How to print an exception in Python 3?

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有刺的猬
有刺的猬 2021-01-31 13:48

Right now, I catch the exception in the except Exception: clause, and do print(exception). The result provides no information since it always prints

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  • 2021-01-31 14:22

    These are the changes since python 2:

        try:
            1 / 0
        except Exception as e: # (as opposed to except Exception, e:)
                               # ^ that will just look for two classes, Exception and e
            # for the repr
            print(repr(e))
            # for just the message, or str(e), since print calls str under the hood
            print(e)
            # the arguments that the exception has been called with. 
            # the first one is usually the message. (OSError is different, though)
            print(e.args)
    

    You can look into the standard library module traceback for fancier stuff.

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  • 2021-01-31 14:30

    Although if you want a code that is compatible with both python2 and python3 you can use this:

    import logging
    try:
        1/0
    except Exception as e:
        if hasattr(e, 'message'):
            logging.warning('python2')
            logging.error(e.message)
        else:
            logging.warning('python3')
            logging.error(e)
    
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  • 2021-01-31 14:34

    I'm guessing that you need to assign the Exception to a variable. As shown in the Python 3 tutorial:

    def fails():
        x = 1 / 0
    
    try:
        fails()
    except Exception as ex:
        print(ex)
    

    To give a brief explanation, as is a pseudo-assignment keyword used in certain compound statements to assign or alias the preceding statement to a variable.

    In this case, as assigns the caught exception to a variable allowing for information about the exception to stored and used later, instead of needing to be dealt with immediately. (This is discussed in detail in the Python 3 Language Reference: The try Statement.)


    The other compound statement using as is the with statement:

    @contextmanager
    def opening(filename):
        f = open(filename)
        try:
            yield f
        finally:
            f.close()
    
    with opening(filename) as f:
        # ...read data from f...
    

    Here, with statements are used to wrap the execution of a block with methods defined by context managers. This functions like an extended try...except...finally statement in a neat generator package, and the as statement assigns the generator-produced result from the context manager to a variable for extended use. (This is discussed in detail in the Python 3 Language Reference: The with Statement.)


    Finally, as can be used when importing modules, to alias a module to a different (usually shorter) name:

    import foo.bar.baz as fbb
    

    This is discussed in detail in the Python 3 Language Reference: The import Statement.

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  • 2021-01-31 14:41

    Try

    try:
        print undefined_var
    except Exception as e:
        print(e)
    

    this will print the representation given by e.__str__():

    "name 'undefined_var' is not defined"

    you can also use:

    print(repr(e))
    

    which will include the Exception class name:

    "NameError("name 'undefined_var' is not defined",)"

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  • 2021-01-31 14:41

    Here is the way I like that prints out all of the error stack.

    import logging
    
    try:
        1 / 0
    except Exception as _e:
        # any one of the follows:
        # print(logging.traceback.format_exc())
        logging.error(logging.traceback.format_exc())
    

    The output looks as the follows:

    ERROR:root:Traceback (most recent call last):
      File "/PATH-TO-YOUR/filename.py", line 4, in <module>
        1 / 0
    ZeroDivisionError: division by zero
    

    LOGGING_FORMAT :

    LOGGING_FORMAT = '%(asctime)s\n  File "%(pathname)s", line %(lineno)d\n  %(levelname)s [%(message)s]'
    
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  • 2021-01-31 14:42

    [In Python3]

    Let's say you want to handle an IndexError and print the traceback, you can do the following:

    from traceback import print_tb 
    empty_list = [] 
    try: 
        x = empty_list[100]
    except IndexError as index_error: 
        print_tb(index_error.__traceback__)
    

    Note: You can use the format_tb function instead of print_tb to get the traceback as a string for logging purposes. Hope this helps.

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