Unless

Unless is a lightweight python library designed to simplify error handling. It introduces the Result class that encapsulates the result of a function call, which can be either the return value, an error, or both.

Install

pip3 install unless-handler

Usage

💡 Remember

Result class has 2 properties and 1 method,

• Properties

  • value - The return value of the function
  • error - Information about the error if there was any
    • It can be either a tuple of the exception type and traceback string, or just the traceback string

• Methods

  • unless - Handle errors and return the value

• Import the Result class and traceback.format_exc for tracebacks

  from unless.result import Result
  
  from traceback import format_exc # Optional
  

• Integrate Result into your function

  • Initialize the Result class (specify the return type for type hints)

    def cool():
        to_return = Result[list]()
                            ^
                        Return type
    

    [list] is the return type of the function and is there so we can have type hints. It is OPTIONAL and Result() works too.

  • Set the return value

    • Use value property of the Result class to set return value

    # <initialized>.value = <value to return>
    to_return.value = [1, 2, 3]
    

  • Catch and set errors

    • Use error property of the Result class to set errors
    • You have the freedom to set any value you like; however, it is recommended to follow the given syntax of <type>, <traceback> as it gives the ability to have caused exception type and the traceback for better error handling

    try:
        ...
    except Exception as e:
        # <initialized>.error = <exception type>, <traceback.format_exc()>
        to_return.error = type(e), format_exc()
    

  • Return the result

    return to_return
    

• Calling your function

  • See result using the value property

    called = my_function()
    
    called.value
    

  • See error using the error property

    called.error
    

  • Or better yet, use the unless method

    called = my_function().unless()
    
    # called is now called.value and errors are handled using handler function
    # for more info check "Examples"
    

Examples

• Basic usage
• Custom error handler
• Integrate with existing functions

Basic usage

def cool():
    to_return = Result[list]()
    try:
        to_return.value = [1, 2, 3]
        raise ValueError("Annoying error...")
    except Exception as e:
        to_return.error = type(e), traceback.format_exc()
    return to_return

# Calling the function
x = cool().unless()
print(x)

Custom error handling

You can call functions with custom error handling logic using Result.unless method that your function returns.

• You can pass any python function to the unless method
• Your handler function must accept at least 1 argument
  • If you're using Result.from_func or following the recommended syntax, the 1st argument will be a tuple consist of <type of exception>, <traceback string>
• Handler function can have keyword arguments (x.unless(func, arg1="first", arg2="second"))

def custom_handler(e, notes):
    _, fmt_traceback = e
    logging.warn(f"{fmt_traceback} \n\nNotes: {notes}")

x = cool().unless(
        custom_handler,
        notes="Probably happend because the function was hot"
    )
print(x)

Integrate with existing functions

You can use Result.from_func method to integrate this with existing functions.

• As a decorator,

  @Result.from_func
  def older(n) -> list:
      return [1, 2, 3, n]
  

• As a function,

  def older(n) -> list:
      return [1, 2, 3, n]
  
  x = Result.from_func(older, list, n=2)
  
  # Important:
  #  if your function doesn't accept arguments, use "()" at the end to call it properly
  
  # Example:
  x = Result.from_func(no_args)()
                                ^
                        Call the function