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So, continuing with our Python Quick Start, let's take a look at how

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conditionals work in Python.

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I am going ahead and make a copy of this conditionals.py, and it's a good idea

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to work with copies.

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I am just going to name this conditionals -working. We'll go ahead and open that.

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There we have an example of a conditional, a very simple conditional. It's if and else.

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And you'll notice that the assignment - it's really cool.

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In Python, you can assign multiple values at a time, so this assigns a 0 to a and

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assigns 1 to b. So that makes a nice little test for the conditional. If a is

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less than b, which it is, then it will print this.

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You'll also notice, in Python, you do the replacement, and this is Python 3;

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Python 2 works differently.

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In Python 3, these curly braces will be replaced by the values in the format, and

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we'll get more into how this works exactly, and some of the details of it, later

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on when we talk about strings.

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But in fact this format is a method of the string object.

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Any place where you have a string, it allows you to do these kinds of

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replacements; it's very useful.

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So, when we run this - and we'll get this one because a is less than b, and unless

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we were to change these values and then maybe we could get this one.

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So let's go ahead and run it, and I'll run it here in Eclipse, and I'll select

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Python Run, and there we go.

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a is less than b, and see there is the value 0 and 1.

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If I change this, if I make this say a 5, and I save that and run it, a is not

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less than b anymore.

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So the syntax of the conditional is, if, and then you have the condition, and you have a colon.

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This is how blocks work in Python. You'll see this same type of syntax and

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loops and functions and classes, where you have the colon and then you have code that's indented.

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There aren't any braces signifying a block of code.

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Blocks are actually called suites in the Python documentation, and they

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simply consist of code that is indented under the level of whatever it is

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that's controlling it.

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So here we are controlling it with a conditional, and we have the colon, and then

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we have this code indented.

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Now four spaces of indenting is considered standard in Python.

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It's actually not required.

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I can make this one space and do the same down here and it would still work,

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save and run, but four is traditional.

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So we are just going to go ahead and go with the tradition here and save that

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and run it, and there we have it.

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Python also has conditional expressions. If you are familiar with any C-based

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languages, you'll be familiar with something that might look like this, where you

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would say print, and you would have a condition a < b and a question mark, and then one value, and a colon,

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and then another value.

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If the condition were true, you would get the first one; if the condition were

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false, you would get the second one.

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Python has something like this, but it works very differently.

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Here, you would say "foo" if a < b else "bar".

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If I save that and run it, then we get bar because a is not less than b. If I put

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this back to a 0, like we had it originally, and save and run,

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now we get foo because a is less than b. So this is how conditional

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expressions work in Python.

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A lot of the syntax that you will see here is a little bit different than in

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other C-based languages.

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And that's because the designers of Python, they didn't consider themselves be

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beholden to tradition, and instead they wanted to do things that seem to flow

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better or make sense in some way.

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And actually, this does have its own set of reason in that you have print "foo"

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just like you would have if you didn't have a condition.

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You only really care about having the condition if you have another value.

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So you have this whole if-else all in one place, and it's spelled out; it's not

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punctuation marks that might just be hard to remember or hard to read.

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It's very clear: "foo" if a < b else "bar"; it's kind of readable.

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So that is how conditions work in Python 3.

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You have the traditional if-else, and you have the conditional expressions.