Pass a Variable to an Exception in Python

To pass a variable to an exception in Python, provide the variable as an argument when raising the exception. For custom exceptions, store the variable in an attribute.

You can pass variables like strings or numbers directly into built-in exceptions to include dynamic data in the error message.

Example: Passing a variable to a ValueError

In this example, we are passing a variable containing an invalid input message to a ValueError -

value = "abc123"
try:
   raise ValueError(f"Invalid input: {value}")
except ValueError as e:
   print("Caught exception:", e)

We get the following output -

Caught exception: Invalid input: abc123

Passing Arguments to an Exception

You can create your own exception class and print values (such as strings, numbers, lists, or even custom objects) or, your own exception message by passing them as a parameter while raising the exception explicitly. The values/variable can be stored as an instance attribute for later access.

Example: Custom Exception with a Variable

In this example, we are passing an error message stored in a variable to a custom exception -

class MyError(Exception):
   def __init__(self, message):
      self.message = message
      super().__init__(message)

msg = "Something went wrong."
try:
   raise MyError(msg)
except MyError as e:
   print("Caught custom exception:", e.message)

Following is the output obtained -

Caught custom exception: Something went wrong.

Passing Multiple Variables

You can also pass multiple variables to an exception and use them to generate a more descriptive error message.

Example

In this example, we are passing a variable for the username and the error reason to a custom exception -

class LoginError(Exception):
   def __init__(self, user, reason):
      self.user = user
      self.reason = reason
      super().__init__(f"User '{user}' failed to login: {reason}")

username = "suresh_raina"
reason = "Invalid password"
try:
   raise LoginError(username, reason)
except LoginError as e:
   print("Caught exception:", e)

We get the output as shown below -

Caught exception: User 'suresh_raina' failed to login: Invalid password

Accessing the Variable Later

If you store the passed variable in an instance variable inside the custom exception class, you can access it even after catching the exception.

Example

In this example, we are accessing the variable stored in the exception object after catching it -

class DataMissingError(Exception):
   def __init__(self, missing_field):
      self.missing_field = missing_field
      super().__init__(f"Missing data: {missing_field}")

try:
   field = "email"
   raise DataMissingError(field)
except DataMissingError as e:
   print("Missing field:", e.missing_field)

We get the following output -

Missing field: email