How to use attr.validators.optional

Question

As per the attrs documentation, optional attributes should be declared as follow:

>>> @attr.s
... class C(object):
...     x = attr.ib(validator=attr.validators.optional(attr.validators.instance_of(int)))
>>> C(42)
C(x=42)
>>> C("42")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None, kw_only=False), <type 'int'>, '42')
>>> C(None)
C(x=None)

However, when I try to use optional attributes, I get the following results:

python
Python 2.7.15 (default, Jul 23 2018, 21:27:06)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin

class with mandatory attribute

class definition

>>> import attr
>>> @attr.s
... class Result(object):
...   log = attr.ib(validator=attr.validators.instance_of(str))
...

Instantiation of the class

with missing attribute

>>> test = Result()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 2 arguments (1 given)

OK

with invalid type

>>> test = Result(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<attrs generated init 086c7a4a2a56d2e9002255c4e881a832c6bc5360>", line 4, in __init__
  File "/.../.venv/lib/python2.7/site-packages/attr/validators.py", line 32, in __call__
    value,
TypeError: ("'log' must be <type 'str'> (got 1 that is a <type 'int'>).", Attribute(name='log', default=NOTHING, validator=<instance_of validator for type <type 'str'>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <type 'str'>, 1)

OK

with correct type

>>> test = Result('aaa')
>>> test.log
'aaa'

OK

class with optional attribute

class definition

>>> import attr
>>> @attr.s
... class Result(object):
...   log = attr.ib(validator=attr.validators.optional(attr.validators.instance_of(str)))
...

Instantiation of the class

with missing attribute

>>> test = Result()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 2 arguments (1 given)

KO

with invalid type

>>> test = Result(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<attrs generated init 7e2ff957cec78e81193f38c89e2f4eb2ff2dad4e>", line 4, in __init__
  File "/.../.venv/lib/python2.7/site-packages/attr/validators.py", line 106, in __call__
    self.validator(inst, attr, value)
  File "/.../.venv/lib/python2.7/site-packages/attr/validators.py", line 32, in __call__
    value,
TypeError: ("'log' must be <type 'str'> (got 1 that is a <type 'int'>).", Attribute(name='log', default=NOTHING, validator=<optional validator for <instance_of validator for type <type 'str'>> or None>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <type 'str'>, 1)

OK

with correct type

>>> test = Result('aaa')
>>> test.log
'aaa'

OK

What am I doing wrong to use optional attributes? Where is my misconception?

E. Jaep · Accepted Answer · 2019-02-07 10:07:24Z

1

The correct class definition is:

>>> @attr.s
... class Result(object):
...   log = attr.ib(default=None, validator=attr.validators.optional(attr.validators.instance_of(str)))

answered Feb 7, 2019 at 10:07

E. Jaep

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4 Comments

hynek Over a year ago

Yes, optional means "can be None" – not "can be missing".

E. Jaep Over a year ago

@hynek Thanks for the pointer. I'm apparently still struggling with the difference between missing and null (coming from the .net world where such a distinction does not exist)

shadowtalker Over a year ago

@E.Jaep Python doesn't have a notion of "missing" as such. You have to consider the underlying data model here. If you never write foo.x = at any point, foo never gets an x attribute. That's what "missing" could mean here -- the constructor/init function would never assign x to the object being constructed/initialized. Another possible meaning could be that the parameter has a default value, meaning you can omit it from the call. This is what default= does in attrs.

OscarVanL Over a year ago

I get No mandatory attributes allowed after an attribute with a default value or factory when using this exact code?!?

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