The standard Python traceback module provides very useful functions to produce useful information about where and why an error occurred. Traceback objects actually contain a great deal more information than the traceback module displays, however. That information can greatly assist in detecting the cause of your error.
Here's an example of an extended traceback printer you might use, followed by a usage example.
Python, 74 lines
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Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 | 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 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.
try:
print value
except:
print "<ERROR WHILE PRINTING VALUE>"
if __name__ == '__main__':
#A simplistic demonstration of the kind of problem this approach can help
#with. Basically, we have a simple function which manipulates all the
#strings in a list. The function doesn't do any error checking, so when
#we pass a list which contains something other than strings, we get an
#error. Figuring out what bad data caused the error is easier with our
#new function.
data = ["1", "2", 3, "4"] #Typo: We 'forget' the quotes on data[2]
def pad4(seq):
"""
Pad each string in seq with zeros, to four places. Note there
is no reason to actually write this function, Python already
does this sort of thing much better.
Just an example.
"""
return_value = []
for thing in seq:
return_value.append("0" * (4 - len(thing)) + thing)
return return_value
#First, show the information we get from a normal traceback.print_exc().
try:
pad4(data)
except:
traceback.print_exc()
print
print "----------------"
print
#Now with our new function. Note how easy it is to see the bad data that
#caused the problem. The variable 'thing' has the value 3, so we know
#that the TypeError we got was because of that. A quick look at the
#value for 'data' shows us we simply forgot the quotes on that item.
try:
pad4(data)
except:
print_exc_plus()
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I use a technique very similar to this in the application I develop. Unexpected errors get logged in a format like this, which makes it a lot easier to figure out what's gone wrong.
This can be used to create a generic debug script. I wrote something similar to this, but this is nicer. I wrapped it in a separate script called debug.py. debug.py takes a parameter which is the script you wish to debug:
Then you can run
debug.py myscript.pyThis is useful for extending editors. e.g. I assign this to F7 in SciTE.
Code to get frames is poor. The code that gets the frames is poor and will fail for certain types of frames. It is also way too complicated and long. Start the function like this and it will on all versions of Python with all types of frame.