Show only first and last line from traceback

Question

I've build a little internal DSL with python. I'm using assert vor validation. If the end user types a wrong parameter the dsl should report whats wrong. At the moment this looks like this:

Traceback (most recent call last):
  File "tests/maskedtimefilter_test/FilterDSL_test.py", line 63, in test_dsl_validation
    input(0): self.regular
  File "/Users/sh/reetz/pythonpath/maskedtimedata/maskedtimefilter.py", line 885, in __lshift__
    kwargs = self.dsl_validation(kwargs)
  File "/Users/sh/reetz/pythonpath/maskedtimedata/maskedtimefilter.py", line 1483, in dsl_validation
    check_if_valid(parameter)
  File "/Users/sh/reetz/pythonpath/maskedtimedata/dsl.py", line 47, in kernel_a
    def kernel_a (x): assert isinstance(x, (list, tuple, np.ndarray)), "kernel must be a list."
AssertionError: kernel must be a list.

But the end users are engineers and no computer scientists. Therefore a minimal Traceback is handy. Is it possible to shrink the Traceback to the essential information (where is the failure and what's the cause) like this?:

Traceback (most recent call last):
  File "tests/maskedtimefilter_test/FilterDSL_test.py", line 63, in test_dsl_validation
    input(0): self.regular
AssertionError: kernel must be a list.

Reluctantly I'd like to use normal prints!

Answer 1

Why not return the traceback data as an array and just work back from that?

import traceback
try:
     codethatwillthrowanexception()
except:
     exceptiondata = traceback.format_exc().splitlines()
     exceptionarray = [exceptiondata[-1]] + exceptiondata[1:-1]

Answer 2

Somewhere in your call stack you can do this:

try:
    my_function_that_can_raise_exception()
except Exception: #or AssertionError
    import traceback
    traceback.print_exc(limit=1)

limit is the depth of how many stack trace entries you want to show. (in your case, 1)

demo:

In [21]: def f():
    ...:     assert False == True, 'Assertion!'
    ...:     

In [22]: try:
    ...:     f()
    ...: except Exception:
    ...:     import traceback
    ...:     traceback.print_exc(limit=1)
    ...:     
Traceback (most recent call last):
  File "<ipython-input-22-78f9db8d5188>", line 2, in <module>
    f()
AssertionError: Assertion!

read more at traceback docs.

Answer 3

As the first part of the traceback is just what you called, you could so something simple, like:

try: 
    input(0): self.regular # base call
except AssertionError as ae: # catch the exception
    print(ae) # print out the message

You can print out whatever you think would be useful in the except block.

Alternatively, you could do something more complex with the traceback module.