using the python traceback type

Question

By calling sys.exc_info() when an exception is handled a 3-tuple is returned containing the exception class, the exception object and the traceback.

This is also evident by the documentation of sys.exc_info :

This function returns a tuple of three values that give information about the exception that is currently being handled. ... the values returned are (type, value, traceback) . ... traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.

I want to use the type traceback which is used to create the third variable in the aforementioned exc_info return value but can't find where it's defined.

My question is, therefore, where is the traceback type available for python scripts?

EDIT:

I would like to use the traceback type to define a PyQt signal . PyQt Signals are defined by specifying the signal name together with the types of parameters passed. I do not need to create an object of that type, only use it in a manner similar to an isinstance call.

Answer 1

bad news, even if you can get the class of the traceback object like this:

import sys

try:
    raise Exception
except:
    tb = sys.exc_info()[2]

print(tb.__class__)

result:

<class 'traceback'>

when you try:

tb.__class__()

you get:

TypeError: cannot create 'traceback' instances

so the traceback type cannot be instanciated externally, probably because you'd need to access python internals to do so (and attributes are read-only even tb_lineno so not possible to "reuse" an instance either)

Answer 2

You can use traceback.format_exc() or sys.exc_info() like :

try:
    raise TypeError("Error !?!")
except Exception:
    print(traceback.format_exc())
    # or
    print(sys.exc_info()[2])

Answer 3

I see the tag includes python2.7 , and the question is old, so apologise for bringing this up again, but just wanted to say that in modern Python, the TracebackType is now in the types module:

>>> isinstance(sys.exc_info()[2], types.TracebackType))
True

Though it seems the class itself thinks it has a different name:

>>> repr(sys.exc_info()[2].__class__)
<class 'traceback'>

What is really interesting, though not the subject of the question, was an interpretation in one of the answers, is that it is now possible (since Python 3.7) to construct a traceback manually. This has interesting implications for accurately reflecting the reported traceback in an exception, and is particularly relevant to deferred/lazy/asynchronous exceptions. The change itself came in https://github.com/python/cpython/pull/4793 - I don't see any good docs/examples of this being done in the wild yet though. Please feel free to leave comments with such articles (or other StackOverflow questions which actually care about this).

Answer 4

That should be defined in C code:

Include/traceback.h

#ifndef Py_LIMITED_API
typedef struct _traceback {
    PyObject_HEAD
    struct _traceback *tb_next;
    struct _frame *tb_frame;
    int tb_lasti;
    int tb_lineno;
} PyTracebackObject;
#endif

Hm.. in fact, I am not very sure what you want to do, so cannot help more..