7.6. Exception Traceback

7.6.1. Rationale

Traceback will help you track down the bug.

>>> raise RuntimeError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError

>>> raise RuntimeError('Huston we have a problem')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Huston we have a problem

7.6.2. Traceback Analysis

• Stacktrace is 8 levels deep, it's not Java's 200 ;)

• Start analysing traceback from bottom-up

  >>> def apollo13():
  ...     raise RuntimeError('Oxygen tank explosion')
  >>>
  >>>
  >>> apollo13()
  Traceback (most recent call last):
    File "<input>", line 5, in <module>
    File "<input>", line 2, in apollo13
  RuntimeError: Oxygen tank explosion
  

  >>> def apollo13():
  ...     raise RuntimeError('Oxygen tank explosion')
  >>>
  >>>
  >>> apollo13()
  Traceback (most recent call last):
    File "/home/watney/myscript.py", line 5, in <module>
      apollo13()
    File "/home/watney/myscript.py", line 2, in apollo13
      raise RuntimeError('Oxygen tank explosion')
  RuntimeError: Oxygen tank explosion
  

  >>> def apollo13():
  ...     raise RuntimeError('Oxygen tank explosion')
  >>>
  >>>
  >>> apollo13()
  Traceback (most recent call last):
    File "<input>", line 1, in <module>
    File "/Applications/PyCharm 2021.1 EAP.app/Contents/helpers/pydev/_pydev_bundle/pydev_umd.py", line 197, in runfile
      pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
    File "/Applications/PyCharm 2021.1 EAP.app/Contents/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
      exec(compile(contents+"\n", file, 'exec'), glob, loc)
    File "/home/watney/myscript.py", line 4, in <module>
      apollo13()
    File "/home/watney/myscript.py", line 2, in apollo13
      raise RuntimeError('Oxygen tank explosion')
  RuntimeError: Oxygen tank explosion
  

7.6.3. Change Verbosity Level

• Change level with sys.tracebacklimit

• From time to time you can have problems somewhere in the middle, but it's rare

• Last lines are the most important, in most cases error is there

  >>> import sys
  >>> sys.tracebacklimit = 2
  >>>
  >>>
  >>> def apollo13():
  ...     raise RuntimeError('Oxygen tank explosion')
  >>>
  >>>
  >>> apollo13()
  Traceback (most recent call last):
    File "/home/watney/myscript.py", line 4, in <module>
      apollo13()
    File "/home/watney/myscript.py", line 2, in apollo13
      raise RuntimeError('Oxygen tank explosion')
  RuntimeError: Oxygen tank explosion