Python in a Nutshell

Name

stack

Synopsis

stack(context=1)

Returns a list of six-item tuples. The first tuple is about stack’s caller, the second tuple is about the caller’s caller, and so on. Each tuple’s items, in order, are: frame object, filename, line number, function name, list of context source code lines around the current line, and index of current line within the list.

For example, suppose that at some point in your program you execute a statement such as:

x.f( )

and unexpectedly receive an AttributeError informing you that object x has no attribute named f. This means that object x is not as you expected, so you want to determine more about x as a preliminary to ascertaining why x is that way and what you should do about it. Change the statement to:

try: x.f( )
except AttributeError:
    import sys, inspect
    sys.stderr.write('x is type %s,(%r)\n'%(type(x),x))
    sys.stderr.write("x's methods are: ")
    for n, v in inspect.getmembers(x, callable):
       sys.stderr.write('%s '%n)
    sys.stderr.write('\n')
    raise

This example uses sys.stderr (covered in Chapter 8), since it’s displaying diagnostic information related to an error, not program results. Function getmembers of module inspect obtains the name of all methods available on x in order to display them. Of course, if you need this kind of diagnostic functionality often, you should package it up into a separate function, such as:

import sys, inspect def show_obj_methods(obj, name, show=sys.stderr.write): show('%s is type %s(%r)\n'%(name,obj,type(obj))) show("%s's methods ...