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Whitespace is significant in Python in ways that you may not be accustomed to,

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if you are familiar with other scripting languages.

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Let's go ahead and make a working copy of syntax.py, and we can look at

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whitespace in Python. Call it syntax-whitespace.py.

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Go ahead and open that up, and you will notice that this main function consists

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of one statement, a print statement, and that print statement is indented four

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spaces, ands that actually tells Python that this print statement is associated

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with this main function.

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It's actually the body of the main function.

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Four spaces are traditional in Python.

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You can use two. You can use one. It will work just as well.

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It does have to be consistent. In other words, if I have more than one line here,

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and if that other line is indented, say three spaces instead of four, if I save

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that and run it, you will see that I get a syntax error, and it says unindent

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does not match any outer indentation level.

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So Python got confused. We indented this one four spaces, we indented this one

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three spaces and it just got confused.

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It didn't know what to do with that. So the indention has to be the same amount.

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Save that and run it, and we will see that that works.

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Now if I take this second line and I indent it back at the same level as the

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function definition, then it is no longer part of the body of that function.

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So what will happen when I run this is it will actually see this line, this is

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another line, before the syntax file and the reason for that is because that will

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get executed on the first pass through here before we call main.

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main gets called at the end of the file.

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So this line will be printed first and then this line.

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So we will save that, and we will run it, and you see this is another line, and

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then this is the syntax file.

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So, by bringing the indentation out to the same level as the definition, it is

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now no longer part of the body of that function.

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We will go ahead and indent that again and save it and run it, and we see that it

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works as we expect with this second line after the first line.

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So indentation as whitespace is significant in Python because Python doesn't use

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curly braces, or any other characters, to indicate the body of a function, or what

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they a call suite - what we might call a block of code that's - associated with a

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particular control structure.

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These indents are used for conditionals. They are used for classes.

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They are used for any place where you might need to associate a block of text

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with a particular control structure.

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Now there is one circumstance where the indentation and the whitespace is not

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actually significant.

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If you have just one line of code in a particular structure - and that's all there

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is, there is not more than one line of code - that one line of code can be on the same line

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as the control structure itself.

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So if I put this print here - and I can put a space, or not put a space, or put several

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spaces - it will work exactly the same.

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I save this and run it, you see that it still works. Just like this 'if'

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statement down here has one line of code, which is the function call for main,

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now this function definition for main has just one line of code, the print, and

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in this case the whitespace is not significant.

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I can have five spaces, I can have one space, and it still runs just the same.

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That's a special case for whether its just one line of code in a

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particular structure.

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Under most circumstances, where you have a function, or a class, or something like

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that, and you have more than one line of code, you will see the indentation used,

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and four spaces is traditional, and the indentation has to be consistent all the

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way through the block or the suite of code.