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Python debugging techniques
- 2. Who am I Tuomas Suutari Software Developer at Anders Inno 8 years Python experience Anders Inno Software Company from Turku specialized in e-commerce www.andersinno.fi 2013-10-21
- 5. Print statements ● ● ● The simplest way to see what's happening Sometimes also the fastest and easiest, so don't overthink it Could cause problems when temporary print statements slip into production 2013-10-21 def do_stuff(): something = do_something() print 'Something:', something do_more(something) print 'Calling do_last' do_last()
- 6. Logging ● ● ● ● Python has powerful and flexible logging module Like print statements with more context information Used through a logger object with name Log messages have levels 2013-10-21 import logging LOG = logging.getLogger(__name__) def do_stuff(): something = do_something() LOG.debug( 'Something: %s', something) do_more(something) LOG.info('Calling do_last') do_last()
- 7. Logging ● Log output can be easily controlled • • ● Outputting to stdout, file, network socket, ... Filtering based on log levels or logger names Logging statements are OK in production 2013-10-21 import logging msg_format = ( '%(asctime)s %(name)s' ' %(levelname)s %(funcName)s' ' %(message)s') logging.basicConfig( format=msg_format, level=logging.DEBUG)
- 9. PDB ● The Python Debugger ● In standard library ● Console interface ● Launch script with PDB or jump into debugger with set_trace() python -m pdb some_script.py def do_stuff(): import pdb; pdb.set_trace() something = do_something() # ... 2013-10-21
- 10. PDB ● Basic commands • c, cont: continue running • s, step: step into function • p, print: print value of variable/expression • b, break: set a breakpoint • u, up: move the current frame one level up • d, down: move the current frame one level down • l, list: list source code for the current file 2013-10-21
- 11. Winpdb ● ● ● A Platform Independent Python Debugger Run Python script from the GUI or embed to script Attaching to embedded debugger is possible from a remote host too 2013-10-21 def do_stuff(): import rpdb2 rpdb2.start_embedded_debugger( 'passwd') something = do_something() # ...
- 13. cProfile ● ● For profiling time consumption of parts of the program python -m cProfile -s cumulative Visualizers for the data could be useful python -m cProfile -o out.pyprof • Run Snake Run • KCacheGrind (with pyprof2calltree) 2013-10-21 some_script.py some_script.py runsnake out.pyprof
- 14. Guppy ● ● For profiling memory consumption of parts of the program # Injecting Guppy memory dumping Data structures in the program that are growing can be detected by dumping memory profile periodically with Guppy and watching the process with profile browser hp.setref() 2013-10-21 # to the program: from guppy import hpy hp = hpy() for x in a_loop_in_the_program: hp.heap().dump('memprof.hpy') # To start Profile Browser: python -c " from guppy import hpy; hpy().pb()"
- 15. Guppy ● Objects can be tracked down by connecting a monitor to running program # Allow monitor to connect to the # program import guppy from guppy.heapy import Remote Remote.on() # To connect with the monitor: python -c "from guppy import hpy; hpy().monitor()" <Monitor> sc 1 <Annex> int >>> hp.heap() 2013-10-21