15.4. Exception Traceback
Traceback will help you track down the bug
>>> def apollo13():
... raise RuntimeError('Oxygen tank explosion')
>>>
>>> apollo13() # doctest +SKIP
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
15.4.1. Fine Grained Error Locations
Since Python 3.11
PEP 657 -- Include Fine Grained Error Locations in Tracebacks
Traceback (most recent call last):
File "calculation.py", line 54, in <module>
result = (x / y / z) * (a / b / c)
~~~~~~^~~
ZeroDivisionError: division by zero
Traceback (most recent call last):
File "distance.py", line 11, in <module>
print(manhattan_distance(p1, p2))
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "distance.py", line 6, in manhattan_distance
return abs(point_1.x - point_2.x) + abs(point_1.y - point_2.y)
^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'x'
Traceback (most recent call last):
File "query.py", line 37, in <module>
magic_arithmetic('foo')
File "query.py", line 18, in magic_arithmetic
return add_counts(x) / 25
^^^^^^^^^^^^^
File "query.py", line 24, in add_counts
return 25 + query_user(user1) + query_user(user2)
^^^^^^^^^^^^^^^^^
File "query.py", line 32, in query_user
return 1 + query_count(db, response['a']['b']['c']['user'], retry=True)
~~~~~~~~~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable
15.4.2. Traceback Analysis
Stacktrace is 8 levels deep, it's not Java's 200 ;)
Start analysing traceback from bottom-up
Figure 15.2. Stacktrace is 8 levels deep, it's not Java's 200 ;) [1]
>>> def apollo13():
... raise RuntimeError('Oxygen tank explosion')
Running script in Python console (REPL):
>>> apollo13() # doctest +SKIP
Traceback (most recent call last):
File "<input>", line 5, in <module>
File "<input>", line 2, in apollo13
RuntimeError: Oxygen tank explosion
Running Python script:
>>> apollo13() # doctest +SKIP
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
Running Python script in PyCharm:
>>> apollo13() # doctest +SKIP
Traceback (most recent call last):
File "<input>", line 1, in <module>
File "/Applications/PyCharm 2023.1.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 2023.1.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
15.4.3. Change Verbosity Level
Change level with
sys.tracebacklimitDefault traceback limit is 8
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
SetUp:
>>> import sys
>>>
>>> def apollo13():
... raise RuntimeError('Oxygen tank explosion')
>>> sys.tracebacklimit = 2
>>>
>>> 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
>>> sys.tracebacklimit = 0
>>>
>>> apollo13()
RuntimeError: Oxygen tank explosion
Note, that sys.tracebacklimit = 0 is useful in Python console (REPL).