Python for Absolute Beginners Transcripts
Chapter: Writing your first lines of code
Lecture: Visualizing variables
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In this next short section, couple sections actually
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we're going to visualize some of the code
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that we're working with.
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Now you've seen it written, you've seen it executed.
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And we've been able to ask questions like
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hey, what data type does it have?
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But I want to make sure you have a really clear understanding
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of what these values and what these variables mean.
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It turns out, getting a good grip on these things
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is actually one of the key steps and one of the cornerstones
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of becoming good at programming.
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Though there's this cool bite here called Python Tutor
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over at pythontutor.com.
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Now, it's not entirely clear how you get started.
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But what we want to do is we want to visualize our code
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or get live help.
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We don't actually care about live help.
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We're just going to write some Python code.
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We could write other languages
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but we're just going to write Python code.
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Now we're just going to work with one data type
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and all we want to see is how different variables
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and values interact.
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So when we say something like person1 = 'Sarah'
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and let's have another person person2 = 'Michael'
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The things on the left, person1 and person2
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these are just, think of them as names.
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And those names can refer to data that exists out in memory
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values that exists with a type
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though the string Sarah and the string Michael
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or later maybe we write down the age
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it could be the integer of 42
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or whatever it was we put in that section.
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We can also take the values of variables
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one variable like person2
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and assign it to something else.
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So what if we want to express somehow that
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Sarah's friend is Michael
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we could rewrite Michael but it makes much more sense
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to take this variable and assign it
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because if this gets changed somewhere along the way
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you know, you want to be able to work with this
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much more dynamically, not just be super, super explicit.
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So let's go ahead and just visualize what this looks like
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so far, then we'll make a few more changes.
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Now in order to do that
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we're going to click visualize execution
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but to get the true picture we're going to pull down
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and say render all objects on the heap
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because hey, that's how Python actually works.
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So let's click on visualize execution.
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Now, look over here, it doesn't just take all the code
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and run it, what it's going to do is it says
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we're going to run one line at a time
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and then hit Next, Next, Next, if you want to run them all
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you can hit last, you could even move this little slider
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to zoom around through your code, okay?
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What we're going to do is we're going to run this
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and we're going to take this variable
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you know, basically create a name
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and then we're going to create Sarah
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which is an object out in memory.
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So here you'll see frames and objects.
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Frames are basically the things
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that hold the variable definitions.
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And then objects are the actual data
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with types out in memory.
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Though let's hit next.
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There's different kinds of frames
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don't worry about what this is
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just means it's not in a function
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or a class or something like that.
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Now, we have our variable person1
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and currently this variable refers to Sarah
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which is a string out in memory.
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We're going to do the same for person2
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hit Next, and we have another variable
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or another piece of data in memory called Michael
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and another variable called person2.
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Now here's where it gets interesting.
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When we say, the friend of Sarah is person2
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instead of creating a copy of Michael, remember
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when we assign the value it says
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Well, what is person2 point at
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it points at this piece of data in memory right there.
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Well, guess what, when you do this line four
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that just means a friend also points to, refers to
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has that thing as its data, as its object
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also called Michael.
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Let's see a few other things we can do here.
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Well, we could come up with a list
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we get a list of all people.
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And don't worry about how we work with list yet
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we'll talk in depth about this.
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But this list lets you hold on too many things at once.
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And we could go to these people and we could say
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I want to put those two people, person1 in
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and I want to put person2 in.
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Let's run that.
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Oh, down there they are, now we're going to create this object
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this list, you can see an empty list out in memory
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that's what people points at.
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And then when we assign it
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just like when we did this friend
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we're going to say, well, one of the things in this list
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is that piece of data called Sarah.
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So notice now the list also points at Sarah
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and then it also points at Michael.
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Pretty cool, right?
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So we have our variables here.
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And then this particular variable is a list
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a complex type, that itself knows about other pieces
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of data, and basically has pointers or variables
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in here that talk about
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hey, Sarah is my first piece of data
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and Michael is the second.
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Let's do one final thing here.
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Let's add a third person.
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Mike or something like that.
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Here, we can run this up to almost the same spot.
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Now we have the same setup
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we've had those two pieces of data
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the two variables that directly pointed at them
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the friend who we said also now is a point there
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and then the list where we put them both in
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and also knows about them.
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So now we just do one more step and say
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there's some other piece of data
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that's not related to this over here.
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I hope this gives you a decent sense of what variables are
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and how they're different from the data or the values
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that they actually have, right.
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So the variables are just names that at some moment
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and point out here, right, and that we saw before
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here, like the people pointed at the list
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but the list didn't happen
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to point at Sarah or Michael just yet.
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Though here's a quick visualization
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of variables and their values as objects in Python.
