Running Python in the Command Line¶

Now that you’ve written Python code, it’s time to see how to run it!

Python in Command Line¶

In this tutorial, we’ll be focusing on how to run Python within the command line. In a following tutorial, we’ll show how to run Python in a Markdown file using Hydrogen.

There are two methods to run Python code in the Command Line: the Interactive Python Interpreter and Python scripts.

Interactive Python Interpreter¶

Remember how the Python Basics tutorial was formatted?

>>> a_dict
{'key': 'value', 'integer': 5, 'float': 2.0, 'boolean': True}
>>> a_dict['key']
value

That was an example of the Interactive Python Interpreter: a way to intuitively run Python within the command line.

To enter the Interactive Python Interpreter, open your command line (MacOS/Linux = Terminal, Windows = PowerShell) and enter python:

$ python
Python 3.6.5
Type "help", "copyright", "credits" or "license" for more information.
>>> _

If the command doesn’t work, see python Command Not Found.

(Note : The $ on the first line indicates that we’re in Terminal - it’s not part of the command that you type. In Powershell, you’d see a > instead.)

Now you can run any lines of Python code that you’d like: try it out for yourself!

>>> a_list = ['can store anything!', 5, 2.0, True]

>>> a_list
['can store anything!', 5, 2.0, True]
>>> a_list[0]
can store anything!
>>> a_list[3]
True

To leave the Interactive Python Interpreter:

>>> quit()
$ _

Note that variables are only stored until you quit the interpreter.

Python Scripts¶

The Interactive Python Interpreter is great for quickly testing lines of Python code, but what if you want to write something more complex?

In the last tutorial, you wrote part of a Python script: a file with the *.py extension. To run the code in a Python script in the command line:

Navigate to the script’s directory with the ls and cd commands. Need a refresher? See Basic Commands
Enter python, followed by a space, followed by the name of the file.

Let’s say we have a script called foo.py containing this line:

print('Hello world!')

Now, let’s run the script in the command line:

$ python foo.py
Hello world!

What this does is run the main body of the script: the lines which are not in functions. To use functions, define them at the beginning of the script and call them in the main body.

foo.py:

def greet():
    print('Hello world!')

greet()

Command line:

$ python foo.py
Hello world!

Python runs code lines in order from start to end; if you don’t define functions at the beginning, Python won’t know what to do when you call them!

Now, run the code in Interactive-Tutorial-2-Python-Basics.py. Fix any TODO’s that fail the tests.