3.2. Your first Python program

Here is some Python code here. You should download this code and put it in a file called hello.py. Throughout these notes you will be given files to download. You should adopt the following practice. Create a directory (folder) in which you place these files and always start up python from the commandline after connecting to that directory.

The file hello.py is a tiny Python program to check that Python is working. Try running this program from the command line like this:

gawron$  python hello.py

(the gawron$ is my commandline prompt; the rest is what I typed on my keyboard, followed by <Enter>). The program should print:

Hello World

Or if you type:

gawron$  python hello.py Alice

it should print:

Hello Alice

Or if you type:

gawron$  python hello.py stupid

it should print:

Hello stupid

If you have a text editor that you feel comfortable with (as discussed in Section Python: Workflow and learning strategy), try editing the hello.py file, changing the ‘Hello’ to ‘Howdy’, and running it again.

Once you have that working, you’re ready for class – you can edit a Python file and run Python code; now you just need to learn Python!

3.2.1. A little more discussion

The following function is what the file contains:

import sys

def main():
   Get the name from the command line, using 'World' as a fallback.

   if len(sys.argv) >= 2:
     name = sys.argv[1]
     name = 'World'
   print 'Hello', name

Skipping line 1 for now, the block of Python code beginning on line 3 is a function definition. A function is a program, and the way you define a function in Python is to write:

def <function_name> ([Arguments]):

followed by an number of lines of indented legal Python code. The arguments are values that are passed into the function that may affect what it does. In the case of main, there are no arguments, and so the arguments list is simply ().

To see how this function works in python, let’s run the Python interactively, so that we end up seeing the python prompt after the program is run. This is done by including a -i on the commandline as follows:

gawron$  python -i hello.py

We then get:

Hello World

We can run the function main again from inside Python, because the first thing Python did on startup was load up all the definitions in the file hello.py. Then it executed main. We can tell it to execute main again as follows:

>>> main()
Hello World

And it still works. In general in this course, we’ll be executing Python code interactively by typing directly to the Python prompt.

This simple function contains a lot of features of Python which we will be discussing in more detail in the next few sections. Just as a small preview, we discuss some of them now:

  1. Line 1 is an import statement. Python comes with a small amount of built in functionality, but most of what you do when you program Python requires importing other files defining new functionality. These helper files are called modules, and any Python distribution, even the bare bones standard distribution, comes with a large number of them. The core set of modules that comes with a standard Python distribution is called the Python standard library. The sys module imported in line 1 is just one of the modules in the Python standard library. It provides access to some variables containing information gathered when python started up, including information about the machine Python is running on and how Python was started. One of these is the variable sys.argv used in line 8.

  2. The variable sys.argv is a list containing any arguments you provided when you called Python from the commandline, including the name of the program you asked Python to run. So if you typed:

    python hello_world.py

    then sys.argv looks like this:

    >>> sys.argv

    if you typed:

    python hello_world.py Alice

    then sys.argv looks like this:

    >>> sys.argv
    ['hello.py', 'Alice']
  3. Line 8 of the program is a test to see if the sys.argv list is long enough to contain a name; its length is 1 if no name was supplied on the commandline and its length is 2 only if a name was supplied. If the test is passed (the length is 2), then the variable name is set to the name supplied on the commandline (line 9). This is because sys.argv[0] gives us the first thing on the list and sys.argv[1] gives us the second. Otherwise (else), the name is set to be ‘World` (line 11).

  4. In line 12, the print command is called. This prints something to the screen. In this case, the print command prints “Hello” followed by a space (signalled by the comma) followed by the value of name.

Each of these points illustrates an important feature of Python. We’ll see them again in the upcoming sections.