how to determine whether pathname1 is below bathname2

[Gelonida]

Hi Thomas,

Thomas Jollans wrote:
> On 07/11/2010 03:37 PM, Gelonida wrote:
>> #################################################
>> import os
>> def is_below_dir(fname,topdir):
>>     relpath = os.path.relpath(fname,topdir)
>>     return not relpath.startswith('..'+os.sep)
>>
>> print is_below_dir(path1,path2)
>> #################################################
>> The basic idea is, if the path name of path1
>> relative to path2 does NOT start with '..', then
>> it must be below path2
>>
>>
>> Does anybody see pitfalls with that solution?
>> Is there by any chance a function, that I overlooked,
>> which does already what I'd like to do?
> 
> It probably won't work on Windows because it isolates volumes (drive
> letters). What does, for example, os.path.relpath do when
> you pass r'c:\foo\bar', r'y:\drive\letters\are\silly' ? I see two
> reasonably correct options:
Thanks, Indeed. different drives raise an ecception.
I could catch the ValueError and let it just return False

> 
> either raise an exception (there is no relative path) or return the
> absolute path, which doesn't start with ..
> 
> On UNIX, the only potential problem I see is that it may or may not do
> what you expect when symlinks are involved somewhere.

Also true. In my case I'd prefer it would not follow, but
it doesn't really matter.


So my function in order to be portable
had now to look like:
#################################################
import os
def is_below_dir(fname,topdir):
    try:
       relpath = os.path.relpath(fname,topdir)
    except ValueError:
        return False
    return not relpath.startswith('..'+os.sep)

print is_below_dir(path1,path2)
#################################################

if I wanted to folow symlinks, then had to apply
os.path.realpath() on
fname AND on topdir