Chapter 1, Lesson 3 Text

Chapter One: Fundamentals of Python Lesson Three: Writing Python Code Each computer programming language has rules or syntax that show exactly how to format your code. In this lesson, we'll explore some of the basic syntax rules for writing Python code. If you do not follow the syntax rules of the programming language, your code cannot be turned into a program that will run successfully. Instead, you will see one or more error messages that let you know there is a problem with your code. Code Statements Python programs are written as a series of statements, or lines of code. How does Python know that you have reached the end of a statement? Once you hit the "Enter" key to move to the next line in your program, Python knows that you are done with that statement. Here is an example statement: myNumber = 1 + 2 Copy In many other languages, a special character such as a semi-colon ( ; ) is used to end a statement. But Python knows the statement has ended when you press "Enter" to start a new line. How can you write a very long statement that doesn't fit easily in a code window? There are two ways to handle this. First, you can put the entire statement on one line. The Python language will allow you to write as long a line as you need. Of course, when viewing this line in a code editor, you will have to scroll the window left to right to see the full statement. You can also choose to break up a long statement into two or more lines. However, you wouldn't press "Enter" to split up your statement into multiple lines as shown below. myNumber = 1 + 2 Copy Python will view this as two lines of code: "myNumber=" and "1 + 2". Neither line makes any sense by itself! If you tried to run this Python program, you would see an error message. To safely break a long statement across multiple lines, add a backslash ( \ ) character at the end of one line to tell Python the statement is continued on the next line. You can break up a single statement into as many lines as you need, if you put the backslash character at the end of each line that has more statement code on the next line. The example below breaks one statement into three lines. myNumber \ = \ 1 + 2 Copy These three lines are understood by Python as a single statement, "myNumber = 1 + 2". Using the "print()" Function A function is a block of code that has been pre-written by you or someone else. Your program can call or run functions to make things happen! Python contains a built-in function called "print()" that will let you display output to the console window. We've already shown simple print() statements like the one below. Click the "Run Code" button to see print() in action! Try It Now print("I'm Alive!") Console Output... To use the print() function on your own, simply create a Python statement that starts with the key word "print", followed by opening and closing parentheses (). Inside the parentheses, you will place a pair of double quotes (""). Write the text you'd like to display to the screen in-between those quotes. Be sure to carefully match each opening parenthesis with a closing parenthesis ( and ). You will also need to use a matched set of double quotes " and " to surround your text. If your print() statement produces an error when you run the program, carefully double-check your code to make sure it meets these syntax rules. Order of Execution Python programs are arranged as a series of statements, one after another. By default, the program will run one statement at a time, starting at the top and then moving down to the next in your source file. What do you think will happen when the following Python program is run? Try it and see the results! Try It Now print("I'm First") print("I'm Second") print("I'm Third") Console Output... That's right, you will see "I'm First" printed to the screen first, followed by "I'm Second" and then "I'm Third". When reading and writing computer code, it's important that you focus on one statement at a time, starting at the top. Ask yourself what will happen when just that top statement runs. Then, look at the next statement and again figure out what it will do after the previous statement has finished. The ability to mentally "trace" your way through program code and understand when and how it will do certain things is an important skill for a programmer! Python Indentation Computer programs will often include blocks of code. A "block" of code is a series of statements that will work together to complete a single task. Some programming languages like Java and C# use curly braces { } or other symbols to define a single block of code. These braces clearly tell the program that the statements within the braces belong to a single block. However, Python code uses indentation to identify statements that belong together as a block. To indent a statement, you would add spaces or other white-space to the beginning of the line, moving the actual statement code to the right. A Python code block starts with the first indented statement and ends with the first un-indented line. The amount of indentation is up to you, but it must be the same throughout that block. In the example below, both the "umbrella = true" and the "sunShining = false" lines of code belong to the same block. They are both indented to the right by the same amount. if isRaining == true: umbrella = true sunShining = false Copy You don't need to worry about indenting any of your code right away. As you begin to write more complex programs and logic, indentation will become more important, so we will return to this concept later. For now, make sure all your Python statements start off on the left side of your source code, without any indentation. Comments Sometimes, program code can be hard to read or understand, especially if someone else wrote the code. Programmers will often leave comments in the source code to help themselves and others understand what the code is doing. Comments contain no code at all, and anything inside a comment is completely ignored when your program runs. To create a comment in Python, start with a single hashtag ( # ). Then, on the remainder of that line, you can write any useful description without worrying about syntax rules. The example below shows two comment lines and a single program statement. # This is going to be very exciting. # We're about to find out what happens when you add two numbers together! myNumber = 1 + 2 Copy You are encouraged to add comments anywhere a program statement might need more explanation, so a human reader can understand what's happening. Comments can begin on their own line or start after the end (to the right) of a Python statement. # Calculate the area of a rectangle, given the length and width length = 2 # set the length width = 4 # set the width myArea = length * width # multiply together to calculate area Copy With comments, a program becomes more understandable to human readers. You have not yet learned how to work with the data and math statements in the example above. However, the comments make it clear what the program is doing, even if the code looks mysterious. Python will ignore any line that starts with the hashtag ( # ) character and there are no limits to the number of comments you can make. It is best practice to add clear comments to your program! Work with Me: Commenting and Printing It's time to test your commenting skills! See if you can add a Python comment to the code below. Place your cursor at the beginning of the "print" line and press "Enter" to move it down a line Move back to the beginning of your new line Add a hashtag (#) and then any other comment you'd like to make Run your program, and ensure it prints out just the expected message. You should not see your comment in the program output. Try It Now alive.py print("I'm Alive!") Console Output... Now, extend this program on your own with a new print() statement. Add another program "print" statement that looks exactly like the first one, and simply display a different message of your choice. Carefully match all opening and closing parentheses and double-quote symbols. Run your code again to confirm you see both messages in the window. Don't forget, you can click on "Show Solution" to see one way to complete each exercise. If you need to start over, simply press "CTRL+F5" on Windows or "Command+R" on Mac OS to refresh your browser window and reload the code defaults. End of Lesson
Last modified: Wednesday, 8 May 2019, 7:50 AM