[python-uk] Unit Testing Problem

Duncan Booth duncan.booth at suttoncourtenay.org.uk
Fri Mar 26 08:43:36 EST 2004


> Hi,
> 
> Trying to get my head around Unit Testing (and Python) and I've come
> across a strange 
> problem when adding some tests to the example from
> pyunit-1.4.1\doc\PyUnit.html. 
> 
> class Widget:
>     def __init__(self, widgetname, size=(50,50)):
>         self.name = widgetname
>         self._size = size
>     def func(self):
>         return 4
>     def name(self):
>         return self.name
> 
> class WidgetTestCase(unittest.TestCase):
>     def setUp(self):
>         self.widget = Widget("The widget")
>     def testFunc(self):
>         assert self.widget.func() == 3, 'Invalid value'
>     def testName(self):
>         assert self.widget.name() == "The widget", "Invalid name"
> 
> I'm getting the error "TypeError: 'str' object is not callable" when I
> run my test suite. 
> It's complaining about the last line of code. 
> However, if I take the function call brackets off (assert
> self.widget.name == "The widget", "Invalid name") 
> it works fine. I expected it to work like the func() example above it.

I suspect you may have copied the example incorrectly, but what is 
happening is this:

You have defined a class Widget with a method called 'name', but in 
your __init__ method you are assigning to an attribute also called 
'name'. The attribute is stored in the instance and this hides the 
method, so when you access self.widget.name you are retrieving the 
string rather than the method.

When you attempt to call the string retrieved by self.widget.name you 
get an error telling you that you cannot call a string.

Remember that functions and methods are first class objects in 
Python, so for example the test:

         assert self.widget.func() == 3, 'Invalid value'

could equally well be written as:

        x = self.widget.func
        assert x() == 3, 'Invalid value'

The compiler does not, and indeed cannot, make a distinction between 
accessing an attribute and calling a method: you could have assigned 
a function or some other callable to self.widget.name by the time it 
tries to access it.

I hope this help.

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Have Shaw & Sons limited considered that appending a confidentiality 
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Duncan Booth

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