Get exception description and stack trace which caused an exception, all as a string
Question:
How to convert a caught Exception
(its description and stack trace) into a str
for external use?
try:
method_that_can_raise_an_exception(params)
except Exception as e:
print(complete_exception_description(e))
Answers:
See the traceback
module, specifically the format_exc()
function. Here.
import traceback
try:
raise ValueError
except ValueError:
tb = traceback.format_exc()
else:
tb = "No error"
finally:
print tb
>>> import sys
>>> import traceback
>>> try:
... 5 / 0
... except ZeroDivisionError as e:
... type_, value_, traceback_ = sys.exc_info()
>>> traceback.format_tb(traceback_)
[' File "<stdin>", line 2, in <module>n']
>>> value_
ZeroDivisionError('integer division or modulo by zero',)
>>> type_
<type 'exceptions.ZeroDivisionError'>
>>>
>>> 5 / 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
You use sys.exc_info() to collect the information and the functions in the traceback
module to format it.
Here are some examples for formatting it.
The whole exception string is at:
>>> ex = traceback.format_exception(type_, value_, traceback_)
>>> ex
['Traceback (most recent call last):n', ' File "<stdin>", line 2, in <module>n', 'ZeroDivisionError: integer division or modulo by zeron']
Let’s create a decently complicated stacktrace, in order to demonstrate that we get the full stacktrace:
def raise_error():
raise RuntimeError('something bad happened!')
def do_something_that_might_error():
raise_error()
Logging the full stacktrace
A best practice is to have a logger set up for your module. It will know the name of the module and be able to change levels (among other attributes, such as handlers)
import logging
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)
And we can use this logger to get the error:
try:
do_something_that_might_error()
except Exception as error:
logger.exception(error)
Which logs:
ERROR:__main__:something bad happened!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
And so we get the same output as when we have an error:
>>> do_something_that_might_error()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
Getting just the string
If you really just want the string, use the traceback.format_exc
function instead, demonstrating logging the string here:
import traceback
try:
do_something_that_might_error()
except Exception as error:
just_the_string = traceback.format_exc()
logger.debug(just_the_string)
Which logs:
DEBUG:__main__:Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
You might also consider using the built-in Python module, cgitb, to get some really good, nicely formatted exception information including local variable values, source code context, function parameters etc..
For instance for this code…
import cgitb
cgitb.enable(format='text')
def func2(a, divisor):
return a / divisor
def func1(a, b):
c = b - 5
return func2(a, c)
func1(1, 5)
we get this exception output…
ZeroDivisionError
Python 3.4.2: C:toolspythonpython.exe
Tue Sep 22 15:29:33 2015
A problem occurred in a Python script. Here is the sequence of
function calls leading up to the error, in the order they occurred.
c:TEMPcgittest2.py in <module>()
7 def func1(a, b):
8 c = b - 5
9 return func2(a, c)
10
11 func1(1, 5)
func1 = <function func1>
c:TEMPcgittest2.py in func1(a=1, b=5)
7 def func1(a, b):
8 c = b - 5
9 return func2(a, c)
10
11 func1(1, 5)
global func2 = <function func2>
a = 1
c = 0
c:TEMPcgittest2.py in func2(a=1, divisor=0)
3
4 def func2(a, divisor):
5 return a / divisor
6
7 def func1(a, b):
