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 information to user.

In the following example I want GET_PARAMS to return me a tuple of parameters supplied to os.chown. Examining the inspect module advised by Alex Martelli, I couldn't find that.

def catch_errors(fn):
    def decorator(*args, **kwargs):
        try:
            return fn(*args, **kwargs)
        except (IOError, OSError):
            msg = sys.exc_info()[2].tb_frame.f_locals['error_message']
            quit(msg.format(SEQUENCE_OF_PARAMETERS_OF_THE_LAST_FUNCTION_CALLED)\
            + '\nError #{0[0]}: {0[1]}'.format(sys.exc_info()[1].args), 1)
    return decorator

@catch_errors
def do_your_job():
    error_message = 'Can\'t change folder ownership \'{0}\' (uid:{1}, gid:{2})'
    os.chown('/root', 1000, 1000) # note that params aren't named vars.

if __name == '__main__' and os.getenv('USERNAME') != 'root':
    do_your_job()

(Thanks to Jim Robert for the decorator)

Answer 1

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 2

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.

Answer 3

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).