9.11 (Optional) Stack Unwinding and Tracebacks

• Each exception object stores information indicating the precise series of function calls that led to the exception
• Helpful when debugging your code

def function1():
    function2()
    

def function2():
    raise Exception('An exception occurred')

• Calling function1 results in the following traceback

function1()

------------------------------------------------------------------------
Exception                              Traceback (most recent call last)
<ipython-input-3-c0b3cafe2087> in <module>
----> 1 function1()

<ipython-input-1-a9f4faeeeb0c> in function1()
      1 def function1():
----> 2     function2()
      3 

<ipython-input-2-ce858d2ad885> in function2()
      1 def function2():
----> 2     raise Exception('An exception occurred')

Exception: An exception occurred

Traceback Details

• Traceback shows the type of exception that occurred (Exception) followed by the complete function call stack that led to the raise point
• The stack’s bottom function call is listed first

Stack Unwinding

• When an exception is not caught in a given function, stack unwinding occurs
• For an uncaught exception, IPython displays the traceback
  • In interactive mode, IPython awaits your next input
  • In a typical script, the script would terminate

Tip for Reading Tracebacks

• When reading a traceback, start from the end of the traceback and read the error message first
• Then, read upward through the traceback, looking for the first line that indicates code you wrote in your program
• Typically, this is the location in your code that led to the exception

Exceptions in finally Suites

• Raising an exception in a finally suite can lead to subtle, hard-to-find problems
• A finally suite should always enclose in a try statement any code that may raise an exception

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