[Tutor] Need help wrapping my head around duck typing

[Cory Teshera-Sterne]

Hello,

Thanks for the input. I guess you're right, this is more of a case of
argument assertion - but then I'm not sure how to do error handling here,
because, for example, multiple issues that should be dealt with in very
different ways could raise the same error (as in the original example).

I'm also not sure how using glob would work any differently here - wouldn't
I still need to iterate over a specified section the directory tree, and
therefore have to figure out how to specify it? (Admittedly, I've only
played with it for a few minutes, so I might be missing something obvious -
and file creation dates don't mean anything in this particular context.)

Thanks again,
Cory

On Tue, May 24, 2011 at 7:57 PM, Izz ad-Din Ruhulessin <
izzaddin.ruhulessin at gmail.com> wrote:

> I forgot to mention strptime<http://docs.python.org/library/datetime.html#strftime-strptime-behavior>
>
>
> 2011/5/25 Izz ad-Din Ruhulessin <izzaddin.ruhulessin at gmail.com>
>
>> Hi Cory,
>>
>> From your description it seems to me that your case is more one of
>> asserting that your function received valid arguments, than a matter of
>> static-vs-ducktyping. Even with type checking, they can still generate
>> errors (out of bounds, incorrect formatting).
>>
>> Now i've seen some discussions where people stated that type checking and
>> argument assertion are bad and that the programmer should just let the code
>> run until it bumps into and error. Python programmers are very inclined to
>> "forgiveness is easier than permission", also referred to in the article you
>> mentioned. However, the principle is only valid in cases where it is
>> guaranteed that an incorrect type or value will produce and exception. For
>> example, if you are interfacing between C and Python this can often not be
>> the case -- or it will crash your programme without any kind of traceback
>> (the dreaded segfault)
>>
>> Will an incorrect type or value raise an exception in your proposed
>> solutions? It probably will, so it is probably best to go for
>> try-except-finally clauses.
>>
>> A sidenote: personnally I would glob the directory and then sort the list
>> by file creation date.
>>
>> Kind regards,
>>
>> Izz ad-Din Ruhulessin
>>
>> 2011/5/24 Cory Teshera-Sterne <ctsterne at gmail.com>
>>
>>> Hi folks,
>>>
>>> Coming from a language background that has a different approach to
>>> variables (*cough* Java *cough*), I'm trying to understand Python's typing
>>> conventions and how to deal with unknown variable types in my code. And as a
>>> newbie, I'm really concerned with not writing code that will make the actual
>>> Python programmer who will be maintaining my code jump out the window from
>>> frustration.
>>>
>>> An example: I have a directory tree containing year directories (in the
>>> form YYYY, ie, "2011"), which contain month directories (in the form MM, ie,
>>> "05"), which in turn contain files. I'm writing a Python function to iterate
>>> over the tree & return a list of the file names from either the full tree
>>> (args==None) or a specified time period within that - ie, Feb. '10 to May
>>> '11 (args==startdate, enddate). I'm not sure what the most pythonic approach
>>> to dealing with the variables here would be. I have several options:
>>>      - accept datetime objects and then get the year/month info from that
>>>      - accept a string (and maybe check its length, add "20" if it's two
>>> digits, or reject it if it's more than 4)
>>>      - accept an integer (and maybe check that it's reasonable)
>>>
>>> My first inclination would be to only accept 4-digit year and 2-digit
>>> month integers and fail the rest, but I understand the value of dealing with
>>> different input types here. My second guess would be to use isinstance() for
>>> each possibility, since different things need to happen in each case. The
>>> "pythonic" approach, as best as I can gather, would be a try/catch block and
>>> use the exception types to do the different processing. However, for
>>> example, both trying to get a year from a string and a len() of an int raise
>>> a TypeError, and in general I'm not sure how to approach this - or am I
>>> overthinking things?
>>>
>>> Finally, I'm aware that I'm really bringing up two topics here:
>>>
>>> 1) What is the best approach in this and similar cases, in terms of
>>> actual, working code that won't make the next user of this code cry? How
>>> do/should I distinguish error types in this case?, and less importantly,
>>>
>>> 2) Can anyone help me get a good handle on the philosophical issues here?
>>> I've read several discussions (both strongly against type-checking<http://www.canonical.org/%7Ekragen/isinstance/>and more
>>> lenient<http://dobesland.wordpress.com/2007/10/07/python-isinstance-considered-useful/>,
>>> as well as good general explanations<http://www.voidspace.org.uk/python/articles/duck_typing.shtml>),
>>> but some of it's over my head, a lot of it says something like "I suggest we
>>> change Python so it does X", and I'm not sure what it all means for me and
>>> my little filename-grabbing script. It's all just a bunch of quaking to me
>>> right now ...
>>>
>>> Thanks so much,
>>> Cory
>>>
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>>
>
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