CODEWITHSUNDEEP
pythonclass
fastbreak78
fastbreak78

Reputation: 95

Calling a class as a function?

I've been reading through some example code for authentication using tokens, and there is a line that I do not understand at all.

client.set_signature_method(SignatureMethod_RSA_SHA1())

Specifically, I'm unsure what "SignatureMethod_RSA_SHA1()" does. Here's the SignatureMethod class, for reference: 

class SignatureMethod_RSA_SHA1(oauth.SignatureMethod):
name = 'RSA-SHA1'

def signing_base(self, request, consumer, token): 
    if not hasattr(request, 'normalized_url') or request.normalized_url      is None: 
        raise ValueError("Base URL for request is not set.")

    sig = ( #Creates the request?
        oauth.escape(request.method), 
        oauth.escape(request.normalized_url), 
        oauth.escape(request.get_normalized_parameters()),
    )

    key = '%s&' % oauth.escape(consumer.secret)#
    if token: 
        key += oauth.escape(token.secret)
    raw = '&'.join(sig)
    return key, raw

def sign(self, request, consumer, token): 
    """Builds the base signature string."""
    key, raw = self.signing_base(request, consumer, token)

    with open('../rsa.pem', 'r') as f: 
        data = f.read()
    privateKeyString = data.strip() 

privatekey = keyfactory.parsePrivateKey(privateKeyString)
    signature = privatekey.hashAndSign(raw) 

    return base64.b64encode(signature)

The syntax suggests that the class is being called as a function, which I have never seen in my short time programming. Does anybody else what this means?

Upvotes: 0

Views: 130

Answers (2)

kylieCatt
kylieCatt

Reputation: 11039

That is how you instantiate a class, it's not being used as a function (although technically it is calling the classes __init__() method).

client.set_signature_method() apparently takes an instance of of some sort of signature method class (not sure if SignatureMethod_RSA_SHA1() is the only possible type) and most likely hangs it on the client object making it accessible through client.signature_method or some other property.

This is the same as doing:

sig_method = SignatureMethod_RSA_SHA1()
client.set_signature_method(sig_method)

It's just a shorter way of doing it if you won't need to access sig_method again outside of the client instance.

More familiar examples would be:

a_list = list()
a_dict = dict()

This way the SignatureMethod_RSA_SHA1() and it's properties and methods are now available to the client instance. For simpler example we can do:

>>> class MyClass:
    def __init__(self, my_list):
        self.my_list = my_list
    def add_to_list(self, a_value):
        self.my_list.append(a_value)

>>> foo = MyClass(list())
>>> foo.my_list
[]

So now the list is available to the class and so are it's methods:

>>> foo.my_list.extend([2,4,1,4,8,0,3,])
>>> foo.my_list
[2, 4, 1, 4, 8, 0, 3]

And more importantly the class can use the list objects methods internally like in our add_to_list() method (which is probably how SignatureMethod_RSA_SHA1() is being used):

>>> foo.add_to_list('a')
>>> foo.my_list
[2, 4, 1, 4, 8, 0, 3, 'a']

Upvotes: 2

lukkol
lukkol

Reputation: 64

SignatureMethod_RSA_SHA1() is just class constructor. Its __init__ function is called.

In the other words - set_signature_method is called with SignatureMethod_RSA_SHA1 instance.

Upvotes: 0

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