Updated: Mar. 13 2026 | Created: Mar. 13 2026
What Is Flask render_template_string() Function?
The Flask render_template_string() function is used to render a template directly from a string instead of a template file.
It processes the template string using the Jinja template engine and returns the rendered output as a string, which is typically valid HTML that the web browser can display.
This function is useful when the template content is generated dynamically in Python code and does not exist as a separate HTML template file.
For example, if a template string contains HTML mixed with Jinja expressions, the render_template_string() function evaluates the Jinja code and returns the final rendered HTML.
If the template string contains only plain HTML, the function simply returns the HTML as it is.
Examples of Flask render_template_string() Function
Example 1: Rendering a Static Template String
In this example, we use the render_template_string() function to render a static template string.
A static template string contains HTML that does not use variables or dynamic content.
Example:
from flask import render_template_string
render_template_string('''<!DOCTYPE html>
<html>
<head>
<title>Demo</title>
</head>
<body>
<h1>Hello, World!</h1>
</body>
</html>''')
Result:
<!DOCTYPE html>
<html>
<head>
<title>Demo</title>
</head>
<body>
<h1>Hello, World!</h1>
</body>
</html>
Since the template string only contains plain HTML, the function returns the same HTML content.
Example 2: Rendering a Dynamic Template String
In this example, the template string contains a Jinja variable that will be replaced with a value passed from Python.
This makes the template dynamic.
Example:
from flask import render_template_string
render_template_string('''<!DOCTYPE html>
<html>
<head>
<title>Demo</title>
</head>
<body>
<h1>Hello, {{ name }}!</h1>
</body>
</html>''', name='John')
Result:
<!DOCTYPE html>
<html>
<head>
<title>Demo</title>
</head>
<body>
<h1>Hello, John!</h1>
</body>
</html>
In this example, the Jinja variable {{ name }} is replaced with the value "John" provided in the function argument.
Syntax of Flask render_template_string() Function
from flask import render_template_string
render_template_string(source, **context)
Parameters of Flask render_template_string() Function
source (str)
The template string that contains the HTML and optional Jinja expressions that will be rendered.
**context (Any)
Optional keyword arguments that define variables available inside the template.
These variables can be accessed using Jinja syntax.
Example:
render_template_string("Hello {{ name }}", name="Alice")
Return Type of Flask render_template_string() Function
str
The function returns the rendered template as a string, which usually contains HTML that can be sent as an HTTP response.
When to Use render_template_string()
You should use render_template_string() when:
- The template is small and generated dynamically in Python
- You do not want to create a separate HTML file
- The template is temporary or simple
However, for most Flask applications, it is recommended to use template files with render_template() because they are easier to maintain and organize.
FAQs About Flask render_template_string() Function
When should I use the Flask render_template_string() function?
Use render_template_string() when you want to render a template directly from a Python string instead of a template file.
This is helpful for quick demos, dynamic templates, or small snippets of HTML.
Can I use render_template_string() to render a template file?
No. The render_template_string() function only renders template strings.
If you want to render an HTML template file stored in the templates directory, you should use the render_template() function instead.
Example:
render_template("index.html")
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