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Python Cookbook by David Ascher, Alex Martelli

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Getting More Information from Tracebacks

Credit: Bryn Keller

Problem

You want to display all of the available information when an uncaught exception is raised.

Solution

A traceback object is basically a linked list of nodes, in which each node refers to a frame object. Frame objects, in turn, form their own linked list opposite the linked list of traceback nodes, so we can walk back and forth if needed. This recipe exploits this structure and the rich amount of information held by frame objects, including the dictionary of local variables for the function corresponding to each frame, in particular:

import sys, traceback

def print_exc_plus(  ):
    """
    Print the usual traceback information, followed by a listing of all the
    local variables in each frame.
    """
    tb = sys.exc_info(  )[2]
    while 1:
        if not tb.tb_next:
            break
        tb = tb.tb_next
    stack = []
    f = tb.tb_frame
    while f:
        stack.append(f)
        f = f.f_back
    stack.reverse(  )
    traceback.print_exc(  )
    print "Locals by frame, innermost last"
    for frame in stack:
        print
        print "Frame %s in %s at line %s" % (frame.f_code.co_name,
                                             frame.f_code.co_filename,
                                             frame.f_lineno)
        for key, value in frame.f_locals.items(  ):
            print "\t%20s = " % key,
            # We have to be VERY careful not to cause a new error in our error
            # printer! Calling str(  ) on an unknown object could cause an
            # error we don't want, so we must use try/except to catch it --
            # we can't stop it from happening, but we can and should
            # stop it from propagating if it does happen!
            try:
                print value
            except:
                print "<ERROR WHILE PRINTING VALUE>"

Discussion

The standard Python traceback module provides useful functions to produce lots of information about where and why an error occurred. However, traceback objects actually contain a great deal more information than the traceback module displays (indirectly, via the frame objects they refer to). This extra information can greatly assist in detecting the cause of some errors you encounter. This recipe gives an example of an extended traceback printer you might use.

Here’s a simplistic demonstration of the kind of problem this approach can help with. Basically, we have a simple function that manipulates all the strings in a list. The function doesn’t do any error checking, so when we pass a list that contains something other than strings, we get an error. Figuring out which bad data caused the error is easier with our new print_exc_plus function to help us:

data = ["1", "2", 3, "4"]     # Typo: we 'forget' the quotes on data[2]
def pad4(seq):
    """
    Pad each string in seq with zeros up to four places. Note that there
    is no reason to actually write this function; Python already
    does this sort of thing much better. It's just an example!
    """
    return_value = []
    for thing in seq:
        return_value.append("0" * (4 - len(thing)) + thing)
    return return_value

Here’s the (limited) information we get from a normal traceback.print_exc:

>>> try:
...     pad4(data)
... except:
...     traceback.print_exc(  )
...
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
  File "<stdin>", line 9, in pad4
TypeError: len(  ) of unsized object

Now here’s how it looks with our new function:

>>> try:
...     pad4(data)
... except:
...     print_exc_plus(  )
...
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
  File "<stdin>", line 9, in pad4
TypeError: len(  ) of unsized object
Locals by frame, innermost last

Frame ? in <stdin> at line 4
                         sys =  <module 'sys' (built-in)>
                        pad4 =  <function pad4 at 0x007C6210>
                _ _builtins_ _ =  <module '_ _builtin_ _' (built-in)>
                    _ _name_ _ =  _ _main_ _
                   traceback =  <module 'traceback' from 'C:\Python22\lib\traceback.py'>
                        data =  ['1', '2', 3, '4']
                     _ _doc_ _ =  None
              print_exc_plus =  <function print_exc_plus at 0x00802038>

Frame pad4 in <stdin> at line 9
                       thing =  3
                return_value =  ['0001', '0002']
                         seq =  ['1', '2', 3, '4']

Note how easy it is to see the bad data that caused the problem. The thing variable has a value of 3, so we know that the TypeError we got was because of this. A quick look at the value for data shows that we simply forgot the quotes on that item.

So we can either fix the data or decide to make pad4 a bit more tolerant (e.g., by changing the loop to for thing in map(str,seq):). This kind of thing is an important design choice, but the point of this recipe is to save you time in understanding what’s going on, so you can make your design choices with all the available information.

The recipe relies on the fact that each traceback object refers to the next traceback object in the stack through the tb_next field, forming a linked list. Each traceback object also refers to a corresponding frame object through the tb_frame field, and each frame refers to the previous frame through the f_back field (a linked list going the other way around from that of the traceback objects).

For simplicity, the recipe accumulates references to all the frame objects in a local list called stack, then loops over the list, emitting information about each frame. For each frame, it first emits some basic information (function name, filename, line number, and so on) then turns to the dictionary representing the local variables of the frame, to which the f_locals field refers. Just like for the dictionaries built and returned by the locals and globals built-in functions, each key is a variable name, and the corresponding value is the variable’s value. The only point of note here is that while printing the name is safe (it’s just a string), printing the value might fail, because it could invoke an arbitrary and buggy _ _str_ _ method of a user-defined object. So the value is printed within a try/except statement to prevent raising an uncaught exception while handling another exception.

I use a technique very similar to this in the applications I develop. Unexpected errors are logged in a format like this, which makes it a lot easier to figure out what’s gone wrong.

See Also

Recipe 14.3 and Recipe 14.4; documentation on the traceback module and the exc_info function in the sys module in the Library Reference.

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