python decorators decorator(arg)(func) # decorator(arg) must be a decorator st ephane vialette...

PythonDecorators

Stephane Vialette

LIGM, Universite Paris-Est Marne-la-Vallee

October 12, 2009

Stephane Vialette (LIGM, Universite Paris-Est Marne-la-Vallee)Python Decorators October 12, 2009 1 / 25

Outline

1 Introduction

2 Decorators without arguments



Introduction

Outline

1 Introduction




Introduction

Decorators

Description

Decorators are just overlays on top of function calls.These overlaysare jsut additional call that are applied when a function or method isdeclared.

The syntax for decorators uses a leading at-sign (@) followed by thedecorator name and optional arguments.

Syntaxe

func = decorator1(decorator2(decorator3(func(arg1, arg2, ...))))


Introduction

Stacking

Description

Stacking multiple decorators

@decorator1@decorator2@decorator3def func (arg1, arg2, ...):

pass

is equivalent to creating acomposite function

func = decorator1(decorator2(decorator3(func(arg1, arg2, ...))))


Decorators without arguments

Outline

1 Introduction






Example

def entry exit(f):def wrapped():

print ">> Entering", f. name , ’>>’ ,f()print "<< Exited", f. name , ’<<’

return wrapped

@entry exitdef func():

print "Inside func",

@entry exitdef func2():

print "inside func2",

if name == ’ main ’:func() # >> Entering func >> Inside func << Exited func <<func2() # >> Entering func2 >> inside func2 << Exited func2 <<



Measuring the running time

Example

import time

def timeit(func):def timed(*args, **kw):

ts = time.time()result = func(*args, **kw)te = time.time()print ’%r (%r, %r) %2.2f sec’ % (func. name , args, kw, te-ts)return result

return timed



Measuring the running time

Example

import timefrom timeit import timeit

class C(object):@timeitdef f(self):

time.sleep(0.2)

@timeitdef g():

time.sleep(1)

c = C()c.f() #’f’ ((< main .C object at 0xb7e2df4c>,), {}) 0.20 secg() # ’g’ ((), {}) 1.00 sec



Check that the supplied argument is an integer

Example

def requireint(func):def wrapper (arg):

assert isinstance(arg, int)return func(arg)

return wrapper

@require intdef g(arg):

...



Decorators as classes

Description

The only constraint upon the object returned by the decorator is thatit can be used as a function – which basically means it must becallable.

Thus, any classes we use as decorators must implement call .

class MyDecorator(object):def init (self, func):

# do something with func ...def call (*args):

# do something args



Classes as decorators

Fibonacci

In [1]: class Decorator(object):...: def init (self, func):...: print ’Decorator. init ’

...: self. func = func

...: def call (self, *args):

...: print ’Decorator. call ’

...: return self. func(args)In [2]: @Decorator

...: def f(arg):

...: print ’f(%s)’ % (arg,)Decorator. initIn [3]: DecoratorOut[3]: <class ’ main .Decorator’>In [4]: fOut[4]: < main .Decorator object at 0x913f12c>In [5]: f(’arg’)Decorator. callf((’arg’,))



Memoized

Fibonacci

def fibonacci(n):print ’fibonacci(%d)’ % n,if 0 <= n <= 1:

return nelse:

return fibonacci(n-1) + fibonacci(n-2)

fibonacci(5)

Output

fibonacci(5) fibonacci(4) fibonacci(3) fibonacci(2) fibonacci(1) fibonacci(0) fibonacci(1)fibonacci(2) fibonacci(1) fibonacci(0) fibonacci(3) fibonacci(2) fibonacci(1) fibonacci(0)fibonacci(1)



Memoized

Fibonacci

class Memoized(object):def init (self, func):

print ’Memoized. init ’self. func = funcself. cache = {}

def call (self, *args):print ’Memoized. call ’,try:

return self. cache[args]except KeyError:

self. cache[args] = value = self. func(*args)return value

except TypeError:return self. func(*args)



Memoized

Fibonaccifrom memoized import Memoized@Memoizeddef fibonacci(n):

print ’fibonacci(%d)’ % n,if 0 <= n <= 1:

return nelse:

return fibonacci(n-1) + fibonacci(n-2)

fibonacci(5)

Output

Memoized. initMemoized. call fibonacci(5) Memoized. call fibonacci(4) Memoized. call fibonacci(3)Memoized. call fibonacci(2) Memoized. call fibonacci(1) Memoized. call fibonacci(0)Memoized. call Memoized. call Memoized. call



Counting function calls

Example

class countcalls(object):

def init (self, func):self. func = funcself. numcalls = 0

def call (self, *args, **kwargs):self. numcalls += 1return self. func(*args, **kwargs)

def count(self):return self. numcalls




Example

@countcallsdef f1():

pass

@countcallsdef f2():

pass

print ’f1:’, f1print ’f2:’, f2

for in range(3):f1()

for in range(2):f1()f2()

print f1.count() # would print 5print f2.count() # would print 2




This does not work !

class countcalls(object):instances = {}

def init (self, func):self. func = funcself. numcalls = 0countcalls. instances[func] = self

def call (self, *args, **kwargs):self. numcalls += 1return self. func(*args, **kwargs)

@staticmethoddef count(func):

return countcalls. instances[func]. numcalls

@staticmethoddef counts():

return dict([(func, countcalls.count(func)) for func in countcalls. instances])



Outline

1 Introduction





Decorators with arguments

Description

Decorators can have arguments, like in

@decorator(arg)def func():

...

is equivalent to creating acomposite function

func = decorator(arg)(func) # decorator(arg) must be a decorator



Adding an attribute

Example

In [1]: def option(value):...: def decorator(func):...: func.attribute = value...: return func...: return decorator...:

In [2]: @option(’a value’)...: def f():...: pass...:

In [3]: f()In [4]: f.attributeOut[4]: ’a value’



Checking type

Example

def return bool(bool value):def wrapper(func):

def wrapped(*args):result = func(*args)if result != bool:

raise TypeErrorreturn result

return wrappedreturn wrapper

@return bool(True)def always true():

return Truealways true() # ok

@return bool(False)def always false():

return Truealways false() # raise TypeError



Easy adding methods to a class instance 1/3

Example

In [1]: import new

In [2]: new.instancemethod?Type: typeBase Class: <type ’type’>String Form: <type ’instancemethod’>Namespace: InteractiveDocstring:

instancemethod(function, instance, class)

Create an instance method object.

In [3]:




Example

# Add a new method to a class instance.# Credits to John Roth.def addto(instance):

def decorator(func):import newfunc = new.instancemethod(func, instance, instance. class )setattr(instance, func.func name, func)return func

return decorator




Example

class C(object):def init (self, val):

self.attribute = val

c1 = C(123)c2 = C(456)

@addto(c1)def print attribute(self):

print self.attribute

c1.print attribute() # would print 123c2.print attribute() # AttributeError: ’C’ object has no attribute ’print attribute’


python decorators decorator(arg)(func) # decorator(arg) must be a decorator st ephane vialette...

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