Why is 'Class ... ' excuted only once when I created several instances?

Question

I wrote codes as follow:

class a:
    print('hello')
    def __init__(self):
        self.name = 'Adam'
b = a()
c = a()

There is only one 'hello' involved, although I creat instances of Class a twice.

hello

It seems that codeblocks below 'class a' only be excuted one time even though I creat the instances twice. I am confused and I want to know how it works.

`print('hello')` is executed when the class is defined, it has nothing to do with instances. When you create an instance, the `__init__()` method is executed. — Barmar, May 02 '22 at 16:08
You'll get that output even if you never create any instances. — Barmar, May 02 '22 at 16:09

score 2 · Accepted Answer · answered May 02 '22 at 16:09

2

The code inside a class is only run once, when the program is run. After that, when you instantiate that class (__init__), the code inside the __init__ method is called.

So, if you have a class like this:

class A:
    print('A run')
    def __init__(self, name):
        print(f'A init {name}')
b = A('B')
c = A('C')

What is printed is:

A run
A init B
A init C

answered May 02 '22 at 16:09

2pichar

1,348
4
17

Thanks a lot and I got it! But I still have and futher question about this problem (that is how and when the memebers in a class was bound to the instance) at [this page](https://stackoverflow.com/questions/72095345/how-other-members-of-a-python-class-are-bound-to-the-instance) – Eason Wang May 03 '22 at 06:07

score 0 · Answer 2 · answered May 02 '22 at 16:11

Just because you initialize a new object, you do not call the code within its class.

You just call the __init__. Method.

When Python first "reads" your code, it runs it all. And if it encounters a print() statement, it will run it too. So, the python interpreter sees your code like this:

# It runs this line - Just a comment, so nothing to run

# A class deceleration, let us look at its body and then store it
class a:
    # A print statement - I will run it. It does not add to the body, but I run all of the code I see
    print('hello')
    # This defines a method, so I will add it to the members of a
    def __init__(self):
        self.name = 'Adam'

#  A new variable, I will call `__init__` - But just `__init__`
# I will not reparse the entire block, I will just jump to the location (Assuming JIT) where that function is stored and run it
b = a()
c = a()

So, we could rewrite your entire code like this:

print("hello")
class a:
    def __init__():
         pass

b = a()
b.name = "Adam"
c = a()
c.name = "Adam"

Thanks a lot, I got it. But I still have and futher question about this problem (that is how and when the memebers in a class was bound to the instance) at [this page](https://stackoverflow.com/questions/72095345/how-other-members-of-a-python-class-are-bound-to-the-instance) — Eason Wang, May 03 '22 at 06:04

Why is 'Class ... ' excuted only once when I created several instances?

2 Answers2