a = 1
divisor = 0
ZeroDivisionError: division by zero
__cause__ = None
__class__ = <class 'ZeroDivisionError'>
__context__ = None
__delattr__ = <method-wrapper '__delattr__' of ZeroDivisionError object>
__dict__ = {}
__dir__ = <built-in method __dir__ of ZeroDivisionError object>
__doc__ = 'Second argument to a division or modulo operation was zero.'
__eq__ = <method-wrapper '__eq__' of ZeroDivisionError object>
__format__ = <built-in method __format__ of ZeroDivisionError object>
__ge__ = <method-wrapper '__ge__' of ZeroDivisionError object>
__getattribute__ = <method-wrapper '__getattribute__' of ZeroDivisionError object>
__gt__ = <method-wrapper '__gt__' of ZeroDivisionError object>
__hash__ = <method-wrapper '__hash__' of ZeroDivisionError object>
__init__ = <method-wrapper '__init__' of ZeroDivisionError object>
__le__ = <method-wrapper '__le__' of ZeroDivisionError object>
__lt__ = <method-wrapper '__lt__' of ZeroDivisionError object>
__ne__ = <method-wrapper '__ne__' of ZeroDivisionError object>
__new__ = <built-in method __new__ of type object>
__reduce__ = <built-in method __reduce__ of ZeroDivisionError object>
__reduce_ex__ = <built-in method __reduce_ex__ of ZeroDivisionError object>
__repr__ = <method-wrapper '__repr__' of ZeroDivisionError object>
__setattr__ = <method-wrapper '__setattr__' of ZeroDivisionError object>
__setstate__ = <built-in method __setstate__ of ZeroDivisionError object>
__sizeof__ = <built-in method __sizeof__ of ZeroDivisionError object>
__str__ = <method-wrapper '__str__' of ZeroDivisionError object>
__subclasshook__ = <built-in method __subclasshook__ of type object>
__suppress_context__ = False
__traceback__ = <traceback object>
args = ('division by zero',)
with_traceback = <built-in method with_traceback of ZeroDivisionError object>
The above is a description of an error in a Python program. Here is
the original traceback:
Traceback (most recent call last):
File "cgittest2.py", line 11, in <module>
func1(1, 5)
File "cgittest2.py", line 9, in func1
return func2(a, c)
File "cgittest2.py", line 5, in func2
return a / divisor
ZeroDivisionError: division by zero
my 2-cents:
import sys, traceback
try:
...
except Exception, e:
T, V, TB = sys.exc_info()
print ''.join(traceback.format_exception(T,V,TB))
With Python 3, the following code will format an Exception
object exactly as would be obtained using traceback.format_exc()
:
import traceback
try:
method_that_can_raise_an_exception(params)
except Exception as ex:
print(''.join(traceback.format_exception(etype=type(ex), value=ex, tb=ex.__traceback__)))
The advantage being that only the Exception
object is needed (thanks to the recorded __traceback__
attribute), and can therefore be more easily passed as an argument to another function for further processing.
For those using Python-3
Using traceback
module and exception.__traceback__
one can extract the stack-trace as follows:
- grab the current stack-trace using
traceback.extract_stack()
- remove the last three elements (as those are entries in the stack that got me to my debug function)
- append the
__traceback__
from the exception object using traceback.extract_tb()
- format the whole thing using
traceback.format_list()
import traceback
def exception_to_string(excp):
stack = traceback.extract_stack()[:-3] + traceback.extract_tb(excp.__traceback__) # add limit=??
pretty = traceback.format_list(stack)
return ''.join(pretty) + 'n {} {}'.format(excp.__class__,excp)
A simple demonstration:
def foo():
try:
something_invalid()
except Exception as e:
print(exception_to_string(e))
def bar():
return foo()
We get the following output when we call bar()
:
File "./test.py", line 57, in <module>
bar()
File "./test.py", line 55, in bar
return foo()
File "./test.py", line 50, in foo
something_invalid()
<class 'NameError'> name 'something_invalid' is not defined
I defined following helper class:
import traceback
class TracedExeptions(object):
def __init__(self):
pass
def __enter__(self):
pass
def __exit__(self, etype, value, tb):
if value :
if not hasattr(value, 'traceString'):
value.traceString = "n".join(traceback.format_exception(etype, value, tb))
return False
return True
Which I can later use like this:
with TracedExeptions():
#some-code-which-might-throw-any-exception
And later can consume it like this:
def log_err(ex):
if hasattr(ex, 'traceString'):
print("ERROR:{}".format(ex.traceString));
else:
print("ERROR:{}".format(ex));
(Background: I was frustraded because of using Promise
s together with Exception
s, which unfortunately passes exceptions raised in one place to a on_rejected handler in another place, and thus it is difficult to get the traceback from original location)
If you would like to get the same information given when an exception isn’t handled you can do something like this. Do import traceback
and then:
try:
...
except Exception as e:
print(traceback.print_tb(e.__traceback__))
I’m using Python 3.7.
