Struct in Python

Question

I'm writing a struct class in Python and was wondering if this were a good way to write it:

class Struct:
    def __init__(self, **kwargs):
        for key, value in kwargs.items():
            if isinstance(getattr(self, key), str):
                setattr(self, key, value)

    def create(*args):
        newStruct = type("Struct", (Struct,), {"__init__": Struct.__init__})
        for arg in args:
            setattr(newStruct, arg, str())
        return newStruct

and to initialize the object:

myStruct = Struct.create('x', 'y')
variable = myStruct(x=2, y=4)

Also, is it actually worth using?

FWIW: github.com/lihaoyi/macropy uses a "case" macro which is similar to what you have. — Dair
– Dair, Commented Feb 13, 2016 at 8:50

200_success · Accepted Answer · 2016-02-13 07:55:02Z

This looks like a mutable version of namedtuple, with some oddities.

Following from your example (myStruct = Struct.create('x', 'y'); variable = myStruct(x=2, y=4)), I would expect variable.z to fail — and indeed it does raise an AttributeError: 'Struct' object has no attribute 'z'. I would also like to see variable.z = 5 fail with the same error, but it doesn't. So, if it doesn't enforce what members can be set, what's the point of this object? Why wouldn't I just use a regular dict?

The __init__ function looks like a constructor, such that I would be tempted to write myStruct = Struct('x', 'y') instead of myStruct = Struct.create('x', 'y')) — but it actually doesn't work that way. Also, I'd expect the constructor to work like namedtuple, accepting a typename, followed by field_names as either a list or a space-delimited string.

Defaulting values to an empty string is weird; I'd expect the default values to be None. You can initialize the dictionary using dict.fromkeys().

It would be nice to have __repr__() overridden, to make it easy to inspect the objects' contents for debugging.

Suggested implementation

def Struct(typename, field_names):
    class StructType:
        def __init__(self, **kwargs):
            for key, value in kwargs.items():
                setattr(self, key, value)

        def __setattr__(self, key, value):
            if hasattr(self, key):
                super().__setattr__(key, value)
            else:
                getattr(self, key) # Trigger AttributeError

        def __repr__(self):
            return repr(self.__dict__)

    return type(
        typename,
        (StructType,),
        dict.fromkeys(field_names.split() if isinstance(field_names, str)
                      else field_names)
    )

Sample run:

>>> myStruct = Struct('myStruct', 'x y')
>>> variable = myStruct(x=2)
>>> variable
{'x': 2}
>>> variable.y
>>> variable.y = 4
>>> variable.y
4
>>> variable.z = 5
Traceback (most recent call last):
  …
AttributeError: 'myStruct' object has no attribute 'z'
>>> variable.z
Traceback (most recent call last):
  …
AttributeError: 'myStruct' object has no attribute 'z'

getattr(self, key) seems needlessly cryptic. How about raise AttributeError(key) instead. — Perry
– Perry, Commented Jun 2, 2016 at 2:01

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