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