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Url_for

As explained in The Well-Grounded Python Developer: How the pros use Python and Flask

url_for is a Flask function that constructs a URL from an endpoint and optional parameter values or file paths.

Hardcoded asset paths can become awkward as a Flask application grows. `url_for` builds a URL from a known endpoint instead, so a template can refer to a file without spelling out its full URL path. section 6.4.4

Link a stylesheet

In `base.html`, set a stylesheet link to `href="{{ url_for('static', filename='css/myblog.css') }}"`. Jinja2 evaluates the expression when it renders the template, placing a valid URL in the link. section 6.4.4

The first argument names the endpoint; `filename` supplies the file’s relative path. Putting this link in a shared base template lets child pages inherit the same stylesheet. section 6.4.4

<!DOCTYPE html>
<html>
<head>
                                            {% block head %} #1
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, 
    initial-scale=1, shrink-to-fit=no">
    <title>Your Second Web Server</title>
    {% block styles %}                                       #2
    <link
        rel="stylesheet"
        type="text/css"
        href="{{ url_for('static', filename='css/myblog.css') }}"
     >                                                     #2
    {% endblock %}                                           #2
    {% endblock %}                                           #1
</head>
 
<body>
    <div id="header">
        <h1>MyBlog Home Page</h1>
    </div>
    <div id="content">
        {% block content %}{% endblock %}                    #3
    </div>
    <div id="footer">
        {% block footer %}                                   #4
        <h4>&copy; Copyright 2020 by MyBlog</h4>    
        {% endblock %}    
    </div>
</body>
 
{% block scripts %}{% endblock %}                            #5
 
</html>

The base-template listing shows `url_for` inside the stylesheet link and its surrounding Jinja2 blocks. section 6.4.4

How The Well-Grounded Python Developer: How the pros use Python and Flask covers it

How deep it goes
Explains endpoint and relative-file arguments through one rendered template expression, without detailing URL-generation internals.
Examples
Shows a `base.html` stylesheet link using `url_for('static', filename='css/myblog.css')`.
What it assumes
Assumes familiarity with HTML links while introducing the Jinja2 expression that calls the Python function.
What you can do after
Apply it via Flask to generate a static-file URL in a Jinja2 template.
How it connects
Connects Flask static files with Jinja2 expression substitution and template inheritance.
Cover of The Well-Grounded Python Developer: How the pros use Python and Flask
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of The Well-Grounded Python Developer: How the pros use Python and Flask. The section links point to the parts of the book it draws on.