Definiing custom exceptions in Python with a meaningful message

custom_errors.py

class InvalidParameterError(ValueError):
    pass

parse_parameters.py

from custom_errors import InvalidParameterError

def validate(p):
  
  for k, v in p.items():
    if v < 0:
      emsg = "Parameter '%s' cannot be negative" %(k,)
      raise InvalidParameterError(emsg)

test_parse_parameters.py

import pytest

from custom_error import InvalidParameterError
from parse_parameters import validate

def test_negative_parameters_raise_error():
  sample_params = {
  'a': 1.43,
  'b': 57,
  'c': -10
  }
  
  with pytest.raises(InvalidParameterError) as excinfo:
    validate(sample_params)
    
  # The object returned by this pytest context manager
  # has a 'value' field assigned to the exception object itself.
  # In addition, all built-in exceptions derived from Exception
  # have an 'args' tuple field assigned to the passed arguments.
  # Hence, when we create a custom error with a string as the first argument,
  # this string is assigned to the first element of the tuple 'args'
  exception_msg = excinfo.value.args[0]
  expected_message = "Parameter 'c' cannot be negative".
  
  assert exception_msg == expected_message