#100DaysOfWeb in Python Transcripts
Chapter: Days 85-88: Serverless Python with AWS Lambda
Lecture: Second Lambda: PEP8 Checker (using pycodestyle)
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0:01
All right, we made our first calculator lambda. We set up some test events. We detected the AWS lambda raised its exceptions earlier than we thought.
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So we adjusted the Bottle code a little bit. And we set up our AWS gateway API and that allowed to receive post requests
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from our app and to invoke our lambda. So we have an end to end working solution which is cool, but the calculator at lambda is not really excited.
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So let's work on a second lambda to submit code through our web form and run PEP 8 on it. While preparing this lesson I noticed
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that the PEP 8 package was renamed to pycodestyle. And I've been using PEP 8 for a while. I got an error on my terminal that said
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that we should really switch to this new package. So I'm going to pip install that and we're going to write our lambda. All right.
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So lets start building our new lambda. Bear with me for a sec. I'm going to import a couple of libraries we're going to use, and explain to you
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in a bit why. I need some constants.
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And we have our default lambda handler. Actually, I can call this function anything but I'm just going to stick with lambda handler.
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Default signature. All right. So, what's the deal? I order to run PEP 8 checks or pycodestyle as the new package is called, we cannot receive a string.
1:57
So we need to run it on a file. And actually, let me show you that first. For example, if we run it on our app I actually get a message that I have
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one line where it expects two. So let me actually fix that on the fly. Into line 10 and we see here one blank line. And as per PEP 8 we should have two
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blank lines between constants and new classes or functions. So save this, run it again. And no news is good news. This means all is okay.
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So this is run from the terminal and if you want to run PEP checks from Python code, pycodestyle has a checker class we can use.
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However, that checker receives a file not a string. So we're going to set up a temporary file. We cannot just save it in the current directory
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because that's not writable in the AWS environment. So I'm going to make that temporary file in /temp, which is writable.
3:05
So now we're going to run the checker. The other complication is that it's output is redirected to standardout.
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So we need a way to capture the standardout and throw that into a variable we then return. So that's why I'm using these modules.
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Temp files are standard library module to create temporary files. So the nice thing is that files created via this method get automatically
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destroyed when done. So we don't have to worry about that. And while preparing this lesson another interesting thing I found was
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the redirect stdout from the context lib which will help us greatly in redirecting standard output to a variable.
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With that said, lets code up our lambda. So this is the same as last time. We need to retrieve the code from the event date.
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I'll just make it default of empty. And I'm going to write to our temporary file as defined in the constant. You have to give it write mode.
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And give it a file handle. Now, to write the code to the temporary file. And I found that we need to add this final new line.
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Otherwise it wouldn't work. And instead of opening it again for reading we can just... So we could open it again to read over the content.
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But you can also use seek. Remember those tapes from the early days? That you rewind the tape. It's kind of the same.
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So you can rewind it and your pointer is at the start again, and then we can consume the content. So it basically is rewriting the file.
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Now the next thing that we want to do is redirect the standard output. And in the Python 3 documentation I found
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this context manager that makes it pretty easy to redirect the standard output. So I can use this snippet directly in my code.
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Don't use anything you already have as a file handle. So I'm going to use another variable name here. And in this body of this with statement
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we're going to do the actual PEP 8 checking. So we're going to use pycodestyle which comes with a checker class
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which can be initialized with the file name. So f is the file handle which has an attribute name. So f.name will be something like /temp/
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some random string. And we give it showsource = True. And lets call this instance PEP. And on PEP then we can call checkall.
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So this gives a bunch of output that goes to standard output. But as we're using a redirect standard output will be captured in the out variable.
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Then out has a method called getvalue which gives a string of the output and we want to split that by new line. And as we saw earlier, in our web app
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we then consume that content which comes back in the body. And we then join it together on new line. Actually we can just consume the string.
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We might not even have to do the split and join. So let me just make this string. Default is to empty string. And then we don't need to split this.
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We just take the full string. And we need to return from the lambda so we're going to return our status code of 200.
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And I believe we can do that because as we saw in the previous video the lambda will raise exceptions early. So if something is wrong it would already
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bail with the error message and error type we set up earlier. So we're already handling that. Getting here means all went okay.
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And we're going to return the body again as expected in the web app. So we have to return the body and that will
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be the output, and I don't think we really need this temporary variable. So we can just do this here. And lets do some local testing here.
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So if name =. Lets do an okay scenario. So starting with some valid code.
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Return = lambda handler passing in the event in empty context, and print the result.
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Oh, I made a typo here. This actually has to be name.
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And for we get a body empty which is as expected. That's how the command line app worked as well. Let's do a second case where we do have
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a PEP 8 violation. Let's get to some indenting. Cool, so here we have the scenario. We have an E111 indentation is not a multiple of four.
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So body has content, which means that would show up in the web app. I'm going to test that next. So we have a lambda.
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What we now just can do is paste that into AWS. We don't need the name. We just need this body. Let's actually make a new one...
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Called PEP checker. Python 3.7. Existing role. My role. Create function. And let's put this code in here.
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Save. And lets set up a test event.
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Okay, let's fix the quoting. Create. Test. Ah, okay. And I did expect this. There's one last thing we need to work around
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and that's to make our lambda function work with external modules. As we saw at the start of this lesson we pip installed pycodestyle and in AWS
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we don't have this module yet. But AWS perfectly supports uploading lambdas with external packages. And in the next video I'm going to show you
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how you can do that.