|
| 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 | +deftest_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 | +defgreeting(name): |
| 22 | +return"Hello, {0}!".format(name) |
| 23 | + |
| 24 | +# This function decorates another functions output with <p> tag. |
| 25 | +defdecorate_with_p(func): |
| 26 | +deffunction_wrapper(name): |
| 27 | +return"<p>{0}</p>".format(func(name)) |
| 28 | +returnfunction_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 | +assertmy_get_text('John')=='<p>Hello, John!</p>'# With decorator. |
| 35 | +assertgreeting('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 | +defgreeting_with_p(name): |
| 44 | +return"Hello, {0}!".format(name) |
| 45 | + |
| 46 | +assertgreeting_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 | +defdecorate_with_div(func): |
| 53 | +deffunction_wrapper(text): |
| 54 | +return"<div>{0}</div>".format(func(text)) |
| 55 | +returnfunction_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 | +defgreeting_with_div_p(name): |
| 64 | +return"Hello, {0}!".format(name) |
| 65 | + |
| 66 | +assertgreeting_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 | +deftags(tag_name): |
| 80 | +deftags_decorator(func): |
| 81 | +deffunc_wrapper(name): |
| 82 | +return"<{0}>{1}</{0}>".format(tag_name,func(name)) |
| 83 | +returnfunc_wrapper |
| 84 | +returntags_decorator |
| 85 | + |
| 86 | +@tags('div') |
| 87 | +@tags('p') |
| 88 | +defgreeting_with_tags(name): |
| 89 | +return"Hello, {0}!".format(name) |
| 90 | + |
| 91 | +assertgreeting_with_tags('John')=='<div><p>Hello, John!</p></div>' |