For Python 3.5+:
So, you can get the stacktrace from your exception as from any other exception. Use traceback.TracebackException
for it (just replace ex
with your exception):
print("".join(traceback.TracebackException.from_exception(ex).format()))
An extended example and other features to do this:
import traceback
try:
1/0
except Exception as ex:
print("".join(traceback.TracebackException.from_exception(ex).format()) == traceback.format_exc() == "".join(traceback.format_exception(type(ex), ex, ex.__traceback__))) # This is True !!
print("".join(traceback.TracebackException.from_exception(ex).format()))
The output will be something like this:
True
Traceback (most recent call last):
File "your_file_name_here.py", line 29, in <module>
1/0
ZeroDivisionError: division by zero
If your goal is to make the exception and stacktrace message look exactly like when python throws an error, the following works in both python 2+3:
import sys, traceback
def format_stacktrace():
parts = ["Traceback (most recent call last):n"]
parts.extend(traceback.format_stack(limit=25)[:-2])
parts.extend(traceback.format_exception(*sys.exc_info())[1:])
return "".join(parts)
# EXAMPLE BELOW...
def a():
b()
def b():
c()
def c():
d()
def d():
assert False, "Noooh don't do it."
print("THIS IS THE FORMATTED STRING")
print("============================n")
try:
a()
except:
stacktrace = format_stacktrace()
print(stacktrace)
print("THIS IS HOW PYTHON DOES IT")
print("==========================n")
a()
It works by removing the last format_stacktrace()
call from the stack and joining the rest. When run, the example above gives the following output:
THIS IS THE FORMATTED STRING
============================
Traceback (most recent call last):
File "test.py", line 31, in <module>
a()
File "test.py", line 12, in a
b()
File "test.py", line 16, in b
c()
File "test.py", line 20, in c
d()
File "test.py", line 24, in d
assert False, "Noooh don't do it."
AssertionError: Noooh don't do it.
THIS IS HOW PYTHON DOES IT
==========================
Traceback (most recent call last):
File "test.py", line 38, in <module>
a()
File "test.py", line 12, in a
b()
File "test.py", line 16, in b
c()
File "test.py", line 20, in c
d()
File "test.py", line 24, in d
assert False, "Noooh don't do it."
AssertionError: Noooh don't do it.
If you would like to convert your traceback to a list of dict (for python > 3.5):
from traceback import TracebackException
def list_traceback(exc_value: BaseException):
result = list()
# get previous fails, so errors are appended by order of execution
if exc_value.__context__:
result += list_traceback(exc_value.__context__)
# convert Exception into TracebackException
tbe = TracebackException.from_exception(exc_value)
# get stacktrace (cascade methods calls)
error_lines = list()
for frame_summary in tbe.stack:
summary_details = {
'filename': frame_summary.filename,
'method' : frame_summary.name,
'lineno' : frame_summary.lineno,
'code' : frame_summary.line
}
error_lines.append(summary_details)
# append error, by order of execution
result.append({"error_lines": error_lines,
"type" : tbe.exc_type.__name__,
"message" : str(tbe)})
return result
This will be (an example of) the result:
[
{
"error_lines": [
{
"filename": "/home/demo/file2.py",
"method": "do_error_2",
"lineno": 18,
"code": "a=1/0"
}
],
"type": "ZeroDivisionError",
"message": "division by zero"
},
{
"error_lines": [
{
"filename": "/home/demo/file_main.py",
"method": "demo2",
"lineno": 50,
"code": "file2.DEMO().do_error_2()"
},
{
"filename": "/home/demo/file2.py",
"method": "do_error_2",
"lineno": 20,
"code": "raise AssertionError("Raised inside the except, after division by zero")"
}
],
"type": "AssertionError",
"message": "Raised inside the except, after division by zero"
}
]
How to convert a caught Exception
(its description and stack trace) into a str
for external use?
