Python: creating class instance

Question

There is some part of code.

application = webapp2.WSGIApplication([
    ('/', MainPage),
    ('/gbook', Guestbook)
])

As I understand it is a list of tuples:

[('/', MainPage), ('/gbook', Guestbook)]

Correct me please if I'm wrong.

And I have question: Where is obvious creation of instance of MainPage class and Guestbook?

Something like that: x = MainPage('/')

If this happens by this tuple ('/', MainPage), then my question: how it's happens?

I need some explanation.

Yes, that is a list of tuples. "Where is obvious creation of instance..." - nowhere in the code you're showing. Explanation of code is off-topic here, particularly if you don't even show the code in question. — jonrsharpe
– jonrsharpe, Commented Oct 21, 2014 at 14:14

tantalum · Accepted Answer · 2014-10-21 14:18:36Z

3

The WSGIApplication it self creates the instances of the classes. In python you can pass classes around, just like you would pass instances of a class. For example:

class A:
    def __init__(self):
        print "A Created"

def foo(cls):
    inst = cls()

foo(A)

If you run this script it will print out "A Created" because you are passing in the class to foo which is creating a new instance from that class.

answered Oct 21, 2014 at 14:18

tantalum

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4 Comments

xiº Over a year ago

Small another question: is there some way to get access to variables of class A?

tantalum Over a year ago

Yes, it's just like everything else in python.

xiº Over a year ago

but in this case I have no any name of instance of class. How it is possible without global var?

tantalum Over a year ago

There is a difference between a class and an instance. In the example of inst is an Object that is an instance of A. If A had static variables or functions than inside foo you could access them with cls.staticFunction().

Stephen Lin · Accepted Answer · 2014-10-21 14:31:21Z

0

You can check source code of webapp2 to verify this.

In __init__ function of WSGIApplication , list '[('/', MainPage),('/gbook', Guestbook)]' will be passed as variable routes, then they will be instantiated by Route class.

answered Oct 21, 2014 at 14:31

Stephen Lin

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