Consider the following code
class Foo:
i = 1 # initialization
def __init__(self):
self.i += 1
t = Foo()
print(t.i)
When exactly does the initialization of i take place? Before the execution of the init method or after it?
2 Answers
Before.
The __init__ method isn't run until Foo is instantiated. i=1 is run whenever the class definition is encountered in the code
You can see this by adding print statements:
print('Before Foo')
class Foo:
i = 1
print(f'Foo.i is now {i}')
def __init__(self):
print('Inside __init__')
self.i += 1
print(f'i is now {self.i}')
print('After Foo')
print('Before __init__')
foo = Foo()
print('After __init__')
which prints:
Before Foo
Foo.i is now 1
After Foo
Before __init__
Inside __init__
i is now 2
After __init__
Notice however, that your self.i += 1 does not modify the class attribute Foo.i.
foo.i # This is 2
Foo.i # This is 1
4 Comments
Mad Physicist
i = 1 would be run in the local scope, but i += 1 just raises an error
Stephen C
@MadPhysicist He added
self, so it's not an error.
dank8
Thanks for the tip about unnamed functions (f''). Like it a lot.
Stephen C
@Dan That is what's commonly called f-strings. It's for string formatting. python.org/dev/peps/pep-0498
The class attribute is initialized the first time you use the ClassName in the source code, also you use the class attribute in the code by doing ClassMethod.attribute
class Foo:
i = 1 # initialization
def __init__(self):
#Use ClassName.attribute to access class attribute
Foo.i += 1
#I used the Name of the class here, Foo.i will be 1
print(Foo.i)
#1
#Instantiated Foo
t = Foo()
#Value of i changes to 2
print(t.i)
#2
5 Comments
Devesh Kumar Singh
Hi @chepner, thanks for the edit but should it be
static class attribute instead?
chepner
No, the only thing in Python with "static" in its name is the static method.
Devesh Kumar Singh
Yes a class method or an attribute is inherently
staticchepner
Define "static" in Python.
Devesh Kumar Singh
That is something I don't have an answer @chepner Could you explain or provide an example :)
Foo.iprior to instantiation.