try:
method_that_can_raise_an_exception(params)
except Exception as e:
print(complete_exception_description(e))
See the traceback
module, specifically the format_exc()
function. Here.
import traceback
try:
raise ValueError
except ValueError:
tb = traceback.format_exc()
else:
tb = "No error"
finally:
print tb
>>> import sys
>>> import traceback
>>> try:
... 5 / 0
... except ZeroDivisionError as e:
... type_, value_, traceback_ = sys.exc_info()
>>> traceback.format_tb(traceback_)
[' File "<stdin>", line 2, in <module>n']
>>> value_
ZeroDivisionError('integer division or modulo by zero',)
>>> type_
<type 'exceptions.ZeroDivisionError'>
>>>
>>> 5 / 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
You use sys.exc_info() to collect the information and the functions in the traceback
module to format it.
Here are some examples for formatting it.
The whole exception string is at:
>>> ex = traceback.format_exception(type_, value_, traceback_)
>>> ex
['Traceback (most recent call last):n', ' File "<stdin>", line 2, in <module>n', 'ZeroDivisionError: integer division or modulo by zeron']
Let’s create a decently complicated stacktrace, in order to demonstrate that we get the full stacktrace:
def raise_error():
raise RuntimeError('something bad happened!')
def do_something_that_might_error():
raise_error()
Logging the full stacktrace
A best practice is to have a logger set up for your module. It will know the name of the module and be able to change levels (among other attributes, such as handlers)
import logging
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)
And we can use this logger to get the error:
try:
do_something_that_might_error()
except Exception as error:
logger.exception(error)
Which logs:
ERROR:__main__:something bad happened!
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
And so we get the same output as when we have an error:
>>> do_something_that_might_error()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
Getting just the string
If you really just want the string, use the traceback.format_exc
function instead, demonstrating logging the string here:
import traceback
try:
do_something_that_might_error()
except Exception as error:
just_the_string = traceback.format_exc()
logger.debug(just_the_string)
Which logs:
DEBUG:__main__:Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "<stdin>", line 2, in do_something_that_might_error
File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!
You might also consider using the built-in Python module, cgitb, to get some really good, nicely formatted exception information including local variable values, source code context, function parameters etc..
For instance for this code…
import cgitb
cgitb.enable(format='text')
def func2(a, divisor):
return a / divisor
def func1(a, b):
c = b - 5
return func2(a, c)
func1(1, 5)
we get this exception output…
ZeroDivisionError
Python 3.4.2: C:toolspythonpython.exe
Tue Sep 22 15:29:33 2015
A problem occurred in a Python script. Here is the sequence of
function calls leading up to the error, in the order they occurred.
c:TEMPcgittest2.py in <module>()
7 def func1(a, b):
8 c = b - 5
9 return func2(a, c)
10
11 func1(1, 5)
func1 = <function func1>
c:TEMPcgittest2.py in func1(a=1, b=5)
7 def func1(a, b):
8 c = b - 5
9 return func2(a, c)
10
11 func1(1, 5)
global func2 = <function func2>
a = 1
c = 0
c:TEMPcgittest2.py in func2(a=1, divisor=0)
3
4 def func2(a, divisor):
5 return a / divisor
6
7 def func1(a, b):
