Pythonic Way to Initialize (Complex) Static Data Members

Question

I have a class with a complex data member that I want to keep "static". I want to initialize it once, using a function. How Pythonic is something like this:

def generate_data():
    ... do some analysis and return complex object e.g. list ...

class Coo:
    data_member = generate_data()
    ... rest of class code ...

The function generate_data takes a long while to complete and returns data that remains constant in the scope of a running program. I don't want it to run every time class Coo is instantiated.

Also, to verify, as long as I don't assign anything to data_member in __init__, it will remain "static"? What if a method in Coo appends some value to data_member (assuming it's a list) - will this addition be available to the rest of the instances?

Thanks

Answer 1

You're right on all counts. data_member will be created once, and will be available to all instances of coo. If any instance modifies it, that modification will be visible to all other instances.

Here's an example that demonstrates all this, with its output shown at the end:

def generate_data():
    print "Generating"
    return [1,2,3]

class coo:
    data_member = generate_data()
    def modify(self):
        self.data_member.append(4)

    def display(self):
        print self.data_member

x = coo()
y = coo()
y.modify()
x.display()

# Output:
# Generating
# [1, 2, 3, 4]

Answer 2

As others have answered you're right -- I'll add one more thing to be aware of: If an instance modifies the object coo.data_member itself, for example

self.data_member.append('foo')

then the modification is seen by the rest of the instances. However if you do

self.data_member = new_object

then a new instance member is created which overrides the class member and is only visible to that instance, not the others. The difference is not always easy to spot, for example self.data_member += 'foo' vs. self.data_member = self.data_member + 'foo'.

To avoid this you probably should always refer to the object as coo.data_member (not through self).