# Audience

These notes were written for students in the 2015 CSE SPIS program at UC San Diego that want to learn about how to do basic web programming in Python using Flask.

We will do this using the student’s ACMS account, i.e. by logging into the systems on the server ieng6-240.ucsd.edu

For students in the SPIS academy program, we don’t assume any prior background in Python or Unix. However, these notes will probably be used a few days into the program, when students have already gained some familiarity with at least a few simple concepts such as:

• Simple function definitions in Python
• Executing a Python file at the Unix command line
• Using the cd, ls and mkdir Unix commands.

Flask is a Python module that provides an easy way to get started with web programming.

The easiest way to get started is to try some small examples. Below is a small example of a Python program that uses Flask. There are seven lines of code here (not including the blank lines), and before the end of this document, we’ll explain every single part of every one of these lines. But first, we’ll do a few things that will allow us to run these seven lines of code, setting up a small web application. That web application doesn’t do much that is very interesting—it puts the words “Hello World!” up in the browser every time you send it a request. But, it provides a starting point that we can then modify to do more interesting things.

• Make a directory in your SPIS account called web-app-intro
• As a reminder, the command is mkdir web-app-intro
• Then, cd into that directory
• Open up IDLE, because we are going to write some code.

Put these lines of code in a file called hello.py

from flask import Flask

@app.route("/")
def hello():
return "Hello World!"

if __name__ == "__main__":
app.run(port=5000)


To get this working, all you have to do is type:

python hello.py


Sadly, it probably won’t work the first time you try it. You’ll probably get a message like this one:

[spis15t7@ieng6-240]:~:107$python hello.py Traceback (most recent call last): File "hello.py", line 1, in from flask import Flask ImportError: No module named flask [spis15t7@ieng6-240]:~:108$


# Why do we have to install Flask? Why isn’t it already there for us to use?

By default, the Flask module is not part of the Python setup. It is fairly common occurrence to need a module that is not part of the default Python installation. There are so many modules out there that extend Python’s capabilities in various ways, that it isn’t a good use of disk space or time to install all of them in advance. Typically, they get installed only “as needed”.

There are many ways to install a Python module, but two of the most common are: (1) Install the module for all users on the system (2) Install it only for a specific user. We are going to choose the second option. The reason is that for some of you that may choose to pursue building a web app as your final SPIS project, you may find that you need a variety of modules that we didn’t anticipate. Doing it this way allows each of you to have your own customized set of add-on modules for Python.

Here’s how to do it.

pip install --user flask


Here is an example of what that would look like. The reason the output is so long is that Flask depends on other pieces of software to work properly. The pip command figures out what those pieces are, and installs them as well along with everything else.

[spis16t3@ieng6-240]:test:185$pip install --user flask Collecting flask Downloading Flask-0.11.1-py2.py3-none-any.whl (80kB) 100% |################################| 81kB 1.2MB/s Requirement already satisfied (use --upgrade to upgrade): click>=2.0 in /software/common64/python-2.7.10/lib/python2.7/site-packages (from flask) Collecting itsdangerous>=0.21 (from flask) Downloading itsdangerous-0.24.tar.gz (46kB) 100% |################################| 49kB 1.5MB/s Collecting Werkzeug>=0.7 (from flask) Downloading Werkzeug-0.11.10-py2.py3-none-any.whl (306kB) 100% |################################| 307kB 506kB/s Requirement already satisfied (use --upgrade to upgrade): Jinja2>=2.4 in /software/common64/python-2.7.10/lib/python2.7/site-packages (from flask) Requirement already satisfied (use --upgrade to upgrade): MarkupSafe in /software/common64/python-2.7.10/lib/python2.7/site-packages (from Jinja2>=2.4->flask) Building wheels for collected packages: itsdangerous Running setup.py bdist_wheel for itsdangerous Stored in directory: /home/linux/ieng6/spis16/spis16t3/.cache/pip/wheels/fc/a8/66/24d655233c757e178d45dea2de22a04c6d92766abfb741129a Successfully built itsdangerous Installing collected packages: itsdangerous, Werkzeug, flask Successfully installed Werkzeug flask itsdangerous You are using pip version 7.1.2, however version 8.1.2 is available. You should consider upgrading via the 'pip install --upgrade pip' command. [spis16t3@ieng6-240]:test:186$

• A note about that itsdangerous stuff: don’t worry. It isn’t “dangerous” to install itsdangerous.
• On the contrary—the itsdangerous python module is designed to help you avoid danger from various security vulnerabilities that webapps can open up. But more on that another time.

Once the Flask module is added to your individual user’s Python installation, you can run the file again by typing:

python hello.py


Here is what that should look like:

