[issue32060] Should an ABC fail if no abstract methods are defined?

[Raymond Hettinger]

Raymond Hettinger  added the comment:

FWIW, I should have add a bit more explanation.  The logic for ABCs 
instantiation is roughly:

if any(method.is_abstract() for method in dir(cls)):
raise TypeError('I refuse to instantiate')
inst = instantiate(cls)

The edge case for any() is to default to False for an empty input sequence and 
this logic applies to ABCs as well.

Alternatively, you can think of the it as using all() style logic, "instantiate 
this class only if all of the methods are non-abstract even if the class has no 
methods at all".  The edge case for all() is that it returns True for an empty 
input.

Another way of describing the logic is that the normal case for instantiating a 
class is to create an instance unless there is still work to be done (as 
evidenced by the presence of one or more abstract methods).  

The abstract methods are essentially a todo list for subclassers.  If there are 
no open todos (even if there were never any todos), instantiation can go 
forward -- "You can't build that building until you have all the permits, but 
there are no permit requirements in this case, so go ahead with the 
construction."

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[issue32060] Should an ABC fail if no abstract methods are defined?

[R. David Murray]

R. David Murray  added the comment:

The docs start out with "as outlined in PEP 3119; see the PEP for why this was 
added to Python".  I think it would be a good doc enhancement request to 
extract the motivation and other description out of that pep and put it in the 
main docs in a suitable form, and make sure this point is included.  We used to 
just treat the peps as extensions of the documentation, but we've found over 
time that it is better to actually move the important parts of the text into 
the main docs, since the actual implementation and sometimes even how we talk 
about the feature tends to evolve away from the original PEP over time.

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[issue32060] Should an ABC fail if no abstract methods are defined?

[Alex Corcoles]

Alex Corcoles  added the comment:

Are you referring to something akin to Java's marker interfaces? That is, a 
class with no methods which you inherit from just for semantic meaning (e.g. 
Java's serializable interface that you implement to indicate that the class is 
designed for pickling).

In that case, I suppose the docs could mention this clearly so this is more 
"discoverable" to noobs like me and prevent further issues like this (I think 
mine wasn't an unreasonable expectation).

In any case, it's a minor point and I don't feel I have enough authority to 
discuss this topic- it's fine to close this.

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[issue32060] Should an ABC fail if no abstract methods are defined?

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I concur.

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nosy: +rhettinger
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue32060] Should an ABC fail if no abstract methods are defined?

[R. David Murray]

R. David Murray  added the comment:

No, you could create an ABC whose only purpose was to be a target of register 
calls.  As with the issue you reference, this is something better implemented 
in a linter.  I'll leave this open for at least one other dev to concur and 
close, though.

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nosy: +r.david.murray

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[issue32060] Should an ABC fail if no abstract methods are defined?

[Alex Corcoles]

New submission from Alex Corcoles :

Hi,

$ python3
Python 3.5.3 (default, Jan 19 2017, 14:11:04) 
>>> import abc
>>> class Foo(abc.ABC):
... pass
... 
>>> Foo()
<__main__.Foo object at 0x7f253e6dcb38>

I think declaring a class as ABC without declaring any abstract method is an 
error.

I've tried searching if this was already discussed, but did not find it. Maybe 
it is related to https://bugs.python.org/issue9731

Cheers,

Álex

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messages: 306433
nosy: Alex Corcoles
priority: normal
severity: normal
status: open
title: Should an ABC fail if no abstract methods are defined?
type: behavior
versions: Python 3.5

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