Decorators¶

A Short Reminder

Functions are things that generate values based on input (arguments).

In Python, functions are first-class objects.

This means that you can bind names to them, pass them around, etc., just like other objects.

Because of this fact, you can write functions that take functions as arguments and/or return functions as values:

def substitute(a_function):
def new_function(*args, **kwargs):
return "I'm not that other function"
return new_function


A Definition¶

There are many things you can do with a simple pattern like this one. So many, that we give it a special name:

Decorator

“A decorator is a function that takes a function as an argument and returns a function as a return value.”

That’s nice and all, but why is that useful?

An Example¶

Imagine you are trying to debug a module with a number of functions like this one:

def add(a, b):
return a + b


You want to see when each function is called, with what arguments and with what result. So you rewrite each function as follows:

def add(a, b):
print("Function 'add' called with args: {}, {}".format(a, b) )
result = a + b
print("\tResult --> {}".format(result))
return result


That’s not particularly nice, especially if you have lots of functions in your module.

Now imagine we defined the following, more generic decorator:

def logged_func(func):
def logged(*args, **kwargs):
print("Function {} called".format(func.__name__))
if args:
print("\twith args: {}".format(args))
if kwargs:
print("\twith kwargs: {}".format(kwargs))
result = func(*args, **kwargs)
print("\t Result --> {}".format(result))
return result
return logged


We could then make logging versions of our module functions:

logging_add = logged_func(add)


Then, where we want to see the results, we can use the logged version:

In [37]: logging_add(3, 4)
with args: (3, 4)
Result --> 7
Out[37]: 7


This is nice, but we have to call the new function wherever we originally had the old one.

It’d be nicer if we could just call the old function and have it log.

Remembering that you can easily rebind symbols in Python using assignment statements leads you to this form:

def logged_func(func):
# implemented above

return a + b


And now you can simply use the code you’ve already written and calls to add will be logged:

In [41]: add(3, 4)
with args: (3, 4)
Result --> 7
Out[41]: 7


Syntax¶

Rebinding the name of a function to the result of calling a decorator on that function is called decoration.

Because this is so common, Python provides a special operator to perform it more declaratively: the @ operator – I told you I’d eventually explain what was going on under the hood with that weird @ symbol.

This is rebinding the name:

def add(a, b):
return a + b


And this means exactly the same thing, with the decoration syntax:

@logged_func
return a + b


The declarative form (called a decorator expression) is far more common, but both have the identical result, and can be used interchangeably.

Here’s another simple example. First we define a decorator – note that it is a function that takes an argument, and returns a function:

In [1]: def my_decorator(func):
...:      def inner():
...:          print('running inner')
...:      return inner
...:


And we can apply it with the regular calling and rebinding syntax:

In [2]: def other_func():
...:     print('running other_func')

In [3]: other_func()
running other_func

In [4]: other_func = my_decorator(other_func)

In [5]: other_func()
In [5]: running inner

In [6]: other_func
Out[6]: <function __main__.my_decorator.<locals>.inner>


Notice that other_func is now the “inner” function, which lives in the “my_decorator” namespace…

And this is the same with the decoration syntax:

In [7]: @my_decorator
...: def other_func():
...:      print('running other_func')
...:

In [8]: other_func()
running inner

In [9]: other_func
Out[9]: <function __main__.my_decorator.<locals>.inner>


Notice that other_func is the “inner” function here as well.

Decorators have the power to replace the decorated function with a different one!

And they do it with compact, declarative syntax that has the decoration right at the top where the function is defined.

Callables¶

Our original definition of a decorator was nice and simple, but a tiny bit incomplete.

In reality, decorators can be used with anything that is callable.

Remember that a callable is a function, a class object, a method in a class, or a instance of a class that implements the __call__ special method.

So in fact the definition should be updated as follows:

“A decorator is a callable that takes a callable as an argument and returns a callable as a return value.”

An Example¶

Consider a decorator that would save the results of calling an expensive function with given arguments so that it would not have to be re-computed with the same input (which is known an memoizing…).

class Memoize:
"""
memoize decorator from avinash.vora
http://avinashv.net/2008/04/python-decorators-syntactic-sugar/
"""
def __init__(self, function):  # runs when memoize class is called
self.function = function
self.memoized = {}

def __call__(self, *args):  # runs when memoize instance is called
try:
return self.memoized[args]
except KeyError:
self.memoized[args] = self.function(*args)
return self.memoized[args]


Let’s try that out with a potentially expensive function:

In [56]: @Memoize
....: def sum2x(n):
....:     return sum(2 * i for i in range(n))
....:

In [57]: sum2x(10000000)
Out[57]: 99999990000000

In [58]: sum2x(10000000)
Out[58]: 99999990000000


Run that code yourself and see how much faster it returns the second time.

It’s nice to see that in action, but what if we want to know exactly how much difference it made?

