[Tutor] Tips

[Mark Lawrence]

On 18/06/2014 20:17, Albert-Jan Roskam wrote:
>
>
> ----- Original Message -----
>
>> From: Mark Lawrence <breamoreboy at yahoo.co.uk>
>> To: tutor at python.org
>> Cc:
>> Sent: Wednesday, June 18, 2014 9:03 PM
>> Subject: Re: [Tutor] Tips
>>
>> On 18/06/2014 15:25, Albert-Jan Roskam wrote:
>>>   ----- Original Message -----
>>>>   From: Alan Gauld <alan.gauld at btinternet.com>
>>>>   To: tutor at python.org
>>>>   Cc:
>>>>   Sent: Wednesday, June 18, 2014 11:47 AM
>>>>   Subject: Re: [Tutor] Tips
>>>>
>>>>   On 18/06/14 01:15, Nanohard wrote:
>>>>>>     On 2014-06-17 13:35, Alan Gauld wrote:
>>>>>>
>>>>>>>     Don't test types, use the interface
>>>>>>
>>>>>>     Can you please explain what you mean by this?
>>>>>
>>>>>     He means use the Python interpreter, by going to your console and
>> typing
>>>>   "python", or in Windows
>>>>>     it's called 'IDLE'.
>>>>
>>>>
>>>>   Nope, I meant what Mark and Danny said.
>>>>
>>>>   For example don't do this:
>>>>
>>>>   def add(a,b):
>>>>         if type(a) == int and type(b) == int:
>>>>            return a+b
>>>>         else:
>>>>            raise TypeError
>>>>
>>>>   Just do this:
>>>>
>>>>   def add(a,b):
>>>>         return a+b
>>>
>>>   Given that the concept of Ducktyping has already been mentioned, is there a
>> reason why you did not mention try-except?
>>>
>>>   def add(a, b):
>>>        try:
>>>            return a + b
>>>        except TypeError:
>>>            raise
>>>
>>>   Btw, given that:
>>>>>>   {}.__add__
>>>   Traceback (most recent call last): File "", line 1, in
>> AttributeError: 'dict' object has no attribute '__add__'
>>>
>>>   Why does one only need to use 'except TypeError', not 'except
>> (TypeError, AttributeError)' in the try-except above?
>>>>>>   {} + 1
>>>   Traceback (most recent call last): File "", line 1, in TypeError:
>> unsupported operand type(s) for +: 'dict' and 'int'
>>>
>>
>> What makes you think that you're calling your add function in either
>> example above?  In the first you're not calling anything as you've
>> missed the brackets.  Even if you add (groan :) them, you'll be trying
>> to call an add method for a dict, not your add function.  In the second
>> example, you're trying to add 1 to an empty dict, again your function
>> doesn't enter into the equation (double groan :)
>
> If I call my add function, then then the return statement would be equivalent to:
> -... if a={] and b=[1]: a.__add__(b)
> -... if a={} and b=1: AttributeError, because the class dict does not have an __add__ method.
> That's why I thought an AttributeError would also have to be caught, just in case the caller is stupid enough to give a dict as the first argument. But indeed (Alan) it was silly of me to just 'raise' and not doing anything else with it.
>

Now you've completely lost me.  Please explain precisely what you think 
your function does, and how it relates to the two examples that you 
tried that gave exceptions.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

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