I know that __call__ method in a class is triggered when the instance of a class is called. However, I have no idea when I can use this special method, because one can simply create a new method and perform the same operation done in __call__ method and instead of calling the instance, you can call the method.
I would really appreciate it if someone gives me a practical usage of this special method.
Django forms module uses __call__ method nicely to implement a consistent API for form validation. You can write your own validator for a form in Django as a function.
def custom_validator(value):
#your validation logic
Django has some default built-in validators such as email validators, url validators etc., which broadly fall under the umbrella of RegEx validators. To implement these cleanly, Django resorts to callable classes (instead of functions). It implements default Regex Validation logic in a RegexValidator and then extends these classes for other validations.
class RegexValidator(object):
def __call__(self, value):
# validation logic
class URLValidator(RegexValidator):
def __call__(self, value):
super(URLValidator, self).__call__(value)
#additional logic
class EmailValidator(RegexValidator):
# some logic
Now both your custom function and built-in EmailValidator can be called with the same syntax.
for v in [custom_validator, EmailValidator()]:
v(value) # <-----
As you can see, this implementation in Django is similar to what others have explained in their answers below. Can this be implemented in any other way? You could, but IMHO it will not be as readable or as easily extensible for a big framework like Django.
This example uses memoization, basically storing values in a table (dictionary in this case) so you can look them up later instead of recalculating them.
Here we use a simple class with a __call__ method to calculate factorials (through a callable object) instead of a factorial function that contains a static variable (as that's not possible in Python).
class Factorial:
def __init__(self):
self.cache = {}
def __call__(self, n):
if n not in self.cache:
if n == 0:
self.cache[n] = 1
else:
self.cache[n] = n * self.__call__(n-1)
return self.cache[n]
fact = Factorial()
Now you have a fact object which is callable, just like every other function. For example
for i in xrange(10):
print("{}! = {}".format(i, fact(i)))
# output
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
And it is also stateful.
I find it useful because it allows me to create APIs that are easy to use (you have some callable object that requires some specific arguments), and are easy to implement because you can use Object Oriented practices.
The following is code I wrote yesterday that makes a version of the hashlib.foo methods that hash entire files rather than strings:
# filehash.py
import hashlib
class Hasher(object):
"""
A wrapper around the hashlib hash algorithms that allows an entire file to
be hashed in a chunked manner.
"""
def __init__(self, algorithm):
self.algorithm = algorithm
def __call__(self, file):
hash = self.algorithm()
with open(file, 'rb') as f:
for chunk in iter(lambda: f.read(4096), ''):
hash.update(chunk)
return hash.hexdigest()
md5 = Hasher(hashlib.md5)
sha1 = Hasher(hashlib.sha1)
sha224 = Hasher(hashlib.sha224)
sha256 = Hasher(hashlib.sha256)
sha384 = Hasher(hashlib.sha384)
sha512 = Hasher(hashlib.sha512)
This implementation allows me to use the functions in a similar fashion to the hashlib.foo functions:
from filehash import sha1
print sha1('somefile.txt')
Of course I could have implemented it a different way, but in this case it seemed like a simple approach.
__call__ is also used to implement decorator classes in python. In this case the instance of the class is called when the method with the decorator is called.
class EnterExitParam(object):
def __init__(self, p1):
self.p1 = p1
def __call__(self, f):
def new_f():
print("Entering", f.__name__)
print("p1=", self.p1)
f()
print("Leaving", f.__name__)
return new_f
@EnterExitParam("foo bar")
def hello():
print("Hello")
if __name__ == "__main__":
hello()
Yes, when you know you're dealing with objects, it's perfectly possible (and in many cases advisable) to use an explicit method call. However, sometimes you deal with code that expects callable objects - typically functions, but thanks to __call__ you can build more complex objects, with instance data and more methods to delegate repetitive tasks, etc. that are still callable.
