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"Hello, world" of Python

If you need help at any time, put your red sticky note on the back of your laptop. When you've finished the steps on all of these pages, put the green sticky note on the back of your laptop.

Using a Python shell

One way to use Python is by typing code into the Python interpreter interactively. To open a Python shell on our server, type

python

in the Bash shell. You should see your prompt change to >>>, like this:

Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

indicating that you're now in a Python shell. In the Python shell, type

print("Hello, world")

and hit \keys{Enter}. Then, to close your Python shell, either use \keys{Ctrl+D}, or type quit() and hit \keys{Enter}.

There are two widely used "major" versions of Python: Python 2 and Python 3. The shell we were given when we typed python without specifying a version was a Python 2 shell. To open a Python 3 shell, type

python3

at the Bash shell prompt, and note the different version number:

Python 3.4.3 (default, Sep 14 2016, 12:36:27) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

You can close the Python 3 shell in the same way: with \keys{Ctrl+D} or quit().

One difference between Python 2 and Python 3 that can be confusing is the print syntax.

Open a python2 shell and verify to yourself that in Python 2, we can use either of these to print a message:

print("Hello, world")
print "Hello, world"

Now, close your python2 shell and open a python3 shell. In Python 3, only the first way is valid:

print("Hello, world")

but when you try running

print "Hello, world"

in Python 3, you will get the error message

SyntaxError: Missing parentheses in call to 'print'

\pagebreak

Writing a Python script

We can also put Python code in a file, and execute it as a script. In the Bash shell, create a file called hello.py:

nano hello.py

and put

print("Hello,world")

in it. Then use \keys{Ctrl + O} to write it to file, hit \keys{Enter} to confirm the file name to which to save, and use \keys{Ctrl + X} to exit nano.

Now you can execute the script from the Bash shell with

python hello.py

(using either a relative path or absolute path to hello.py).

To specifically run our script with Python 2 or Python 3 (rather than accepting whatever the system default is), we can use

python2 hello.py

or

python3 hello.py

As we've learned previously, if we want to be able to execute a script without having to specify the interpreter with which to execute it, we need to

  • add a shebang at the top of the file specifying which interpreter to use to run it, and
  • make the script executable.

Open your hello.py file with nano again, and add the shebang to the top:

#!/usr/bin/env python

Save the file and close nano, then make the file executable with

chmod a+x hello.py

Now you can run it with

./hello.py

or by typing the full path to hello.py in your Bash shell.

To explicitly specify whether you want to run your script with Python 2 or Python 3, you can change the shebang to

!#/usr/bin/env python2

or

#!/usr/bin/env python3

respectively.

\pagebreak

Using a Python notebook

Another popular way to use Python is with a Python notebook. A notebook lets you write Python code, execute it, and see the results (text output or images), all from a web browser. We have set up a Python notebook server that you can access for the duration of this workshop.

Open a browser. (Chrome is recommended; some students have had problems trying to use our notebook server with Safari.) In the browser, visit the URL for the server you're working with today:

You may have to tell your browser to add an exception to allow access to this site, even though it uses an untrusted certificate. When you reach the login screen, log in with your net ID and the same password that you use to SSH into the server.

When you log in, you will be "located" in your home directory. To start a new Python 3 notebook there, choose \menu{New > Python 3}. At the top of the page in your new notebook, click on where it says "Untitled" and change the notebook name to "hello".

Then in the first "cell" of the notebook, type

print("Hello, world")

and click the \menu{\ForwardToEnd} button or \menu{Cell > Run Cells} to run the code in that cell.

Running "Hello, world" in a Python notebook

Two nice features of Python notebooks are:

  • You can keep the code and results of its execution together,
  • You can add Markdown cells - use \menu{Cell > Cell Type > Markdown} - to add an explanation of your work, making it a complete document.