[Tutor] Need help wrapping my head around duck typing

[Izz ad-Din Ruhulessin]

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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