Throw custom exception in Python with input parameters to print

Question

I have seen a few examples like this, this and this, which show how to throw custom exception in Python. But what I'm hoping to accomplish is something like this:

## in main.py
import my_errors

if  os.path.exists(filename):
    # here, I'd like to pass in 'filename' as parameter to FileNotFound
    raise my_errors.FileNotFound(filename)

## in my_errors.py
class MyError(Exception):
    pass

class FileNotFound(MyError):
    # here, how do I make this FileNotFound class to accept/receive 'filename' as parameter so that I can print something informative like below
    print("FileNotFound ERROR: Please make sure that the following file exists:", filename)

Thank you in advance for your help!

Michael Bianconi · Accepted Answer · 2019-08-01T18:20:45.343

1

You'll want to implement your own __init__() function.

class FileNotFound(MyError):

    def __init__(self, filename):
        super().__init__("FileNotFound ERROR: Please make sure that the following file exists:" % filename)

Note that this will set the error message of the exception. To print normally:

class FileNotFound(MyError):

    def __init__(self, filename):
        print("FileNotFound ERROR: Please make sure that the following file exists:", filename)

        super().__init__(filename)

edited Aug 01 '19 at 18:20

answered Aug 01 '19 at 18:14

Michael Bianconi

5,072
1
10
25

1

Setting the error message is sufficient; don't force an error message to be displayed, because someone catching the error might not care that the file is missing, and simply create it, or ignore that it is missing. – chepner Aug 01 '19 at 18:19
1

Absolutely. However, his question uses ```print``` so I included that as a solution. – Michael Bianconi Aug 01 '19 at 18:20
@MichaelBianconi, Thank you! Now when I throw `raise my_errors.FileNotFound('test_file.csv')`, I receive `Traceback` ending with something like `my_errors.FileNotFound: ('ERROR: Please make sure that the following file(s) exists:', 'test_file.csv')`. How can I make that last line appear in a string instead of a tuple (e.g., like this `my_errors.FileNotFound: ERROR: Please make sure that the following file(s) exists: test_file.csv`? Is that possible? – user1330974 Aug 01 '19 at 18:27
1

@user1330974 You shouldn't really care about how the traceback is displayed, because proper code shouldn't be *showing* a traceback; it should be catching the exception and handling it properly (which may, of course, simply mean printing an error message based on the exception and exiting). – chepner Aug 01 '19 at 18:30
@chepner, Totally understand your point and I would follow that advice in other code modules. But in this specific case, I am not planning to catch these errors in my code. I want them to fail with informative error message to the stdout. Hope that make sense. :) – user1330974 Aug 01 '19 at 18:39
@MichaelBianconi I figured out my follow-up question. I have defined `def __str__(self): return ''.join(self.args)` in the `FileNotFound` class to print it as a string (not as a tuple). Thank you for your help providing the solution above! – user1330974 Aug 01 '19 at 18:43

Throw custom exception in Python with input parameters to print

1 Answers1