Get last function's call arguments from traceback?

Question

Can I get the parameters of the last function called in traceback? How?

I want to make a catcher for standard errors to make readable code, yet provide detailed info

Answer 1

Here is an example of such function and some problems that you can't get around:

import sys

def get_params(tb):
    while tb.tb_next:
        tb = tb.tb_next
    frame = tb.tb_frame
    code = frame.f_code
    argcount = code.co_argcount
    if code.co_flags & 4: # *args
        argcount += 1
    if code.co_flags & 8: # **kwargs
        argcount += 1
    names = code.co_varnames[:argcount]
    params = {}
    for name in names:
        params[name] = frame.f_locals.get(name, '<deleted>')
    return params


def f(a, b=2, c=3, *d, **e):
    del c
    c = 4
    e['g'] = 6
    assert False

try:
    f(1, f=5)
except:
    print get_params(sys.exc_info()[2])

The output is:

{'a': 1, 'c': 4, 'b': 2, 'e': {'g': 6, 'f': 5}, 'd': ()}

I didn't used inspect.getinnerframes() to show another way to get needed frame. Although it simplifies a bit, it also do some extra work that is not needed for you while being relatively slow (inspect.getinnerframes() reads source file for every module in traceback; this is not important for one debugging call, but could be an issue in other cases).

Answer 2

The problem with using a decorator for what you're trying to achieve is that the frame the exception handler gets is do_your_job()s, not os.listdir()s, os.makedirs()s or os.chown()s. So the information you'll be printing out is the arguments to do_your_job(). In order to get the behavior I think you intend, you would have to decorate all the library functions you're calling.

Answer 3

For such inspection tasks, always think first of module inspect in the standard library. Here, inspect.getargvalues gives you the argument values given a frame, and inspect.getinnerframes gives you the frames of interest from a traceback object.