Exceptions and Tracebacks¶

so far we’ve ignored situations where errors occurred, but real software needs to handle errors or unexpected conditions all the time

>>> value = ' Aquamarine Falcon '
>>> float( value )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float:  Aquamarine Falcon

when functions call other functions, the system creates a “stack” of “frames”
- A stack is like a list where you can only add to the end or remove from the end
- A frame is the namespace where a function is run (including which line of the function is running)
An uncaught error in Python will, by default, print out a “traceback” of these frames to let you see what was happening when your error occurred.
When something goes wrong, you use the traceback to help you find out where and what the problem was
in python the traceback is ordered from “top” to “bottom”, that is, the “frame” printed first in the traceback (“<stdin>” in the example below) is the “top level” caller
in python, the last line of the traceback is a string representation of the Exception which was raised, which generally attempts to be a useful description of what went wrong

>>> def will_fail():
...     1/0
...
>>> def will_call():
...     will_fail()
...
>>> will_call()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "<console>", line 2, in will_call
  File "<console>", line 2, in will_fail
ZeroDivisionError: division by zero

it is possible to catch these Exceptions in Python by using a special type of block around the code in which the exception may occur

>>> value = '  Aquamarine Falcon '
>>> try:
...     value = float( value )
... except ValueError as err:
...     value = value.strip()
...
>>> value
'Aquamarine Falcon'

Note

We can catch multiple Exception types using except (ValueError,TypeError) as err instead.