---
## Learning Objectives:
*After this lesson, you will be able to:*
- Write a basic Flask application.
---
## Discussion: Commonalities
What do you think these websites have in common?
- [Pinterest](http://www.pinterest.com)
- [Instagram](http://www.instagram.com)
- [LinkedIn](http://linkedin.com/)
They're each:
- High on user interactivity.
- Handling a large server load.
What else?
---
## They All Use **Flask**
Some quick notes about Flask:
- It's a Python micro web framework.
- It can create and write the entire back-end in Python!
- It can do small tasks (e.g., create a microblog or stand up a simple API).
- It can do complex tasks (e.g., Pinterest's API or create a Twitter clone).
---
## Flask Syntax
How?
We just make a normal Python app.
It looks like:
```python
# Import Flask class from flask library. (Note the upper/lowercase convention.)
from flask import Flask
# Initialize an instance of the Flask class.
# This starts the website!
app = Flask(__name__)
# The default URL ends in / ("my-website.com/").
# Could be instead "my-website.com/about" or anything - more on this later.
@app.route('/')
# Function that returns the page: Display "Hello, World!"
def index():
return 'Hello, World!'
# Run the app when the program starts!
if __name__ == '__main__':
app.run(debug=True)
```
---
## We Do: Let's Try!
We'll run the Flask app like any other app.
- We need to install Flask!
- `pip install flask`
Create a file called `my_website.py`.
Start with:
```python
# Import Flask class from flask library.
from flask import Flask
```
---
## We Do: The Main Flask App
Let's add:
```python
# Initialize an instance of the Flask class.
# This starts the website!
app = Flask(__name__)
# The default URL ends in / ("my-website.com/").
@app.route('/')
# Function that returns the page: Display "Hello, World!"
def index():
return 'Hello, World!'
# Run the app when the program starts!
if __name__ == '__main__':
app.run(debug=True)
```
---
## We Do: Flask App — Try it!
Run the app like normal:
`python my_website.py`
Go to:
`http://localhost:5000/`
You made a web app!
Let's change the string:
```python
def index():
# The "return" determines what's displayed.
return 'Hello, World!'
```
---
## I Do: Displaying the App
It's just Python — we can write any code.
- But `return` essentially just takes strings.
```python
def index():
my_list = ["Hey", "check", "this", "out"]
return my_list[0] # Works!
```
Conversely:
```python
def index():
my_list = ["Hey", "check", "this", "out"]
return my_list # WON'T WORK
```
---
## We Do: Flask Variations
`app` and `index` are just naming conventions.
- `def index():` could be `def monkey():`.
- `app` could be `guitar`.
- Be sure to change it in all places!
But, naming variables sensibly is important!
```python
from flask import Flask
guitar = Flask(__name__)
@guitar.route('/')
def monkey():
return 'Hello, World!'
if __name__ == '__main__':
guitar.run(debug=True)
```
---
## Flask History
Let's back up. Where did Flask come from?
- Before 2010:
- No easy method for Python websites.
- 2010:
- A few developers built Flask to fix this.
Flask is built on two libraries:
- *Werkzeug*:
- Interfaces with the web.
- Helps handle request and connections.
- *Jinja*:
- We'll be using this later!
- We can write templates for all pages across our web app.
---
## Summary: Flask
- A Python micro web framework
- Developed in 2010
Looks like this:
```python
# Import Flask class from flask library.
from flask import Flask
# Initialize an instance of the Flask class.
app = Flask(__name__)
# The default URL ends in / ("my-website.com/").
@app.route('/')
# Function that returns the page: Display "Hello, World!"
def index():
return 'Hello, World!'
# Run the app when the program starts!
if __name__ == '__main__':
app.run(debug=True)
```
---
## Additional Reading
- [Flask Documentation](http://flask.pocoo.org/docs/0.11/)
- [A Flask Tutorial to Follow Along With](https://github.com/miguelgrinberg/flask-pycon2014)
- [The Flask Mega-Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-ii-templates)
- [A Great Guide to Those Weird "Decorators"](http://simeonfranklin.com/blog/2012/jul/1/python-decorators-in-12-steps/)