Page 155 -

persistence

When an exception is raised and handled by your except suite, the Python interpreter passes an exception object
into the suite. A small change makes this exception object available to your code as an identifier:

Give your exception object
except IOError as err: a name…
print('File error: ' + err) …then use it as part of
your error message.

But when you try to run your code with this change made, another exception is raised:

Traceback (most recent call last):
File "<pyshell#18>", line 5, in <module> Whoops! Yet another
exception; this time it’s a
print('File error:' + err) “TypeError”.
TypeError: Can't convert 'IOError' object to str implicitly

This time your error message didn’t appear at all. It turns out exception objects and strings are not compatible
types, so trying to concatenate one with the other leads to problems. You can convert (or cast) one to the other
using the str() BIF:
Use the “str()” BIF to force the
except IOError as err: exception object to behave like a string.
print('File error: ' + str(err))

And you now get a specific error
Now, with this final change, your code is behaving exactly as expected:
message that tells you exactly
File error: [Errno 2] No such file or directory: 'missing.txt' what went wrong.

Of course, all this extra logic is starting to obscure the
real meaning of your code.
Wouldn’t it be dreamy if there
were a way to take advantage of
these mechanisms without the code
bloat? I guess it’s just a fantasy...

you are here 4 119