[spis15t7@ieng6-240]:~:179$python hello.py * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)  If that works, great! But if it doesn’t work, it is likely that the problem will be this error message: socket.error: [Errno 98] Address already in use  Actually, that error message will be the last line of a long sequence of messages, that look like these (the … means I’ve left out many lines of output). Traceback (most recent call last): File "hello.py", line 9, in app.run() File "/home/linux/ieng6/spis15/spis15t7/.local/lib/python2.7/site-packages/flask/app.py", line 772, in run run_simple(host, port, self, **options) ... File "/software/common/python-2.7.10/lib/python2.7/socket.py", line 228, in meth return getattr(self._sock,name)(*args) socket.error: [Errno 98] Address already in use  There will be times later when we will want to dive deep into that long “Traceback”, but this is not one of those times. The last line, in this case, tells us everything we need to know. It tells us that the default port number that Flask listens for connections on, 5000, is already being used by some other program on ieng6-240.ucsd.edu. In that case, no worries. We’ll just choose a different number. We do that by change the line of code: app.run(port=5000) to app.run(port=5001). We keep doing this, i.e. adding one to the port number, until we find a port number that works for us. • (Actually, you don’t have to go in order, 5002, 5003, etc., and it probably is better not to. Instead skip up to 10000, 12345, 20000–basically any number between 5000 and 65535. The more you change it up, the less likely you are choosing a number already in use by someone else. • There is a very specific reason that the maximum number is exactly 65535. Can you guess what it is? It has to do with our lecture on number representation. And yes, if you thought: wait, can we write some code to do this, instead of doing by hand—then congratuations! You are thinking like a programmer! Eventually, we’ll look at that, but if you just can’t wait, check this out. Now let’s suppose it “works”, meaning that when you type in python hello.py you get a message such as this one: [spis15t7@ieng6-240]:~:179$ python hello.py
* Running on http://127.0.0.1:5001/ (Press CTRL+C to quit)


This may not be very impressive. It gets a little more awesome when you realize that you’ve just put up a web server that you can connect to with a browser.

• Bring up a browser on the SAME machine as the server
• To do this, bring up a browser, but—and this part is important—on the SAME machine as where you are running this server. For now, your server is not available to the entire internet—it is only available to browsers running on the same machine as the server. We’ll say what to do with that browser in a minute.

If you are sitting in a computer lab in the CSE building while you try this, this will be easy since the browser will automatically be on the same machine.

If instead, you are using a terminal program to remotely access the ieng6-240.ucsd.edu server via Windows or Mac, then to bring up a server on the same machine, you’ll probably need to either:

• Use mobaxterm on Windows
• Use ssh -X on Mac or Linux

Once you have your browser up, enter the web address http://localhost:5000 or http://127.0.0.1:5000

This is the web address where your server is located. If you enter this in your browser, you should see the message “Hello World” come up in the browser, as shown here:

# What next?

Now, here are a few things to try, and some questions to answer. As you try these things and think through what is happening, you should begin to understand a few things about how this code is working.

Before you start, arrange the windows on your screen so that you have the window where you typed python hello.py on the left side of your screen (we’ll call this the server window), and the browser window on the right side of your screen.

• In the browser window, hit ‘refresh’ a few times. What do you see in the server window?
• In the window where you originally typed python hello.py, type a CTRL+C (that is, “Control-C”). Then try hitting “refresh” in the browser where you typed localhost:5000, and see what happens. Then, type the python hello.py command again, and try hitting refresh in the browser again.
• Use CTRL+C on the server side to stop the server, edit the hello.py file to change the string “Hello World” to something else, for example, if your name is Cory, make it say: “Hello from Cory’s server”. Start the server up again and then hit refresh in your browser a few times.

# What to do next?

There are two choices.

The next page of this tutorial suggest a few things we can do next to make our first Flask web app a bit more interesting. If you are content to move ahead, and figure things out as you go, just skip the rest of this page, and click to go to Web Apps Intro Part 2

If you are the type that wants to understand everything before moving on, the section below explains each line of code in our hello.py file, one at a time. If you prefer, read through that first, before moving on.

# Explaining all the lines of code

Ok, here is an explanation of each of the lines of code. Then, we’ll walk through how to do a few things that are a bit more interesting.

line explanation
1 This code pulls in the definition of a new datatype called Flask (note the uppercase F) from a module called flask (note the lowercase f.) In Python, when you import something, you are telling the Python interpreter that you want to build your software on top of some software already written by someone else.
2 This line of code is an assignment statement. The right hand side is evaluated, and then the variable on the left is set to the result of the right hand side. The right hand side, Flask(name) creates a new object of type Flask. The __name__ parameter is explained further in the box “About the First Parameter” on this page of the api documentation. We’ll defer the details for now, and just say: for very simple Flask applications, name is always the correct choice.
4 The @ signals that what follows is a Python annotation. It indicates that there is some special way the following function definition should be handled. In this case, the annotation app.route("/") indicates that this function is called whenever we ask for the main page on this web server (i.e. "/". This will make more sense when we add other pages later.
5 Lines 5 and 6 are a regular plain old Python function definition. We can see that the function is called "hello", and takes no parameters.
6 This function returns a string, "Hello World!", which is used as the page to display for the main page of the web server ("/").
8 Line 8 has an if test that checks the special variable __name__ to see if it has the special value “main”. This is the mysterious “conditional script stanza”, which we aren’t going to explain in detail here. Instead, we’ll just say that whatever “main thing” a Python code is supposed to do when you select “Run” in IDLE, or type python filename.py, in this case python hello.py, should typically be wrapped in this if test. That makes your file much more useful, because then the definitions it contains can be included as a module in another file.
9 Line 9 is the line that actually causes our web server to start running. The dot-notation app.run() tells us that run is a method of the object app. By putting () after it, we are making a function call to that method, and starting things in motion. The port=5000 part indicates the port number we are going to listen for connections on. By default, web servers listen for connections on port 80, and web browsers send requests to port 80. We don’t have the necessary permissions to set up a server on port 80 (port numbers lower than 1024 are restricted to system administrators), but we can set up a server on a “high numbered port”, basically anywhere between 1024 and 65536. For various reasons, its better to choose a number starting at 5000 (less likely to conflict with some other network service.) Being able to specify a port number both on the server side and the browser side allows many users to set up servers and connect to them all on the same machine.

Click to access: Web Apps Intro, Part 2.