Also, sometimes you're using both objects for complex tasks (where it makes sense to write a dedicated class) and objects for simple tasks (that already exist in functions, or are more easily written as functions). To have a common interface, you either have to write tiny classes wrapping those functions with the expected interface, or you keep the functions functions and make the more complex objects callable. Let's take threads as example. The Thread objects from the standard libary module threading want a callable as target argument (i.e. as action to be done in the new thread). With a callable object, you are not restricted to functions, you can pass other objects as well, such as a relatively complex worker that gets tasks to do from other threads and executes them sequentially:
class Worker(object):
def __init__(self, *args, **kwargs):
self.queue = queue.Queue()
self.args = args
self.kwargs = kwargs
def add_task(self, task):
self.queue.put(task)
def __call__(self):
while True:
next_action = self.queue.get()
success = next_action(*self.args, **self.kwargs)
if not success:
self.add_task(next_action)
This is just an example off the top of my head, but I think it is already complex enough to warrant the class. Doing this only with functions is hard, at least it requires returning two functions and that's slowly getting complex. One could rename __call__ to something else and pass a bound method, but that makes the code creating the thread slightly less obvious, and doesn't add any value.
Class-based decorators use __call__ to reference the wrapped function. E.g.:
class Deco(object):
def __init__(self,f):
self.f = f
def __call__(self, *args, **kwargs):
print args
print kwargs
self.f(*args, **kwargs)
There is a good description of the various options here at Artima.com
IMHO __call__ method and closures give us a natural way to create STRATEGY design pattern in Python. We define a family of algorithms, encapsulate each one, make them interchangeable and in the end we can execute a common set of steps and, for example, calculate a hash for a file.
I just stumbled upon a usage of __call__() in concert with __getattr__() which I think is beautiful. It allows you to hide multiple levels of a JSON/HTTP/(however_serialized) API inside an object.
The __getattr__() part takes care of iteratively returning a modified instance of the same class, filling in one more attribute at a time. Then, after all information has been exhausted, __call__() takes over with whatever arguments you passed in.
Using this model, you can for example make a call like api.v2.volumes.ssd.update(size=20), which ends up in a PUT request to https://some.tld/api/v2/volumes/ssd/update.
The particular code is a block storage driver for a certain volume backend in OpenStack, you can check it out here: https://github.com/openstack/cinder/blob/master/cinder/volume/drivers/nexenta/jsonrpc.py
EDIT: Updated the link to point to master revision.
Specify a __metaclass__ and override the __call__ method, and have the specified meta classes' __new__ method return an instance of the class, viola you have a "function" with methods.
We can use __call__ method to use other class methods as static methods.
class _Callable:
def __init__(self, anycallable):
self.__call__ = anycallable
class Model:
def get_instance(conn, table_name):
""" do something"""
get_instance = _Callable(get_instance)
provs_fac = Model.get_instance(connection, "users")
One common example is the __call__ in functools.partial, here is a simplified version (with Python >= 3.5):
class partial:
"""New function with partial application of the given arguments and keywords."""
def __new__(cls, func, *args, **kwargs):
if not callable(func):
raise TypeError("the first argument must be callable")
self = super().__new__(cls)
self.func = func
self.args = args
self.kwargs = kwargs
return self
def __call__(self, *args, **kwargs):
return self.func(*self.args, *args, **self.kwargs, **kwargs)
Usage:
def add(x, y):
return x + y
inc = partial(add, y=1)
print(inc(41)) # 42
The function call operator.
class Foo:
def __call__(self, a, b, c):
# do something
x = Foo()
x(1, 2, 3)
The __call__ method can be used to redefined/re-initialize the same object. It also facilitates the use of instances/objects of a class as functions by passing arguments to the objects.
I find a good place to use callable objects, those that define __call__(), is when using the functional programming capabilities in Python, such as map(), filter(), reduce().
The best time to use a callable object over a plain function or a lambda function is when the logic is complex and needs to retain some state or uses other info that in not passed to the __call__() function.
Here's some code that filters file names based upon their filename extension using a callable object and filter().
Callable:
import os
class FileAcceptor(object):
def __init__(self, accepted_extensions):
self.accepted_extensions = accepted_extensions
def __call__(self, filename):
base, ext = os.path.splitext(filename)
return ext in self.accepted_extensions
class ImageFileAcceptor(FileAcceptor):
def __init__(self):
image_extensions = ('.jpg', '.jpeg', '.gif', '.bmp')
super(ImageFileAcceptor, self).__init__(image_extensions)
Usage:
filenames = [
'me.jpg',
'me.txt',
'friend1.jpg',
'friend2.bmp',
'you.jpeg',
'you.xml']
acceptor = ImageFileAcceptor()
image_filenames = filter(acceptor, filenames)
print image_filenames
Output:
['me.jpg', 'friend1.jpg', 'friend2.bmp', 'you.jpeg']
来源：https://stackoverflow.com/questions/5824881/python-call-special-method-practical-example