Unittest - testing properties (read-only attributes)

[Paul Moore]

I have a class with a read-only attribute, and I want to add a unit
test to ensure that it really *is* read-only. I can do this as

def test_readonly(self):
"""Value and multiplier must be readonly"""
try:
self.combat.value = 1
self.fail("Value is not read only")
except AttributeError:
pass

That works, but it seems a bit clumsy. Is there a better way?

Thanks,
Paul.

 

[Diez B. Roggisch]

I have a class with a read-only attribute, and I want to add a unit
test to ensure that it really *is* read-only. I can do this as

def test_readonly(self):
"""Value and multiplier must be readonly"""
try:
self.combat.value = 1
self.fail("Value is not read only")
except AttributeError:
pass

That works, but it seems a bit clumsy. Is there a better way?

By using setattr, you could refactor the above code into a function. Looks
like this (untested):

def test_readonly(self, instance, attribute, value=1):
"""Value and multiplier must be readonly"""
try:
setattr(instance, attribute, value)
self.fail("Value is not read only")
except AttributeError:
pass


Then the testing becomes one line:

self.test_readonly(self.combat, "value")

 

[Roy Smith]

I have a class with a read-only attribute, and I want to add a unit
test to ensure that it really *is* read-only. I can do this as

def test_readonly(self):
"""Value and multiplier must be readonly"""
try:
self.combat.value = 1
self.fail("Value is not read only")
except AttributeError:
pass

That works, but it seems a bit clumsy. Is there a better way?

Thanks,
Paul.

You want something like

self.assertRaises(AttributeError, lambda: self.combat.value = 1)

 

[Peter Hansen]

You want something like

self.assertRaises(AttributeError, lambda: self.combat.value = 1)

Or, combining the two responses and avoiding the lambda:

self.assertRaises(AttributeError, setattr, self.combat, 'value', 1)

Hmm... this might be a case where the lambda form is actually the
more readable one...

-Peter

 

[Paul Rubin]

Or, combining the two responses and avoiding the lambda:

self.assertRaises(AttributeError, setattr, self.combat, 'value', 1)

Hmm... this might be a case where the lambda form is actually the
more readable one...

Yes, assignment expressions could make code more readable, if Python
supported them.

 

[Paul Moore]

Or, combining the two responses and avoiding the lambda:

self.assertRaises(AttributeError, setattr, self.combat, 'value', 1)

Hmm... this might be a case where the lambda form is actually the
more readable one...

Thanks, I hadn't thought of setattr. I was bitten by the "assignment
is a statement, so can't be used in a lambda" issue, as well

Paul.

 

[Duncan Booth]

Yes, assignment expressions could make code more readable, if Python
supported them.

An assignment expression, if such a thing existed wouldn't help here.

The point being that the expression must be evaluated inside the exception
handler in assertRaises, so you either need to delay the evaluation with a
lambda, or by passing the function and arguments in separately. If you had
an assignment expression it would be roughly equivalent to:

self.assertRaises(AttributeError, setattr(self.combat, 'value', 1))

which will throw the AttributeError instead of passing the test.

 

[Duncan Booth]

An assignment expression, if such a thing existed wouldn't help here.

Although of course it would help if still inside a lambda.

 

[Paul Rubin]

An assignment expression, if such a thing existed wouldn't help here.

The point being that the expression must be evaluated inside the exception
handler in assertRaises, so you either need to delay the evaluation with a
lambda, or by passing the function and arguments in separately. If you had
an assignment expression it would be roughly equivalent to:

self.assertRaises(AttributeError, setattr(self.combat, 'value', 1))

which will throw the AttributeError instead of passing the test.

Yes. The example I quoted used an assignment expression inside a
lambda. The person who posted it, and the person who followed it up
with the setattr alternative, both didn't notice that the assignment
expression wasn't valid Python. However, my post came out sounding
grumpier than I intended .

 

[Roy Smith]

The example I quoted used an assignment expression inside a
lambda. The person who posted it,

That was me.

and the person who followed it up
with the setattr alternative, both didn't notice that the assignment
expression wasn't valid Python.

Ugh. No, I hadn't noticed it. Thanks for pointing it out.

I don't use lambdas much. In fact, the only time I ever do use them is for
an assertRaises unit test. I've always thought that "assignment is a
statement not an expression" was one of Python's warts, and this is just
another example of why it is. Of course, "lambda isn't just a def body" is
a wart too

However, my post came out sounding
grumpier than I intended .

Actually, I hadn't noticed that either.