Shell commands are basically either aliases or programs stored on disk. You can write your programs put them in some directory and add that directory path to the shell's PATH variable.
Let's say you have a program called create.py which creates the directories. You can follow these two ways to make them available as command on a shell
Assume create.py is present in /home/bob/scripts directory
Create a wrapper script
Create a file called createDirectory with below content in /home/bob/scripts
python /home/bob/scripts/create.py $*
 
Add /home/bob/scripts to the PATH
export PATH="$PATH:/home/bob/scripts"
 
Using aliases
- Run the alias command
alias createDirectory="python /home/bob/scripts/create.py"
 
Usage
createDirectory <whatever> <arguments> <your> <program> <expects>
NOTE: You can add this alias command and export command to ~/.bashrc file so that it is run when you start a shell