1.4. Executing Python in this Book¶
This book provides two special ways to execute Python programs. Both techniques are designed to assist you as you learn the Python programming language. They will help you increase your understanding of how Python programs work.
First, you can write, modify, and execute programs using a unique activecode interpreter that allows you to execute Python code right in the text itself (right from the web browser). Although this is certainly not the way real programs are written, it provides an excellent environment for learning a programming language like Python since you can experiment with the language as you are reading.
Take a look at the activecode interpreter in action.
What you see depends on whether you are logged in or not! Code that you write and run is saved for all future sessions only if you are logged in! If you are logged in, you should see a green Save & Run button. If you are not logged in you see only Run. In the next discussion we will refer to both variants as the Run button.
If we use the Python code from the previous example and make it active, you will see that it can be executed directly by pressing the Run button. Try pressing the Run (or Save & Run) button above.
Now try modifying the activecode program shown above. First, modify the string in the first print statement by changing the word adds to the word multiplies. Now press Run. You can see that the result of the program has changed. However, it still prints “5” as the answer. Modify the second print statement by changing the addition symbol, the “+”, to the multiplication symbol, “*”. Press Run to see the new results.
As the name suggests, Save & Run also saves your latest version of the code, and you can recover it even in later sessions when logged in. If not logged in, Run saves versions only until your browser leaves the current web page, and then you lose all modifications.
After you have run your code the first time, a Load History button that was beside the Run button turns into a slider. If you click on the slider location box, you can use your left and right arrow buttons to switch to other versions you ran. Alternately you can drag the box on the slider. Now move the slider to see a previously saved state. You can just run it by pressing Run, or edit and then save and run it as the latest version.
In addition to activecode, you can also execute Python code with the assistance of a unique visualization tool. This tool, known as codelens, allows you to control the step by step execution of a program. It also lets you see the values of all variables (introduced in Variables) as they are created and modified. The following example shows codelens in action on the same program as we saw above. Note that in activecode, the source code executes from beginning to end and you can see the final result. In codelens you can see and control the step by step progress. Note that the red arrow always points to the next line of code that is going to be executed. The light green arrow points to the line that was just executed.
The examples in this book use a mixture of the standard Python interpreter, source code, activecode, and codelens. You will be able to tell which is which by looking for either the Python prompt in the case of a shell mode program, the run button for the activecode, or the forward/backward buttons for codelens.
Check your understanding