C W <tmrs...@gmail.com> writes:

> I am new to OOP.

Welcome, and congratulations on learning Python.

> I'm a bit confused about the following code.
>     def print_time(self):

Begins a function definition. The function will receive one argument
(the class instance), and bind the name ‘self’ to that.

>         time = '6:30'

Creates a new text string, and binds the name ‘time’ to that.

Nothing else ever uses that local name.

>         print(self.time)

Gets the value of ‘self.time’ – which means, get the object referred to
by ‘self’, look up an attribute named ‘time’, and get the object that
name is bound to – then pass that object as an argument to call ‘print’.

The ‘print’ function will get a text representation of the object, and
emit that text to output.

Nothing else happens in the function; so, the local name ‘time’ falls
out of scope and is never used.

The function then returns ‘None’.

> clock = Clock('5:30')
> clock.print_time()
> 5:30
> I set time to 6:30

No, you bound *a* name ‘time’ locally inside the ‘print_time’ function;
but you never used that afterward.

The local name ‘time’ is a different reference from the ‘self.time’

> How does line-by-line execution run inside a frame? How does __init__
> work?

After Python creates an instance of a class (using that class's
‘__new__’ method as the constructor), it then tells the instance to
initialise itself; it calls the object's ‘__init__’ method as the

So the initialiser, named ‘__init__’, is called once the object exists,
but before the caller gets to use that object. The initialiser's job is
to initialise the state of the object; your class does this by setting
the per-instance ‘time’ attribute.

So, by the time your statement binds the name ‘clock’ to the new Clock
instance, that instance already has an attribute ‘clock.time’ with the
value ‘'5:30'’.

That attribute is then available when something else uses that object;
for example, the ‘print_time’ method accesses that attribute and prints
it out.

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Ben Finney


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