Creating simple script files

While we can use all of Python from the REPL, this is not a good way to produce a final application. Most of what we do with Python will be done via script files. We'll look at script files in detail in Chapter 12, Scripts, Modules, Packages, Libraries, and Applications. For now, we'll look at a few features.

A script file has to follow a few rules:

For example, we'll try to focus on names such as test_1_2.py. We can't as easily use a file named test-1.2.py; the base name isn't a valid identifier—this name looks like a Python expression. While the second name is acceptable for a top-level script, it won't work as a module or package.

We'll look at some Python syntax rules in the next section. For now, we can create a simple script file named ex_1.py that has one line:

print("π≈", 355/113)

We can also use "\u03c0\u2248" instead of "π≈". The string "\N{GREEK SMALL LETTER PI}\N{ALMOST EQUAL TO}" will also work.

Once we have this file, we can have Python execute the file as follows:

MacBookPro-SLott:Chapter_1 slott$ python3 ex_1.py
π≈ 3.1415929203539825

We've provided a filename, ex_1.py, as the positional argument to the python3 program. Python reads the file and executes each line. The output that we see is the text printed to the console by the print() function.

The file is found by Python using ordinary OS rules for locating files, starting with the current working directory. This will work with any kind of filename.

If we followed the naming rules for our file—the filename is an identifier and the extension is .py—we can also use the following command to execute a Python module:

MacBookPro-SLott:Chapter_1 slott$ python3 -m ex_1
π≈ 3.1415929203539825

The -m ex_1 option forces Python to search for a module named ex_1. The file associated with this module is named ex_1.py. Python has a search path that it uses to find the requested module. Unless special arrangements are made, Python will search the local directory first, and then will search the library directories. This allows us to run our scripts and Python's built-in applications with a simple, uniform syntax. It also allows us to add our own applications and modules by modifying the PYTHONPATH environment variable.

We'll look at the search path in Chapter 12, Scripts, Modules, Packages, Libraries, and Applications. The detailed documentation for the search path is part of the site package.

print(
   "Hello world",
   "π≈",
   355/113
)