Python for Absolute Beginners Transcripts
Chapter: Writing your first lines of code
Lecture: Visualizing variables

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0:00 In this next short section, couple sections actually
0:03 we're going to visualize some of the code
0:06 that we're working with.
0:07 Now you've seen it written, you've seen it executed.
0:11 And we've been able to ask questions like
0:12 hey, what data type does it have?
0:14 But I want to make sure you have a really clear understanding
0:16 of what these values and what these variables mean.
0:19 It turns out, getting a good grip on these things
0:22 is actually one of the key steps and one of the cornerstones
0:26 of becoming good at programming.
0:27 Though there's this cool bite here called Python Tutor
0:31 over at pythontutor.com.
0:33 Now, it's not entirely clear how you get started.
0:35 But what we want to do is we want to visualize our code
0:38 or get live help.
0:39 We don't actually care about live help.
0:40 We're just going to write some Python code.
0:42 We could write other languages
0:44 but we're just going to write Python code.
0:47 Now we're just going to work with one data type
0:48 and all we want to see is how different variables
0:51 and values interact.
0:53 So when we say something like person1 = 'Sarah'
0:58 and let's have another person person2 = 'Michael'
1:04 The things on the left, person1 and person2
1:06 these are just, think of them as names.
1:09 And those names can refer to data that exists out in memory
1:13 values that exists with a type
1:15 though the string Sarah and the string Michael
1:18 or later maybe we write down the age
1:20 it could be the integer of 42
1:22 or whatever it was we put in that section.
1:25 We can also take the values of variables
1:28 one variable like person2
1:30 and assign it to something else.
1:31 So what if we want to express somehow that
1:33 Sarah's friend is Michael
1:36 we could rewrite Michael but it makes much more sense
1:39 to take this variable and assign it
1:41 because if this gets changed somewhere along the way
1:44 you know, you want to be able to work with this
1:45 much more dynamically, not just be super, super explicit.
1:49 So let's go ahead and just visualize what this looks like
1:52 so far, then we'll make a few more changes.
1:54 Now in order to do that
1:55 we're going to click visualize execution
1:58 but to get the true picture we're going to pull down
2:01 and say render all objects on the heap
2:03 because hey, that's how Python actually works.
2:06 So let's click on visualize execution.
2:08 Now, look over here, it doesn't just take all the code
2:11 and run it, what it's going to do is it says
2:13 we're going to run one line at a time
2:15 and then hit Next, Next, Next, if you want to run them all
2:18 you can hit last, you could even move this little slider
2:20 to zoom around through your code, okay?
2:23 What we're going to do is we're going to run this
2:25 and we're going to take this variable
2:28 you know, basically create a name
2:30 and then we're going to create Sarah
2:32 which is an object out in memory.
2:34 So here you'll see frames and objects.
2:37 Frames are basically the things
2:38 that hold the variable definitions.
2:40 And then objects are the actual data
2:42 with types out in memory.
2:44 Though let's hit next.
2:45 There's different kinds of frames
2:46 don't worry about what this is
2:48 just means it's not in a function
2:49 or a class or something like that.
2:50 Now, we have our variable person1
2:54 and currently this variable refers to Sarah
2:57 which is a string out in memory.
3:00 We're going to do the same for person2
3:01 hit Next, and we have another variable
3:03 or another piece of data in memory called Michael
3:05 and another variable called person2.
3:08 Now here's where it gets interesting.
3:10 When we say, the friend of Sarah is person2
3:15 instead of creating a copy of Michael, remember
3:18 when we assign the value it says
3:19 Well, what is person2 point at
3:21 it points at this piece of data in memory right there.
3:24 Well, guess what, when you do this line four
3:27 that just means a friend also points to, refers to
3:30 has that thing as its data, as its object
3:34 also called Michael.
3:36 Let's see a few other things we can do here.
3:39 Well, we could come up with a list
3:41 we get a list of all people.
3:43 And don't worry about how we work with list yet
3:44 we'll talk in depth about this.
3:47 But this list lets you hold on too many things at once.
3:50 And we could go to these people and we could say
3:52 I want to put those two people, person1 in
3:57 and I want to put person2 in.
3:58 Let's run that.
4:01 Oh, down there they are, now we're going to create this object
4:04 this list, you can see an empty list out in memory
4:07 that's what people points at.
4:09 And then when we assign it
4:10 just like when we did this friend
4:12 we're going to say, well, one of the things in this list
4:15 is that piece of data called Sarah.
4:17 So notice now the list also points at Sarah
4:21 and then it also points at Michael.
4:24 Pretty cool, right?
4:25 So we have our variables here.
4:27 And then this particular variable is a list
4:29 a complex type, that itself knows about other pieces
4:32 of data, and basically has pointers or variables
4:35 in here that talk about
4:37 hey, Sarah is my first piece of data
4:39 and Michael is the second.
4:41 Let's do one final thing here.
4:43 Let's add a third person.
4:48 Mike or something like that.
4:51 Here, we can run this up to almost the same spot.
4:55 Now we have the same setup
4:57 we've had those two pieces of data
4:58 the two variables that directly pointed at them
5:01 the friend who we said also now is a point there
5:03 and then the list where we put them both in
5:05 and also knows about them.
5:06 So now we just do one more step and say
5:08 there's some other piece of data
5:10 that's not related to this over here.
5:12 I hope this gives you a decent sense of what variables are
5:16 and how they're different from the data or the values
5:19 that they actually have, right.
5:20 So the variables are just names that at some moment
5:24 and point out here, right, and that we saw before
5:28 here, like the people pointed at the list
5:30 but the list didn't happen
5:31 to point at Sarah or Michael just yet.
5:34 Though here's a quick visualization
5:36 of variables and their values as objects in Python.