[Tutor] Built in functions

[Mark Lawrence]

On 07/08/2012 10:48, Joel Goldstick wrote:
> On Tue, Aug 7, 2012 at 4:37 AM, Peter Otten <__peter__ at web.de> wrote:
>> Dane Saltzman wrote:
>>
>>> I'm new to python and I was wondering if you could tell me how I would:
>>>
>>> first, define a function,distance_from_zero, with one parameter (choose
>>> any parameter name you like). Second, have that function do the following:
>>> 1. Check the type of the input it receives.
>>>
>>> 2. If the type is int or float, the function should return the absolute
>>> value of the function input.
>>>
>>> 3. If the type is any other type, the function should return "This isn't
>>> an integer or a float!"
>>
>> I'm assuming that the subject line is a hint from your professor where you
>> should look for functions that
>>
>> (a) check if a value is of a particular type
>> (b) calculate an absolute value
>>
>> I suggest that you scan the table at
>>
>> http://docs.python.org/library/functions.html
>>
>> for candidates and try to come up with a suggestion for the implementation
>> of distance_from_zero() yourself. Come back with the code you have if you
>> run into problems. We will be glad to help you fix them.
>>
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>
>
> This looks like homework.  So, I'll let Peter's response stand as what
> looks like good advice to answer your question.
>
> While you can do it that way, and that way seems to be what your
> instructor wants you to learn, among python programmers there is a
> saying that its better to ask forgiveness than permission.  Try to do
> something, and if it raises an error, take some other action.
>
> If you open up a python shell you can experiment:
>
>>>> abs(-3)
> 3
> 4.5
>>>> abs(-5)
> 5
>>>> abs (1, 3)
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: abs() takes exactly one argument (2 given)
>>>> abs('b0b')
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: bad operand type for abs(): 'str'
>>>>
>
> With this knowledge you can do something like this:
>
> def distance_from_zero(value):
>      try:
>          return abs(value)
>      except TypeError:
>          return "Sorry, can't find an abs of a type like that"
>
> Rather than testing if you can find an absolute value of the type you
> pass (there maybe other types that you want to use later)
>
> for instance complex numbers also have distance values:
>
>>>> abs(complex(3,4))
> 5.0
>
>
>

For the benefit of the OP and possibly others research the contrast in 
approach between LBYL and EAFP.  Should keep you off of the streets for 
a few minutes :)

-- 
Cheers.

Mark Lawrence.