# Flask
```python
from flask import Flask, render_template
app = Flask(__name__, template_folder="path_to_folder") # create app
# template folder contains html pages
@app.route("/") # define URLs
def index():
return render_template("index.html") # parse HTML page and return it
if __name__ == "__main__":
# run server if server is single file
app.run(debug=True, host="0.0.0.0")
```
`@app.route("/page/")` enables to access the page with `url/page` and `url/page/`. The same is possible using `app.add_url_rule("/", "page", function)`.
## Variable Rules
You can add variable sections to a URL by marking sections with `<variable_name>`.
Your function then receives the `<variable_name>` as a keyword argument.
Optionally, you can use a converter to specify the type of the argument like `<converter:variable_name>`.
Converter Type | Accepts
---------------|------------------------------
`string` | any text without a slash (default option)
`int` | positive integers
`float` | positive floating point values
`path` | strings with slashes
`uuid` | UUID strings
```python
@app.route("/user/<string:username>") # handle URL at runtime
def profile(username):
return f"{escape(username)}'s profile'"
```
## Redirection
`url_for(endpoint, **values)` is used to redirect passing keyworded arguments. It can be used in combination with `@app.route("/<value>")` to accept the passed arguments.
```py
from flask import Flask, redirect, url_for
@app.route("/url")
def func():
return redirect(url_for("html_file/function")) # redirect to other page
```
## Jinja Template Rendering (Parsing Python Code in HTML, CSS)
* `{% ... %}` for **Statements**
* `{{ ... }}` for **Expressions** to print to the template output
* `{# ... #}` for **Comments** not included in the template output
* `# ... ##` for **Line Statements**
Use `{% block block_code %}` to put a line python code inside HTML.
Use `{% end<block> %}` to end a block of code.
In `page.html`;
```py
<html>
{% for item in content %}
<p>{{item}}</p>
{% endfor %}
</html>
```
In `file.py`:
```py
@app.route("/page/)
def func():
return render_template("page.html", content=["A", "B", "C"])
```
### Hyperlinks
In `file.py`:
```py
@app.route('/linked_page/')
def cool_form():
return render_template('linked_page.html')
```
In `page.html`:
```html
<!doctype html>
<html>
<head>
</head>
<body>
<a href="{{ url_for('linked_page') }}">link text</a>
</body>
</html>
```
### CSS
Put `style.css` inside `/static/style`.
In `page.html`:
```html
<!doctype html>
<html>
<head>
</head>
<link rel="stylesheet" href="{{ url_for('static', filename='style/style.css') }}">
<body>
</body>
</html>
```
## Template Inheritance
In `parent_template.html`:
```html
<html>
<!-- html content -->
{% block block_name %}
{% endblock %}
<!-- html content -->
</html>
```
The content of the block will be filled by the child class.
In `child_template.html`:
```html
{% extends "parent_template.html" %}
{% block block_name}
{{ super() }} <!-- use parent's contents -->
<!-- block content -->
{% endblock %}
```