This one will solve all your problems not only on Mac but to find it on Linux also ( & every basic shell).

TL;DR (you don't have to go through all the answer - just the 1st half).
LET'S GO

Run in terminal:

which python3

On Mac you should get:

/usr/local/bin/python3

WAIT!!! It's prob a symbolic link, how do you know? Run:

ls -al /usr/local/bin/python3 

and you'll get (if you've installed Python w/ Brew):

/usr/local/bin/python3 -> /usr/local/Cellar/python/3.6.4_4/bin/python3

which means that your

/usr/local/bin/python3 

is actually pointing to (the real location)

/usr/local/Cellar/python/3.6.4_4/bin/python3

That's it!

Longer version (optional): If for some reason, your

/usr/local/bin/python3 

is not pointing to the place you want, which is in our case:

/usr/local/Cellar/python/3.6.4_4/bin/python3

just back it up (+cool trick to add .orig suffix to file):

cp /usr/local/bin/python3{,.orig} 

and run:

rm -rf /usr/local/bin/python3

now create a new symbolic link:

ln -s /usr/local/Cellar/python/3.6.4_4/bin/python3 /usr/local/bin/python3 

and now your

/usr/local/bin/python3

is pointing to

/usr/local/Cellar/python/3.6.4_4/bin/python3 

Check it out by running:

ls -al /usr/local/bin/python3

Modifications to sys.path only apply for the life of that Python interpreter. If you want to do it permanently you need to modify the PYTHONPATH environment variable:

PYTHONPATH="/Me/Documents/mydir:$PYTHONPATH"
export PYTHONPATH

Note that PATH is the system path for executables, which is completely separate.

**You can write the above in ~/.bash_profile and the source it using source ~/.bash_profile

The command you are running is python3 and you are giving that command the string Hello.py as an argument. The python3 executable will therefore try to find a file named Hello.py in the current directory. The PATH variable is completely irrelevant here, that is only useful to find commands (executables), not files you give as arguments to commands.

If you want to be able to run the script directly, then you need to make sure it has a shebang line. Open Hello.py and make sure the first line is (assuming your env is in /usr/bin, let me know if not):

#!/usr/bin/env python3

Then, make the file executable:

chmod a+x /Users/user/mu_code/Hello.py

Now, since you have /Users/user/mu_code in your PATH variable, you can run the script by simply executing

Hello.py

If you want to run it as an argument to python3, then you will always need to run either python3 /full/path/to/Hello.py or first cd /full/path/to/ and then python3 Hello.py.

I just solved this problem by installing Python 3.2.2 on my Mac OS X Lion so I thought I would post how I did it in the hopes someone may find it helpful.

Firstly, I agree with shadesandcolour in that you should not modify the default copy of python that came with your mac because there could be scripts that may not work properly when run using the Python 3.0+ interpreter. Having said that, to install that latest Python (3.2.2 at the time of this post), you should first download the Python installer from the official website. Install as per normal. Run the "Update Shell Profile.command" file in /Applications/Python(ver) folder.

After python has been installed, you need to edit your path by typing "sudo nano /etc/paths" at the command line. Next, enter your password when prompted. Next just "cut" the line "/usr/local/bin" and "paste" it BEFORE the occurrence of "/usr/bin". Save the file, close any open terminals and run a new instance of terminal. Now when you type "python" at the command line, the shell will search through /usr/local/bin first and run the latest copy of python.