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POPython: Decorators: How does the following code work?
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copied!<p><strong>Are the comments in the following code correct? Particularly the "instance =..." one?</strong></p> <pre><code># This does nothing. class donothing(object): def __init__(self, func): """ The 'func' argument is the function being decorated because in this case, we're not instantiating the decorator class. Instead we are just using the class object as a callable (a class is always callable as this is how an instance is returned) to use as a decorator, which means that it is being instantiated upon definition of the decorated function and the decorated function is being passed in as an argument to the class's __init__ method. """ self.func = func def __call__(self, *args, **kwargs): """ The __call__ function is called when the decorated function is called because the function has be eaten by the decorator class. Now it's up to the this method to return a call to the original function. The arguments are passed in as args, kwargs to be manipulated. """ # Returns original function call with original arguments. return self.func(*args, **kwargs) @donothing def printer(text): print(text) printer('hello world') # The printer function is now an alias for the donothing instance created, so # the preceding was the same as: # # instance = donothing(printer) # instance('hello world') # # Next example: class checkforkeysinparams(object): def __init__(self, required): self.required = set(required) def __call__(self, params): def wrapper(params): missing = self.required.difference(params) if missing: raise TypeError('Missing from "params" argument: %s' % ', '.join(sorted(missing))) return wrapper # Apply decorator class, passing in the __init__'s 'required' argument. @checkforkeysinparams(['name', 'pass', 'code']) def complex_function(params): # Obviously these three are needed or a KeyError will be raised. print(params['name']) print(params['pass']) print(params['code']) # Create params to pass in. Note, I've commented out one of the required params. params = { 'name': 'John Doe', 'pass': 'OpenSesame', #'code': '1134', } # This call will output: TypeError: Missing from "params" argument: code complex_function(params=params) </code></pre>

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