How to print an exception in Python 3?

Right now, I catch the exception in the except Exception: clause, and do print(exception). The result provides no information since it always prints <class 'Exception'>. I knew this used to work in python 2, but how do I do it in python3?

Solution 1:

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.)

There are other compound statements that use as. The first 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.)

As of Python 3.10, match statements also use as:

from random import randint

match randint(0, 2):
    case 0|1 as low:
        print(f"{low} is a low number")
    case _:
        print("not a low number")

match statements take an expression (in this case, randint(0, 2)) and compare its value to each case branch one at a time until one of them succeeds, at which point it executes that branch's block. In a case branch, as can be used to assign the value of the branch to a variable if that branch succeeds. If it doesn't succeed, it is not bound.

(The match statement is covered by the tutorial and discussed in detail in the Python 3 Language Reference: match Statements.)

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.

Solution 2:

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.