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Classes

Метки: python

Like everything else in Python, the definition of a class uses a minimum of additional syntax. You use the class keyword, and inside the body you use def to create methods. Here’s a simple class:

# PythonForProgrammers/SimpleClass.py
class Simple:
    def __init__(self, str):
        print("Inside the Simple constructor")
        self.s = str
    # Two methods:
    def show(self):
        print(self.s)
    def showMsg(self, msg):
        print(msg + ':',
        self.show()) # Calling another method

if __name__ == "__main__":
    # Create an object:
    x = Simple("constructor argument")
    x.show()
    x.showMsg("A message")

Both methods have self as their first argument. C++ and Java both have a hidden first argument in their class methods, which points to the object that the method was called for and can be accessed using the keyword this. Python methods also use a reference to the current object, but when you are defining a method you must explicitly specify the reference as the first argument. Traditionally, the reference is called self but you could use any identifier you want (if you do not use self you will probably confuse a lot of people, however). If you need to refer to fields in the object or other methods in the object, you must use self in the expression. However, when you call a method for an object as in x.show( ), you do not hand it the reference to the object - that is done for you.

Here, the first method is special, as is any identifier that begins and ends with double underscores. In this case, it defines the constructor, which is automatically called when the object is created, just like in C++ and Java. However, at the bottom of the example you can see that the creation of an object looks just like a function call using the class name. Python’s spare syntax makes you realize that the new keyword isn’t really necessary in C++ or Java, either.

All the code at the bottom is set off by an if clause, which checks to see if something called __name__ is equivalent to __main__. Again, the double underscores indicate special names. The reason for the if is that any file can also be used as a library module within another program (modules are described shortly). In that case, you just want the classes defined, but you don’t want the code at the bottom of the file to be executed. This particular ifstatement is only true when you are running this file directly; that is,  if you say on the command line:

Python SimpleClass.py

However, if this file is imported as a module into another program, the __main__ code is not executed.

Something that’s a little surprising at first is that while in C++ or Java you declare object level fields outside of the methods, you do not declare them in Python. To create an object field, you just name it - using self - inside of one of the methods (usually in the constructor, but not always), and space is created when that method is run. This seems a little strange coming from C++ or Java where you must decide ahead of time how much space your object is going to occupy, but it turns out to be a very flexible way to program. If you declare fields using the C++/Java style, they implicitly become class level fields (similar to the static fields in C++/Java)