Professional IronPython

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CHAPTER 12
Debugging ironPython APPlicAtions
low-level functions in the 
sys
module. The following list is low-level modules you can use for special 
needs, but won’t normally use in your application.
➤➤
sys.excepthook(
type
, 
value
, 
traceback
)
: The system calls this low-level function each 
time it generates an exception. To use this function, you supply the same tuple of values as 
you receive when you call 
sys.exc_info()
.
➤
➤
sys._getframe([
depth
])
: The system calls this low-level function to display a frame 
object from the call stack. If the caller supplies a 
depth
value, the frame object is at that 
call stack depth. The default 
depth
value setting is 0. IronPython doesn’t appear to imple-
ment this function, but you may encounter it in other versions of Python, so it pays to 
know about this function.
If you want to control how much information the interpreter provides when you request a 
traceback
, 
you can always set the 
sys.tracebacklimit
attribute. The 
sys.tracebacklimit
attribute defaults to 
1,000. It doesn’t actually appear when you perform a 
dir()
command. In fact, until you set it, printing 
the 
sys.tracebacklimit
attribute returns an 
AttributeError
. Use code like this
sys.tracebacklimit = 3
to modify the 
traceback
level. Now when you try to print the 
sys.tracebacklimit
attribute, you 
get back the value you supplied.
Using the traceback Module
The 
traceback
module adds to the capabilities of the 
sys
module described in the “Using the sys 
Module” section of the chapter. In addition, it adds to the standard exception handling capabilities 
of IronPython by making it easier to obtain complex information about exceptions in general. The 
traceback
module does focus on 
traceback
s, which are the IronPython equivalent of a call stack.
The most common call is 
traceback.print_exc()
. Essentially, this call prints out the current 
exception information. You can use it in a 
try...except
structure, much as you’d use the
sys.exc_info()
function, but with fewer limitations. Figure 12-6 shows a typical view of
the 
traceback.print_exc()
function in action.
FIgURE 12-6: 
Obtain traceback information with ease using the traceback.print_exc() function.
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