1.8. Writing a program

Typing commands into the Python interpreter is a great way to experiment with Python’s features, but it is not recommended for solving more complex problems.

When we want to write a program, we use a text editor to write the Python instructions into a file, which is called a script. By convention, Python scripts have names that end with .py.

To execute the script, you have to tell the Python interpreter the name of the file. In a command window, you would type python hello.py as follows:

$ cat hello.py
print('Hello world!')
$ python hello.py
Hello world!

The “$” is the operating system prompt, and the “cat hello.py” is showing us that the file “hello.py” has a one-line Python program to print a string.

We call the Python interpreter and tell it to read its source code from the file “hello.py” instead of prompting us for lines of Python code interactively.

You will notice that there was no need to have quit() at the end of the Python program in the file. When Python is reading your source code from a file, it knows to stop when it reaches the end of the file.

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