[python-uk] Unit Testing Problem

[Duncan Booth]

> 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.

> CONFIDENTIALITY NOTICE
> 
> This communication contains information which is confidential and may
> also be privileged. It is for the exclusive use of the intended
> recipient(s). If you are not the intended recipient(s) please note that
> any distribution, copying or use of this communication or the
> information in it is strictly prohibited. If you have received this
> communication in error please notify us by e-mail or by telephone
> (+44(0) 1322 621100) and then delete the e-mail and any copies of it. 
> 
> This communication is from Shaw & Sons Limited whose registered office
> is at Shaway House, 21 Bourne Park, Bourne Road, Crayford, Kent DA1 4BZ.
> The views expressed in this communication may not be the views held by
> Shaw & Sons Limited.
> 
> This message has been checked for all known viruses by McAfee VirusScan.

Have Shaw & Sons limited considered that appending a confidentiality 
notice to emails that are patently not intended to be confidential 
makes it almost impossible for them to defend their position if 
anyone does distribute an email that really was intended to be 
confidential?

Duncan Booth

-- 

IMPORTANT: This email is intended for the use of  the  individual
addressee(s)  named  above  and  may  contain information that is
confidential,  privileged  or  unsuitable  for  overly  sensitive
persons  with  low  self-esteem, no sense of humour or irrational
religious beliefs. If you are not  the  intended  recipient,  any
dissemination,  distribution  or  copying  of  this  email is not
authorised (either explicitly or implicitly) and  constitutes  an
irritating  social  faux  pas. Unless the word absquatulation has
been used in its correct context somewhere  other  than  in  this
warning, it does not have any legal or grammatical use and may be
ignored. No animals were  harmed  in  the  transmission  of  this
email,  although  the  yorkshire  terrier  next door is living on
borrowed time, let me tell you. Those of you with an overwhelming
fear  of  the unknown will be gratified to learn that there is no
hidden message revealed by reading  this  warning  backwards,  so
just ignore that Alert Notice from Microsoft: However, by pouring
a complete circle of salt around yourself and your  computer  you
can  ensure  that  no harm befalls you and your pets. If you have
received this email in error, please  add  some  nutmeg  and  egg
whites  and place it in a warm oven for 40 minutes. Whisk briefly
and   let    it    stand    for    2    hours    before    icing.