[Tutor] how to check a user input path for dirve existence

Brian van den Broek bvande at po-box.mcgill.ca
Mon Apr 5 22:11:28 EDT 2004


thanks for the response. I have some reactions interspersed below.

Alan Gauld said unto the world upon 05/04/2004 19:46:
>>check to an attempt to create the directory, I won't have ruled
>>out trying to create, say, J:\spam on a system without a J: drive.
> This presupposes that drive letters can be part of a path, which 
> kind of limits things to CP/M(do we get Python for CP/M?) and 
> Windows/DOS. (I think they are the only OS's that use drive 
> letters as part of a path? Macs use drives but they have 
> names and no other distinguishing features...)

Yes to the presupposition, but a qualified no to the limit, I 
think. (Perhaps my intent was ill-described.) What I want is to 
rule out non-existent drives being referenced in the user input 
path. Since I work mostly with Win (never have managed to install 
Linux on my laptop :-( ) that is most relevant to me. But what I 
was aiming at was to disallow any path which begins X: where X is 
some string and X: is not a valid drive letter. I might be wrong, 
but for any OS not using L: (L a letter) as a drive name, the test 
will still work, albeit trivially since no such string is ever a 
valid drive name. But you are right, it won't catch non-existent 
drive references on a box with a different OS.

> Given that limitation we know that the drive info can only consist 
> of a single letter and a colon. So the only thing to test for is:
> if path[1] == ':'

That is the limitation on valid drivenames. But I also want to 
rule out user input like:


which attempts to use 'users:' as a drive name.

So, I don't think

if path[1] == ':':
	do some stuff

will work. That only gets it if the input respects the Windows/DOS 
'drive names have a single letter' convention.

Maybe I am over-fretting about possible input errors, but I am 
quite new to programming and recently read a text on general style 
which suggested trying to 'user-error proof' your code as much as 

Thinking about it more, though, I believe I've answered the 
question of which method (the one here discussed or try/except 
clauses) is better. Going this way will have to build a block of 
code for testing for valid drive letters, one for testing that no 
disallowed characters ('*', '?', etc.) are included and so forth. 
Seems exception handling will be the way to go.


> Also my brain is still adjusting to Python & Programming after 
> a weeks break so maybe I'm missing some complexity somewhere 
> - like network drives maybe...

Break? I've heard of them, but thought they were a myth ;-)

> Alan G.

Thanks again,

Brian vdB

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