Nested Decorators¶

You can stack decorator expressions. The result is like calling each decorator in order, from bottom to top:

@decorator_two
@decorator_one
def func(x):
pass

# is exactly equal to:
def func(x):
pass
func = decorator_two(decorator_one(func))


Let’s define another decorator that will time how long a given call takes:

import time
def timed_func(func):
def timed(*args, **kwargs):
start = time.time()
result = func(*args, **kwargs)
elapsed = time.time() - start
print("time expired: {}".format(elapsed))
return result
return timed


And now we can use this new decorator stacked along with our memoizing decorator:

In [71]: @timed_func
....: @Memoize
....: def sum2x(n):
....:     return sum(2 * i for i in range(n))
In [72]: sum2x(10000000)
time expired: 0.997071027756
Out[72]: 99999990000000
In [73]: sum2x(10000000)
time expired: 4.05311584473e-06
Out[73]: 99999990000000


Parameterized Decorators¶

The purpose of the outer function in the decorator is to receive the function to be decorated, adding anything to scope that should be there before the decorated function is called.

The inner function runs the function being decorated, so its inputs are the same as the function being decorated.

How do we add more input parameters to our decorator? Like this example from Django:

@register.filter(name='cut')
def cut(value, arg):
return value.replace(arg, '')


Add yet another function in scope:

def decorator(arg1, arg2):
def real_decorator(function):
def wrapper(*args, **kwargs):
print("Congratulations. You decorated a function that does
something with {} and {}".format(arg1, arg2))
function(*args, **kwargs)
return wrapper
return real_decorator

@decorator("arg1", "arg2")
def print_args(*args):
for arg in args:
print(arg)


Last example from: http://scottlobdell.me/2015/04/decorators-arguments-python/

Examples from the Standard Library¶

It’s going to be a lot more common for you to use pre-defined decorators than for you to be writing your own.

For example, @staticmethod and @classmethod can also be used as simple callables, without the nifty decorator expression:

class C:
@staticmethod
return a + b


Is exactly the same as:

class C:
return a + b


Note that the “def” binds the name add, then the next line rebinds it.

[Note that this is exactly how you defined a staticmethod before the decoration syntax was added in python 2.4]

The classmethod() builtin can do the same thing:

# in declarative style
class C:
@classmethod
def from_iterable(cls, seq):
# method body

# in imperative style:
class C:
def from_iterable(cls, seq):
# method body
from_iterable = classmethod(from_iterable)


property()¶

Remember the property() builtin?

Perhaps most commonly, you’ll see the property() builtin used this way.

Previously, we saw this code:

class C:
def __init__(self):
self._x = None
@property
def x(self):
return self._x
@x.setter
def x(self, value):
self._x = value
@x.deleter
def x(self):
del self._x


But this could also be accomplished like so:

class C:
def __init__(self):
self._x = None
def getx(self):
return self._x
def setx(self, value):
self._x = value
def delx(self):
del self._x
x = property(getx, setx, delx,
"I'm the 'x' property.")


property_ugly.py

Note that in this case, the decorator object returned by the property decorator itself implements additional decorators as attributes on the returned method object. So you could actually do this:

class C:
def __init__(self):
self._x = None
def x(self):
return self._x
x = property(x)
def _set_x(self, value):
self._x = value
x = x.setter(_set_x)
def _del_x(self):
del self._x
x = x.deleter(_del_x)


But that’s getting really ugly! Makes you appreciate the @, doesn’t it?

Import Time vs. Run Time¶

Decorators are run at import time. Run this code and see what happens when:

play_with_imports.py

What if my decorated function uses unknown inputs?¶

If you don’t know what parameters the decorated function will take (and you usually don’t), you want to make sure the inner function that you are replacing the decorated function with takes ANY arguments, and passes them on to the decorated function.

*args, **kwargs is your friend here:

A decorator that wraps an html <p> tag around the output of any decorated function.

def p_decorate(func):
def func_wrapper(*args, **kwargs):
return "<p>{0}</p>".format(func(*args, **kwargs))
return func_wrapper

@p_decorate
def get_fullname(first_name, last_name):
return f"{first_name} {last_name}"

In [124]: get_fullname('Chris', 'Barker')
Out[124]: '<p>Chris Barker</p>'


Functools Library¶

Single dispatch:
• create many functions that do the same sort of thing, but based on type

• decorator determines type, and decides which function is run

https://docs.python.org/3/library/functools.html#functools.singledispatch

Memoize decorator we created earlier is in Functools:

https://docs.python.org/3/library/functools.html#functools.lru_cache

LAB¶

A little excercise. See the “p_decorate” decorator defined above – it wrapped an html <p> tag (paragraph) around the results of any function that returned a string.

Can you make a version that will wrap any other tag – specified as a parameter of the decorator itself? For example:

@add_tag('p')
def get_fullname(first_name, last_name):
return f"{first_name} {last_name}"

In [124]: get_fullname('Chris', 'Barker')
Out[124]: '<p>Chris Barker</p>'


Just like the p_decorate one above.

But:

@add_tag('div')
def get_fullname(first_name, last_name):
return f"{first_name} {last_name}"

In [124]: get_fullname('Chris', 'Barker')
Out[124]: '<div>Chris Barker</div>'


and you could pass any tag in.

This can be accomplished either with a closure –nesting another level of functions in the decorator, or with a callable class, like the memoize example. Maybe try both, and decide which you like better.