How do I raise the same Exception with a custom message in Python?

Question

I have this try block in my code:

try:
    do_something_that_might_raise_an_exception()
except ValueError as err:
    errmsg = \'My custom error         


        

Answer 1

Update: For Python 3, check Ben's answer

---

To attach a message to the current exception and re-raise it: (the outer try/except is just to show the effect)

For python 2.x where x>=6:

try:
    try:
      raise ValueError  # something bad...
    except ValueError as err:
      err.message=err.message+" hello"
      raise              # re-raise current exception
except ValueError as e:
    print(" got error of type "+ str(type(e))+" with message " +e.message)

This will also do the right thing if err is derived from ValueError. For example UnicodeDecodeError.

Note that you can add whatever you like to err. For example err.problematic_array=[1,2,3].

---

Edit: @Ducan points in a comment the above does not work with python 3 since .message is not a member of ValueError. Instead you could use this (valid python 2.6 or later or 3.x):

try:
    try:
      raise ValueError
    except ValueError as err:
       if not err.args: 
           err.args=('',)
       err.args = err.args + ("hello",)
       raise 
except ValueError as e:
    print(" error was "+ str(type(e))+str(e.args))

Edit2:

Depending on what the purpose is, you can also opt for adding the extra information under your own variable name. For both python2 and python3:

try:
    try:
      raise ValueError
    except ValueError as err:
       err.extra_info = "hello"
       raise 
except ValueError as e:
    print(" error was "+ str(type(e))+str(e))
    if 'extra_info' in dir(e):
       print e.extra_info