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In Python, an expression is anything that
can be calculated to return a value.

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So on line 4, we've got the expression a + b.

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Python evaluates that expression and prints the value 15.

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On the next line, we've got a - b, and that evaluates to the value 9.

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So working down the code, a multiplied by b is also an expression, evaluating to 36.

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Now the next two are similar, a / b and a integer divide by b.

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Now they evaluate to 4, but the first one is a floating point value and the second is an int.

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And finally on line 9, we've got the expression a and its remainder operator b.

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Oh, I said finally there, but it's not actually the last expression in the code.

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There are two more expressions on lines 1 and 2,

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and it may not be obvious but, strictly speaking, 12 & 3 are also expressions.

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They evaluate to the values 12 and 3, respectively, but they're still, technically, expressions.

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That means you can use a literal value, anywhere that Python expects an expression.

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Now there's also four more expressions at the end of the program,

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but I'd only expect you to identify two of them, at the moment.

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So can you spot more expressions in the code, after line 9?

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Pause the video if you need to, and come back when you've found two more.

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Alright so welcome back, did you spot them?

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Well firstly, on line 13, we've got the literal value 1 inside the parentheses,

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and it evaluates, not surprisingly, to the value 1, but again, it's still an expression.

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Now also inside the parentheses, we've got a integer divide by b, and that's the two slashes.

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At the moment, that evaluates to 4 because a is currently 12 and b is 3.

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So these are the two expressions I
expected you to find, but there are more.

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So still on line 13, the two expressions
we've identified are part of another expression.

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So the code range parentheses, and then it's 1, a // by b.

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That evaluates to a range of numbers from 1 to 3.

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So python has to evaluate that, in order to work out what values to use in the loop.

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So it evaluates the expression 1 to get the
value 1, and then a // b to get the value 4.

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Once it's done that, it has to evaluate range(1, 4).

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So that's another expression.

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We haven't covered for loops and ranges yet, which is why I wouldn't have expected you to find that expression,

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But there's also another one on line 14.

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The variable i has to be evaluated so that Python knows what to print.

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The first time around the loop, it evaluates to the value 1, next time around it's 2 and the last time around it evaluates to 3.

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But hang on a minute, if i on line 14 is an expression,

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what about a and b on line 4?

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Aren't they also an expression?

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Yes they are.

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Python evaluates a to get the value 12,
and b to get the value 3.

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It then adds them, evaluating the expression 12 + 3, and the same thing happens on lines 5 through 9.

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So expressions can themselves, be made up of expressions.

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In total there were 26 expressions in this code.

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Now we'll look at more complex expressions in the next video,

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but I'll finish this one by talking about a and b on lines 1 and 2.

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They're not expressions.

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You can't have an expression on the left of an assignment.

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The expression on the right of the equals sign is evaluated,

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and the variable on the left is bound to the result.

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So a common way of saying that is to say, the value 12 is assigned to the variable a,

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but I want to get you used to the idea that we bind, or attach variables to values.

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The result is the same. Whenever we refer to a in the code, Python evaluates it as 12.

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Okay, so what about i on line 13.

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Well that's also not an expression.

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i is being bound to each of the values produced by range in turn.

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So if I write this code long handed instead of using a for loop,

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it should become more obvious. So I'll come down here and I'll start on line 16.

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So put i equals 1,

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and on line 17 I'm gonna do print parentheses i, then i equals 2,

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print i, i equals 3, print i.

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Alright, I'll just move that up a bit.

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So that does exactly the same thing, where that would take six lines of code instead of two,

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which I've got on lines 13 and 14.

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But when I run the program,

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we get the same three values printed out again.

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So if i on lines 16, and i'll just scroll us up so we can see it,

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so lines 16, 18 and 20. So if I on those lines isn't an expression,

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then it should be clear that it's not an expression on line 13 either.

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Alright, so you'll see why we don't have i = 4 later.

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I'll stop the video here now, though, and we'll look at operator precedence in the next video.