Python for Absolute Beginners Transcripts
Chapter: The big ideas of software development
Lecture: Code execution diagram

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0:00 So, we have our source code
0:01 that we're going to talk about, but not yet.
0:03 And it's written into this file called urlify.py.
0:07 To run it, we just say, python, here's some Python code.
0:09 Run this little application.
0:11 And we type this on our command prompt.
0:13 What happens next?
0:15 Well, obviously, we start out with the urlify.py file.
0:19 This is the Python source code.
0:20 And this is what most people think of
0:22 as the thing that is running maybe right at the beginning.
0:25 But in fact, this is not what is run.
0:28 This is transformed several times
0:30 into something else that then is actually run by Python.
0:35 So this is parsed or compiled by Python.
0:38 Basically, Python goes through and understands all the words
0:42 and symbols and stuff in the source file
0:44 and it converts it into a lower level thing that
0:48 it doesn't have to try to interpret how you've written it.
0:51 It knows now there's an if statement.
0:53 Here's the thing we're testing, test it.
0:55 Stuff like that, like very simple, low level
0:58 byte code that Python can use.
1:01 Now this step is not hugely expensive
1:02 but it's not for free
1:03 and you're reading potentially
1:05 very large and complicated text files
1:07 and turning them into this form that's been validated
1:10 and there's a canonical representation
1:12 of all the stuff in there.
1:13 So what happens next is this is actually saved
1:17 into a local folder called double __pycache.
1:21 And the reason it does this is
1:23 the next time you run this file
1:25 second, third, and fourth time you run it
1:27 Python can skip that first step.
1:28 And it goes, this file corresponds to this Python byte code.
1:32 We're just going to start from there and run it
1:34 as a way to make sure it hasn't changed
1:36 and things like that.
1:37 But it'll just grab this pre read pre understood version
1:42 and start running that. What does it do with this?
1:44 Well, it fires up the Python runtime or virtual machine.
1:49 Sometimes it's called a Python interpreter.
1:51 It's going to take this Python byte code
1:53 and feed it to there and say
1:54 Okay, what I want you to do
1:56 is go through this step by step
1:58 getting the data wherever you need it to get from.
2:00 Does it come off a file?
2:01 Do you ask the user?
2:02 Do you get it from a web service?
2:04 And use that to help make decisions and choose
2:07 which byte instruction, bytecode piece to run next?
2:10 Usually, when you write some Python code
2:12 like if this thing is greater than
2:14 if the age is less than 14
2:16 that might turn into three or four different
2:18 little instructions to make that test actually happen.
2:22 What Python going to do
2:23 is it's going to take that byte code
2:25 and it's going to feed it into a loop that just goes around
2:27 and around and around and around, basically, endlessly
2:31 as long as there's more byte code to be run.
2:33 And it's going to break it up into step by step by step
2:35 and says, Okay, this step is to load up a value, age.
2:39 This step is to load up another value 14.
2:41 The next step is to test are the two loaded values
2:44 less than equal less than each other.
2:47 Is the first one less than the second one.
2:49 And it just goes through things like that.
2:51 So it has this byte code
2:52 it's going to break it up into little pieces.
2:55 Some of those might say we need to talk to the file system
2:58 and go pull in some data from there.
3:00 To open a file stream
3:01 and we're going to start working on that.
3:03 Some of it might create some memory structure
3:05 that lives in RAM.
3:07 And it's going to read from it and write from it.
3:08 And that'll help it keep track of what it's doing.
3:11 So their code might say
3:12 I also need to read from that thing
3:13 let me go up there and see, you know
3:15 what the name of the logged in user is, for example.
3:18 There's other stuff that happens to.
3:20 We don't have to explicitly say
3:22 I need to create this place in RAM to put the data
3:25 and then now I'm done with it.
3:26 Some languages actually do.
3:27 But Python automatically cleans it up.
3:30 So there's two levels of what is called garbage collection
3:33 that just goes and throws away the data
3:35 returns it back to the operating system.
3:37 If you're done using it.
3:38 Though, maybe I created the name of somebody
3:40 got the name of the user, put it into HTML
3:43 then I'm done with that variable.
3:44 Well, something has to give
3:46 that memory back to the computer.
3:47 So there's some processes called
3:49 garbage collection that take care of that.
3:52 This is only a small part of what happens
3:54 when you run your Python code.
3:56 Code is also turned into this thing called a tree.
3:59 So that it can be understood in a more generalized way.
4:02 There's a lot of stuff going on.
4:05 But this is a pretty good mental model to have.
4:08 When you type Python my source code file
4:10 what is happening, this step from left to right
4:13 and that loop and loop and loop again and again and again.
4:16 Figure out what each byte code instruction says to do.
4:20 This is what happens when we run our code.