How to check whether a directory is a sub directory of another directory

I like to write a template system in Python, which allows to include files. e.g. ``` This is a template You can safely include files with safe_include`othertemplate.rst` ``` As you know, including files might be dangerous. For example, if I use the template system in a web application which allows users to create their own templates, they might do something like ``` I want your passwords: safe_include`/etc/password` ``` So therefore, I have to restrict the inclusion of files to files which are for example in a certain subdirectory (e.g. `/home/user/templates`) The question is now: How can I check, whether `/home/user/templates/includes/inc1.rst` is in a subdirectory of `/home/user/templates`? Would the following code work and be secure? ``` import os.path def in_directory(file, directory, allow_symlink = False): #make both absolute directory = os.path.abspath(directory) file = os.path.abspath(file) #check whether file is a symbolic link, if yes, return false if they are not allowed if not allow_symlink and os.path.islink(file): return False #return true, if the common prefix of both is equal to directory #e.g. /a/b/c/d.rst and directory is /a/b, the common prefix is /a/b return os.path.commonprefix([file, directory]) == directory ``` As long, as `allow_symlink` is False, it should be secure, I think. Allowing symlinks of course would make it insecure if the user is able to create such links. UPDATE - Solution The code above does not work, if intermediate directories are symbolic links. To prevent this, you have to use `realpath` instead of `abspath`. UPDATE: adding a trailing / to directory to solve the problem with commonprefix() Reorx pointed out. This also makes `allow_symlink` unnecessary as symlinks are expanded to their real destination ``` import os.path def in_directory(file, directory): #make both absolute directory = os.path.join(os.path.realpath(directory), '') file = os.path.realpath(file) #return true, if the common prefix of both is equal to directory #e.g. /a/b/c/d.rst and directory is /a/b, the common prefix is /a/b return os.path.commonprefix([file, directory]) == directory ```