Trjtdtk

This is Bash. The behavior is similar in fish.

$ which python
/usr/bin/python
$ alias py=python
$ type py
py is aliased to `python'

​And then, running type -P py prints nothing, where as I expected to print /usr/bin/pyton in a similar fashion to what is seen below.​

$ type ls
ls is aliased to `ls --color=auto'
$ type -P ls
/bin/ls

The documentation for the -P option reads

 -P force a PATH search for each NAME, even if it is an alias,
 builtin, or function, and returns the name of the disk file
 that would be executed

I've confirmed that /usr/bin (the directory where python is located) is in PATH.

What is going on here?

This:

force a PATH search for each NAME, even if it is an alias,

does not mean that bash will expand the alias and then search for the expanded command. It means that, if there were a command foo, and also an alias foo, the type -P foo will still look for the command named foo, even though there's an alias masking it. So bash isn't expanding py in type -P py to be python, and it won't show /usr/bin/python.

What’s going on is that your shell is looking for a binary named py in each directory on your PATH, and not finding any.

type -P doesn’t interpret aliases or functions; it forces the given name to be searched on the path, ignoring any other available command not of type “file” with the same name.

(There’s an added subtlety with type -p and type -P: they take hashes into account, so they will show a hashed value if one exists, without looking in the PATH. But that’s not involved here.)