# Python Expressions – Intro to Computer Science So, the Python grammar is much stricter than the English grammar or most natural language grammars. In English, if we say something nongrammatical like me go to the store, another English speaker might laugh at us. But they can probably understand what we meant. In Python, the code must match the language grammar exactly. Here we saw when we tried to evaluate 2 plus without the extra operand, we got a syntax error, that means it’s not grammatical. If we said 2 plus to someone in English well they might be able to figure out that that means the same thing as 2. In Python it needs to match the grammar exactly. So now we’re going to look at the Python grammar for arithmetic expressions. We won’t see the entire Python grammar here. But enough to get an idea of why the expressions we’ve seen were valid and why the 2 plus is not. So first of all, an expression is something that has a value. We’re going to start our grammar with an non-terminal expression. And we can make an expression by combining two expressions with an operator. This is sort of like the sentence rule we have for English where we could make a sentence by combining a subject, a verb and an object. This is a lot more interesting though, because we have expressions both on the left side and the right side. This looks kind of circular. Because we have other rules, it’s not completely circular. This is what’s called a recursive definition. We’ll talk a lot more about that in a later class, in unit six we will, but for now, we’re going to just see how we can use that to make the Python grammar. And to make a good recursive definition, we need at least two rules. We need one where we can keep going, using the same thing on the right side as the left side. And we need one where we can stop. So, another thing that an expression can be is a number. And this is why we can make expressions like 1 plus 1. This expression will become a number which will become one. The operator will become plus, and the other expression will become one. So let’s write a few more rules to see how that works. So we can replace the operator with plus. We can also replace operator with times. And there’s several more operators in Python. Those are enough for now, we just need those two. We can also make numbers. And the complete rule for making numbers is sort of complicated, but let’s assume we know what numbers are. We can make numbers any number of digits. There different rules to make all those numbers, but we know what they are. So, this looks like a very simple grammar. It’s quite small, but it can express infinitely many things. And the reason for that is because we have expression on both sides here. That we can replace an expression with a derivative expression, and keep going. So here’s an example derivation. We can start with expression. We can follow the rule, replacing that with expression operator expression. And then, we can follow the rules again. We can replace the first expression with a number and replace that number with the actual number one. And we notice the actual numbers are terminal. So, once we get to the number one, we’re done. There’s no more replacements to do. We can replace the operator with plus. Plus is also a terminal, and we can replace this expression with a number, and replace that number with the number one. So this is how we got the expression 1 plus 1 in the Python grammar and that’s why that was valid. We can do a lot more than this though. Instead of replacing this expression with a number, we can replace this expression with another expression, operator expression. So we’re going to use the first rule replacing the expression with expression, operator expression here. I’m going to start abbreviating so Expr is an abbreviation for expression. Op for operator and Expr but, we’re just using this rule. And now we can do the same thing again, replacing this expression with a number and replacing that number, let’s say with number two. We could replace this operator with the multiplication operator. And we can replace this expression with a number. Or you could replace it with an expression, operator, expression. Let’s do that, and build increasingly complicated expressions this way. We can keep doing this. We are not done until everything is a terminal. So let’s say, we skipped a few steps here, but this could be 3 plus 3. So this is how we can build up complicated expressions from very simple rules. So I am going to add one more rule to our python grammar, that will be enough to produce all the expressions that we have used so far. And that rule that we need is for parentheses. And this is actually quite a simple rule. It just says we can make an expression by taking any expression we have and putting parentheses around it. So, this is a pretty small grammar, but this is actually grammar that’s powerful enough to produce all the Python expressions we’ve seen so far. As well as infinitely many different expressions. We’re not going to cover the entire Python grammar this way, but this should give you an idea of the power of writing our grammar this way. So we’re going to have a quiz to see that everyone understands the grammar. So the question is which of the following are valid Python expressions that can be produced starting from expression using the grammar rules. So here are the choices; there are five possibilities, check all the ones that are valid expressions that can be produced from this grammar starting from expression. You can try your answers in the Python interpreter if you want, but first see if you can figure it out for yourself, which ones are valid.