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| 1 | +"""Function Decorators. |
| 2 | + |
| 3 | +@see: https://www.thecodeship.com/patterns/guide-to-python-function-decorators/ |
| 4 | + |
| 5 | +Function decorators are simply wrappers to existing functions. In the context of design patterns, |
| 6 | +decorators dynamically alter the functionality of a function, method or class without having to |
| 7 | +directly use subclasses. This is ideal when you need to extend the functionality of functions that |
| 8 | +you don't want to modify. We can implement the decorator pattern anywhere, but Python facilitates |
| 9 | +the implementation by providing much more expressive features and syntax for that. |
| 10 | +""" |
| 11 | + |
| 12 | + |
| 13 | +def test_function_decorators(): |
| 14 | + """Function Decorators.""" |
| 15 | + |
| 16 | + # Function decorators are simply wrappers to existing functions. Putting the ideas mentioned |
| 17 | + # above together, we can build a decorator. In this example let's consider a function that |
| 18 | + # wraps the string output of another function by p tags. |
| 19 | + |
| 20 | + # This is the function that we цфте to decorate. |
| 21 | + def greeting(name): |
| 22 | + return "Hello, {0}!".format(name) |
| 23 | + |
| 24 | + # This function decorates another functions output with <p> tag. |
| 25 | + def decorate_with_p(func): |
| 26 | + def function_wrapper(name): |
| 27 | + return "<p>{0}</p>".format(func(name)) |
| 28 | + return function_wrapper |
| 29 | + |
| 30 | + # Now, let's call our decorator and pass the function we want decorate to it. |
| 31 | + my_get_text = decorate_with_p(greeting) |
| 32 | + |
| 33 | + # Here we go, we've just decorated the function output without changing the function itself. |
| 34 | + assert my_get_text('John') == '<p>Hello, John!</p>' # With decorator. |
| 35 | + assert greeting('John') == 'Hello, John!' # Without decorator. |
| 36 | + |
| 37 | + # Now, Python makes creating and using decorators a bit cleaner and nicer for the programmer |
| 38 | + # through some syntactic sugar There is a neat shortcut for that, which is to mention the |
| 39 | + # name of the decorating function before the function to be decorated. The name of the |
| 40 | + # decorator should be prepended with an @ symbol. |
| 41 | + |
| 42 | + @decorate_with_p |
| 43 | + def greeting_with_p(name): |
| 44 | + return "Hello, {0}!".format(name) |
| 45 | + |
| 46 | + assert greeting_with_p('John') == '<p>Hello, John!</p>' |
| 47 | + |
| 48 | + # Now let's consider we wanted to decorate our greeting function by one more functions to wrap a |
| 49 | + # div the string output. |
| 50 | + |
| 51 | + # This will be our second decorator. |
| 52 | + def decorate_with_div(func): |
| 53 | + def function_wrapper(text): |
| 54 | + return "<div>{0}</div>".format(func(text)) |
| 55 | + return function_wrapper |
| 56 | + |
| 57 | + # With the basic approach, decorating get_text would be along the lines of |
| 58 | + # greeting_with_div_p = decorate_with_div(decorate_with_p(greeting_with_p)) |
| 59 | + |
| 60 | + # With Python's decorator syntax, same thing can be achieved with much more expressive power. |
| 61 | + @decorate_with_div |
| 62 | + @decorate_with_p |
| 63 | + def greeting_with_div_p(name): |
| 64 | + return "Hello, {0}!".format(name) |
| 65 | + |
| 66 | + assert greeting_with_div_p('John') == '<div><p>Hello, John!</p></div>' |
| 67 | + |
| 68 | + # One important thing to notice here is that the order of setting our decorators matters. |
| 69 | + # If the order was different in the example above, the output would have been different. |
| 70 | + |
| 71 | + # Passing arguments to decorators. |
| 72 | + |
| 73 | + # Looking back at the example before, you can notice how redundant the decorators in the |
| 74 | + # example are. 2 decorators(decorate_with_div, decorate_with_p) each with the same |
| 75 | + # functionality but wrapping the string with different tags. We can definitely do much better |
| 76 | + # than that. Why not have a more general implementation for one that takes the tag to wrap |
| 77 | + # with as a string? Yes please! |
| 78 | + |
| 79 | + def tags(tag_name): |
| 80 | + def tags_decorator(func): |
| 81 | + def func_wrapper(name): |
| 82 | + return "<{0}>{1}</{0}>".format(tag_name, func(name)) |
| 83 | + return func_wrapper |
| 84 | + return tags_decorator |
| 85 | + |
| 86 | + @tags('div') |
| 87 | + @tags('p') |
| 88 | + def greeting_with_tags(name): |
| 89 | + return "Hello, {0}!".format(name) |
| 90 | + |
| 91 | + assert greeting_with_tags('John') == '<div><p>Hello, John!</p></div>' |
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