[Python-ideas] introspectable assertions

[Steven D'Aprano]

On Thu, Feb 19, 2015 at 08:16:33AM +0100, Ronny Pfannschmidt wrote:
> 
> Hi,
> 
> the idea is that if 
> 
> -> from __future__ import assertion_introspection
> 
> then 
> 
> -> assert foo.bar(baz) == abc.def(), msg
> DetailedAssertionError: {msg}
> foo -> <foo...>
> baz -> <bar ...>
> foo.bar() -> <...>
> abc -> ...
> abc.def() -> ...
> foo.bar(baz) == abc.def() -> ...
> 
> it would help introspection and testing utilities

I don't think we need a __future__ import, or to special case assert. 
Python already has hooks for customizing tracebacks, see the cgitb 
module. With it:

py> assert 23 == 24
AssertionError
Python 3.3.0rc3: /usr/local/bin/python3.3
Thu Feb 19 21:51:04 2015

A problem occurred in a Python script.  Here is the sequence of
function calls leading up to the error, in the order they occurred.

 /home/steve/<stdin> in <module>()

AssertionError:
    __cause__ = None
    __class__ = <class 'AssertionError'>
    __context__ = None
    __delattr__ = <method-wrapper '__delattr__' of AssertionError object>
    __dict__ = {}
    __dir__ = <built-in method __dir__ of AssertionError object>
    __doc__ = 'Assertion failed.'
    __eq__ = <method-wrapper '__eq__' of AssertionError object>
    __format__ = <built-in method __format__ of AssertionError object>
    __ge__ = <method-wrapper '__ge__' of AssertionError object>
    __getattribute__ = <method-wrapper '__getattribute__' of AssertionError object>
    __gt__ = <method-wrapper '__gt__' of AssertionError object>
    __hash__ = <method-wrapper '__hash__' of AssertionError object>
    __init__ = <method-wrapper '__init__' of AssertionError object>
    __le__ = <method-wrapper '__le__' of AssertionError object>
    __lt__ = <method-wrapper '__lt__' of AssertionError object>
    __ne__ = <method-wrapper '__ne__' of AssertionError object>
    __new__ = <built-in method __new__ of type object>
    __reduce__ = <built-in method __reduce__ of AssertionError object>
    __reduce_ex__ = <built-in method __reduce_ex__ of AssertionError object>
    __repr__ = <method-wrapper '__repr__' of AssertionError object>
    __setattr__ = <method-wrapper '__setattr__' of AssertionError object>
    __setstate__ = <built-in method __setstate__ of AssertionError object>
    __sizeof__ = <built-in method __sizeof__ of AssertionError object>
    __str__ = <method-wrapper '__str__' of AssertionError object>
    __subclasshook__ = <built-in method __subclasshook__ of type object>
    __suppress_context__ = False
    __traceback__ = <traceback object>
    args = ()
    with_traceback = <built-in method with_traceback of AssertionError object>

The above is a description of an error in a Python program.  Here is
the original traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError



I don't know that I would actually use the cgitb module in production, 
it appears to display a lot of noise, but I think it is a good place to 
start.


-- 
Steve