a = 1
divisor = 0
ZeroDivisionError: division by zero
__cause__ = None
__class__ = <class 'ZeroDivisionError'>
__context__ = None
__delattr__ = <method-wrapper '__delattr__' of ZeroDivisionError object>
__dict__ = {}
__dir__ = <built-in method __dir__ of ZeroDivisionError object>
__doc__ = 'Second argument to a division or modulo operation was zero.'
__eq__ = <method-wrapper '__eq__' of ZeroDivisionError object>
__format__ = <built-in method __format__ of ZeroDivisionError object>
__ge__ = <method-wrapper '__ge__' of ZeroDivisionError object>
__getattribute__ = <method-wrapper '__getattribute__' of ZeroDivisionError object>
__gt__ = <method-wrapper '__gt__' of ZeroDivisionError object>
__hash__ = <method-wrapper '__hash__' of ZeroDivisionError object>
__init__ = <method-wrapper '__init__' of ZeroDivisionError object>
__le__ = <method-wrapper '__le__' of ZeroDivisionError object>
__lt__ = <method-wrapper '__lt__' of ZeroDivisionError object>
__ne__ = <method-wrapper '__ne__' of ZeroDivisionError object>
__new__ = <built-in method __new__ of type object>
__reduce__ = <built-in method __reduce__ of ZeroDivisionError object>
__reduce_ex__ = <built-in method __reduce_ex__ of ZeroDivisionError object>
__repr__ = <method-wrapper '__repr__' of ZeroDivisionError object>
__setattr__ = <method-wrapper '__setattr__' of ZeroDivisionError object>
__setstate__ = <built-in method __setstate__ of ZeroDivisionError object>
__sizeof__ = <built-in method __sizeof__ of ZeroDivisionError object>
__str__ = <method-wrapper '__str__' of ZeroDivisionError object>
__subclasshook__ = <built-in method __subclasshook__ of type object>
__suppress_context__ = False
__traceback__ = <traceback object>
args = ('division by zero',)
with_traceback = <built-in method with_traceback of ZeroDivisionError object>
The above is a description of an error in a Python program. Here is
the original traceback:
Traceback (most recent call last):
File "cgittest2.py", line 11, in <module>
func1(1, 5)
File "cgittest2.py", line 9, in func1
return func2(a, c)
File "cgittest2.py", line 5, in func2
return a / divisor
ZeroDivisionError: division by zero
my 2-cents:
import sys, traceback
try:
...
except Exception, e:
T, V, TB = sys.exc_info()
print ''.join(traceback.format_exception(T,V,TB))
With Python 3, the following code will format an Exception
object exactly as would be obtained using traceback.format_exc()
:
import traceback
try:
method_that_can_raise_an_exception(params)
except Exception as ex:
print(''.join(traceback.format_exception(etype=type(ex), value=ex, tb=ex.__traceback__)))
The advantage being that only the Exception
object is needed (thanks to the recorded __traceback__
attribute), and can therefore be more easily passed as an argument to another function for further processing.
For those using Python-3
Using traceback
module and exception.__traceback__
one can extract the stack-trace as follows:
- grab the current stack-trace using
traceback.extract_stack()
- remove the last three elements (as those are entries in the stack that got me to my debug function)
- append the
__traceback__
from the exception object usingtraceback.extract_tb()
- format the whole thing using
traceback.format_list()
import traceback
def exception_to_string(excp):
stack = traceback.extract_stack()[:-3] + traceback.extract_tb(excp.__traceback__) # add limit=??
pretty = traceback.format_list(stack)
return ''.join(pretty) + 'n {} {}'.format(excp.__class__,excp)
A simple demonstration:
def foo():
try:
something_invalid()
except Exception as e:
print(exception_to_string(e))
def bar():
return foo()
We get the following output when we call bar()
:
File "./test.py", line 57, in <module>
bar()
File "./test.py", line 55, in bar
return foo()
File "./test.py", line 50, in foo
something_invalid()
<class 'NameError'> name 'something_invalid' is not defined
I defined following helper class:
import traceback
class TracedExeptions(object):
def __init__(self):
pass
def __enter__(self):
pass
def __exit__(self, etype, value, tb):
if value :
if not hasattr(value, 'traceString'):
value.traceString = "n".join(traceback.format_exception(etype, value, tb))
return False
return True
Which I can later use like this:
with TracedExeptions():
#some-code-which-might-throw-any-exception
And later can consume it like this:
def log_err(ex):
if hasattr(ex, 'traceString'):
print("ERROR:{}".format(ex.traceString));
else:
print("ERROR:{}".format(ex));
(Background: I was frustraded because of using Promise
s together with Exception
s, which unfortunately passes exceptions raised in one place to a on_rejected handler in another place, and thus it is difficult to get the traceback from original location)
If you would like to get the same information given when an exception isn’t handled you can do something like this. Do import traceback
and then:
try:
...
except Exception as e:
print(traceback.print_tb(e.__traceback__))
I’m using Python 3.7.
