How to iterate over a the attributes of a class, in the order they were defined? [duplicate]

Answer 1

This information is not accessible, since attributes are stored in a dict, which is inherently unordered. Python does not store this information about order anywhere.

If you want your serialization format to store things in a certain order you would specify that order explicitly. You would not use dir, which contains things you don't know or care about, you would explicitly provide a list (or whatever) describing what to serialize.

Answer 2

I'm new in python, so my solution may be not effective

class Q:
def __init__(self):
    self.a = 5
    self.b = 10

if __name__ == "__main__":
    w = Q() 
    keys = w.__dict__.keys()
    for k in keys:
        print "Q.%s = %d"%(k,w.__dict__[k])

Output is:

Q.a = 5
Q.b = 10

Answer 3

put this method in your class

def __setattr__(self, attr, value):
    if attr not in dir(self):
        if attr == "__ordered_fields__":
            super.__setattr__(self, attr, value)
        else:
            if not hasattr(self, "__ordered_fields__"):
                setattr(self, "__ordered_fields__", [])
            self.__ordered_fields__.append(attr)
            super.__setattr__(self, attr, value)

and to get the fields in order, just do something like:

print(self.__ordered_fields__)

Answer 4

I don't think this is possible in Python 2.x. When the class members are provided to the __new__ method they are given as a dictionary, so the order has already been lost at that point. Therefore even metaclasses can't help you here (unless there are additional features that I missed).

In Python 3 you can use the new __prepare__ special method to create an ordered dict (this is even given as an example in PEP 3115).