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The first error is called the syntax error.

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Just like in English where we have syntax as an important part of grammar,

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a syntax error in Python means that the statements just aren't well formed,

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the interpreter can't parse it.

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In this case, we have the word print and we have

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the open parenthesis and then something to print

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but we're missing the closing parentheses and

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the Python interpreter tries to keep looking further on,

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as well there's no closed parentheses here but

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maybe it'll be here or down here and it keeps going to look for it,

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it can't find it and it just says, "This is an error."

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I don't know what the programmer had in mind here.

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Doesn't try to guess.

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It says, "This is a syntax error."

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So, let's see what that looks like.

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We actually get an error message and it tells us,

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syntax error and then it tells us something about what's going on and it says, "EOF."

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That's an acronym EOF end of file.

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Of course we don't really have a file,

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we just have a little window but it thinks of the contents of

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that window as being a file that has commands in it,

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it got to the end of that file it

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was in the middle of a multi-line statement the statement started on line

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one and it was waiting for that statement to end with

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that close parentheses and it never got to the end,

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you got to line three,

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the end of the file without ever seeing a closed parenthesis.

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Now, in the textbook when you're executing Python,

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we've actually tried to translate some of the error messages that you would

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get in a normal python interpreter and make them

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easier to understand for someone who is just getting started programming.

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So, believe it or not this is actually already a little

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bit more human-friendly than you might get normally,

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and we've tried to give you a little description and

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some suggestions of what you should do to fix it.

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In this case the things that are suggesting aren't quite what we need,

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what we really needed was

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a close parenthesis The other thing to notice here is that it's telling us line three,

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there isn't even a line three but certainly the place where we ought to

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add the parenthesis is on line one not on line three,

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is just that the problem started on line

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one and the interpreter wasn't really sure it was

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a problem until it got to the end of

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the file line three without getting the close parenthesis.

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So, if I put this here,

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now it will run just fine.

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An alternative, I could have put it here and that also would be fine.

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Normally, there would be no reason to do that,

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it would be much easier to read if you put it there.

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Now here I'm going to get another error.

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It says a bad input on line three because I have this extra close parenthesis,

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and these are all syntax errors that make it impossible for

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the interpreter to figure out what the program was supposed to do.

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The second type of error are runtime errors.

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These occur when the interpreter is able to parse the program,

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but during the running of the program some illegal operation happens.

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For example, let me do one of those.

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Suppose I print three divided by zero.

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What is three divided by zero?

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Infinity maybe.

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It's not illegal operation doesn't make sense to divide by zero and if I try to do that,

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I get this zero division error where it says,

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the integer division or modulo by zero on line one.

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So, that's an example of a runtime error.

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It can figure out what you're trying to do but it's not a legal operation.

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The third type of error is called the semantic error.

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That's when the Python interpreter is able to parse the code,

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it's actually able to run it but it doesn't

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produce the thing that the programmer intended.

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So for example, suppose I try to print something that

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explains the conversion of a fraction to a percentage but I do it a little bit wrong.

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So, I say 1.5 as a percentage

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is and then I say one over two.

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I'd like it to say 50 percent but it's really going to say 0.5.

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What I should've done here is multiply it by a 100 to turn it into a percentage.

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So, 1.5 is a percentage is 50 percent.

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I apologize for the line break there but you can figure out what it means, 50 percent.

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So, when I left off the 100 and I just

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said one over two that was a semantic error,

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was a programmer's error that produced

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the wrong thing but the program was able to run all the way to completion,

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it just didn't do what I wanted it to do.

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So, those are our three types of errors syntax, runtime and semantic.

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In this multiple choice question which you'll get a version of

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an each of the pages for the three different error types we've got one example of each.

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So, attempting to divide by zero,

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that is a runtime error.

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Forgetting a semicolon at the end of a statement where one is required,

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that would make it impossible to parse what you've written

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and that would be a syntax error.

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If you forget to divide by a 100 or

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multiplied by a 100 when you're printing a percentage amount,

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that would be a semantic error.