mypy: base class has no attribute x, how to type hint in base class

Question

I recently discovered mypy and I want my code to be type checked with it.

I have a Connector base class:

class Connector():
    ... some methods, but no __init__ ...

And I have several subclasses, they are all connectors, but of different types:

class Siphon(Connector)
    def __init__():
        short_name = "S"


class Tube(Connector)
    def __init__():
        short_name = "T"

When I use these objects, I normally put them in a list:

c1 = Siphon()
c2 = Tube()
list_connectors: List[Connector] = list()
list_connectors.append(c1)
list_connectors.append(c2)

Now let's say I want to write a function to return all the short names of all the connectors, as a list. I'd write something like that:

def get_names(list_connectors: List[Connector]) -> List[str]:
    tmp_list: List[str] = list()
    for c in list_connectors:
        tmp_list.append(c.short_name)
    return tmp_list

When I do that, mypy complains:

error: "Connector" has no attribute "short_name"

Which is true, the base Class Connector doesn't have this attribute, only the subclasses. But all Connector subclasses will have this attribute.

How should I correct that? I can"t use a class attribute here since all my subclasses will need their own short_name attribute.

Should I use a Union in the type hinting of my get_names function (in my real life situation, there are much more than 2 types of connectors, and the user of my API could add his own)?

I'm also not sure I could write a base __init_ function and override it in the subclasses, because the subclasses all have a different init

Answer 1

You'd add that attribute to the base type; you don't need to give it a value:

class Connector:
    short_name: str

This uses Python 3.6's Variable Annotation syntax, which is new in Python 3.6 or newer. It defines the type of an instance attribute, not a class attribute (for which there is a separate syntax).

You can use a comment otherwise, at which point you do have to give the attribute an initial value, and is a class attribute:

class Connector:
   short_name = ''  # type: str