For instance, consider the case: <pre class="prettyprint"><code>>>> a = [] >>> a[12] Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range </code></pre> the exception does not print the value that was out of range. My guess is that we don't know if the <code>__str__</code> function of whatever was passed in raises an exception, so we don't touch it?

Why do python exceptions typically not print offending values?

For instance, consider the case:

>>> a = []
>>> a[12]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

the exception does not print the value that was out of range.

My guess is that we don't know if the __str__ function of whatever was passed in raises an exception, so we don't touch it?

The value could be printed, it just isn't.

However, there is a tb module on PyPI that actually will print the values of the variables in the stack trace.