Print and errors

Create a file ~/pyladies/02/printing.py (in the editor) and write the following commands:

print(1)
print(1, 2, 3)
print(1 + 1)
print(3 * 8)
print(10 - 2.2)
print(3 + (4 + 6) * 8 / 2 - 1)
print('*' * 80)
print("Hello" + " " + "PyLadies!")
print("Sum of numbers 3 and 8 is", 3 + 8)
print('Twinkle, twinkle, little star')
print(How I wonder what you are.)

Now run the program. Does it work?

How to read error messages

You will often find out that the code you wrote isn't working on its first run. It's because the computer is not that smart and you have to write the commands in the exact way, according to the Python rules. Don't worry, it happens even to experienced programmers. The important thing is to know how to find out what is wrong. The error messages will help you with that. For example, after we run our program, it will print this:

  File "~/pyladies/02/printing.py", line 11
    print(How I wonder what you are.)
               ^
SyntaxError: invalid syntax

First, Python prints the name of the file and line number, where the error is. Then it prints the whole line with the mistake. And finally the error type (in our case it's "syntax error") and eventually some more info.

Error messages can be hard to understand in the beginning, but you will get used to them with practice. For now, the important thing for you will be the line number. When you know that the mistake is on line 11, you can look at that line and try to find it.

When you can't find it there, it can also be a few lines above or below. Python sometimes doesn't share human views where the mistake actually is and it shows you where it noticed the mistake.

In our case the mistake is that we don't have quotation marks around the string. So add them there and run the program again. If it works, congratulations! If not, try to correct the program and repeat until it works :)

How the program works

Now that our program works we can take a closer look at what is happening when it's running. It's quite simple: Commands are performed one after the other from top to bottom. The program is like a cooking recipe: a list of instructions that tells you what to do.

Soon your programs will look more like a sorcerer's potion (wait for the full moon and if Mars is in conjunction with Jupiter then stir three times) but the principle is still the same: the computer reads the commands from top to bottom and performs them one after the other.

Print and expressions

And which instructions make up our "recipe"?

That print which we use is a function. We will talk about functions later, now all you need to know is that if you type print and after that, in parentheses, some expressions separated by comma, the values of those expressions will be printed.

And what are those expressions? You have some examples in our code: An expression can be a number, a string, or some (e.g. mathematical) operations composed of multiple expressions. For example the expression 3 + 8 will add up 3 and 8.

We will focus on expressions and their values more in the lesson about variables.

print("Hello!")

print(1, 2, 3)

print(2 + 8)
print("One and a half is ", 1 + 1/2)