For Python 3.5+:
So, you can get the stacktrace from your exception as from any other exception. Use traceback.TracebackException
for it (just replace ex
with your exception):
print("".join(traceback.TracebackException.from_exception(ex).format()))
An extended example and other features to do this:
import traceback
try:
1/0
except Exception as ex:
print("".join(traceback.TracebackException.from_exception(ex).format()) == traceback.format_exc() == "".join(traceback.format_exception(type(ex), ex, ex.__traceback__))) # This is True !!
print("".join(traceback.TracebackException.from_exception(ex).format()))
The output will be something like this:
True
Traceback (most recent call last):
File "your_file_name_here.py", line 29, in <module>
1/0
ZeroDivisionError: division by zero
If your goal is to make the exception and stacktrace message look exactly like when python throws an error, the following works in both python 2+3:
import sys, traceback
def format_stacktrace():
parts = ["Traceback (most recent call last):n"]
parts.extend(traceback.format_stack(limit=25)[:-2])
parts.extend(traceback.format_exception(*sys.exc_info())[1:])
return "".join(parts)
# EXAMPLE BELOW...
def a():
b()
def b():
c()
def c():
d()
def d():
assert False, "Noooh don't do it."
print("THIS IS THE FORMATTED STRING")
print("============================n")
try:
a()
except:
stacktrace = format_stacktrace()
print(stacktrace)
print("THIS IS HOW PYTHON DOES IT")
print("==========================n")
a()
It works by removing the last format_stacktrace()
call from the stack and joining the rest. When run, the example above gives the following output:
THIS IS THE FORMATTED STRING
============================
Traceback (most recent call last):
File "test.py", line 31, in <module>
a()
File "test.py", line 12, in a
b()
File "test.py", line 16, in b
c()
File "test.py", line 20, in c
d()
File "test.py", line 24, in d
assert False, "Noooh don't do it."
AssertionError: Noooh don't do it.
THIS IS HOW PYTHON DOES IT
==========================
Traceback (most recent call last):
File "test.py", line 38, in <module>
a()
File "test.py", line 12, in a
b()
File "test.py", line 16, in b
c()
File "test.py", line 20, in c
d()
File "test.py", line 24, in d
assert False, "Noooh don't do it."
AssertionError: Noooh don't do it.
If you would like to convert your traceback to a list of dict (for python > 3.5):
from traceback import TracebackException
def list_traceback(exc_value: BaseException):
result = list()
# get previous fails, so errors are appended by order of execution
if exc_value.__context__:
result += list_traceback(exc_value.__context__)
# convert Exception into TracebackException
tbe = TracebackException.from_exception(exc_value)
# get stacktrace (cascade methods calls)
error_lines = list()
for frame_summary in tbe.stack:
summary_details = {
'filename': frame_summary.filename,
'method' : frame_summary.name,
'lineno' : frame_summary.lineno,
'code' : frame_summary.line
}
error_lines.append(summary_details)
# append error, by order of execution
result.append({"error_lines": error_lines,
"type" : tbe.exc_type.__name__,
"message" : str(tbe)})
return result
This will be (an example of) the result:
[
{
"error_lines": [
{
"filename": "/home/demo/file2.py",
"method": "do_error_2",
"lineno": 18,
"code": "a=1/0"
}
],
"type": "ZeroDivisionError",
"message": "division by zero"
},
{
"error_lines": [
{
"filename": "/home/demo/file_main.py",
"method": "demo2",
"lineno": 50,
"code": "file2.DEMO().do_error_2()"
},
{
"filename": "/home/demo/file2.py",
"method": "do_error_2",
"lineno": 20,
"code": "raise AssertionError("Raised inside the except, after division by zero")"
}
],
"type": "AssertionError",
"message": "Raised inside the except, after division by zero"
}
]