[Tutor] Is there a simpler way to remove from a set?

[Mats Wichmann]

On 5/6/21 6:52 PM, Leam Hall wrote:
> Base code:
> 
> ###
> def build_dir_set(my_list, exclude_dirs):
>    my_set  = set()
>    for item in my_list:
>      path = Path(item)
>      parent = str(path.parent)
>      my_set.add(parent)
>    my_new_set = set()
>    for exclude_dir in exclude_dirs:
>      for parent in my_set:
>        if re.match(exclude_dir, parent):
>          my_new_set.add(parent)
>    return my_set - my_new_set
> ###
> 
> Where "my_list" and "exclude_dirs" are lists of directories
> 
> For example, "my_list" might include:
>      /opt/backup/home/me/Desktop/Documents/my_cool_file.txt
>      /opt/backup/home/me/lang/git/software/src/cute_image.jpg
>      /home/me/lang/git/this/noop.py
>      /home/me/lang/git/that/anop.py
> 
> And "exclude_dirs" might be"
>      /home/me/lang/git
> 
> The return should be a set of directories that do not start with any
> of the excluded directory names:
>      /opt/backup/home/me/Desktop/Documents
>      /opt/backup/home/me/lang/git/software/src
> 
> I couldn't remove from the set during iteration, python complained. 
> Hence having to make a second set.

To take Alan's reply down to an even shorter one:

The "simpler way" is to not add entries that don't qualify, then you 
have nothing to remove.  That requires a slightly different way of 
thinking about your code.

There is, however, nothing wrong in general with using set operations 
like you do to remove unwanted entries from a set.