Zope/Plone: bin/test -D
 pytest & nosetest: pytest / nosetest --pdb
 unittest: need to wrap in try/except ▸ Test-Driven Debug-Driven Development TIPS AND TRICKS Post-Mortem Testing
  • 27. ▸ add a config-file ~/.pdbrc ▸ alias inspect import inspect;
 print inspect.getsource(%1) ▸ alias help !print help(%1) TIPS AND TRICKS Customize your pdb
  • 28. TEXT Sentry ▸ Collect tracebacks from production-environments ▸ The traceback includes all arguments for all stacks ▸ OpenSource / Django ▸ github.com/getsentry/sentry ▸ Available as a service: sentry.io ▸ starzel.de is working on a DebOps/Ansible role for Sentry
  • 30. SUMMARY What to take away ▸ learn the basics ▸ try pdb++ ▸ learn introspection ▸ try debug callable() ▸ use post-mortem-debugging ▸ always use pretty-print ▸ use Sentry to debug errors on production
  • 31. QUESTIONS Philip Bauer @StarzelDe pbauer All Fotos by Philip Bauer. Taken in Boothbay, Maine October 2016