Context Processors

A context processor adds variables to the template context for every render.

Useful for:

• site name
• current year
• navigation links
• feature flags

diagram two contexts, not one mermaid

  flowchart TD
  A["a request arrives"] --> B["push application context"]
  B --> C["push request context"]
  C --> D["your view runs"]
  D --> E["pop request context"]
  E --> F["pop application context"]
  B -.-> G["current_app, g available"]
  C -.-> H["request, session available"]
  I["a CLI command or a background job"] --> J["with app.app_context():"]
  J --> G
  J -.->|"still NOT available"| H

Flask pushes an application context and, for an actual request, a request context inside it. Which proxies work depends on which of the two is active -- current_app and g need the application context, request and session need the request context. That is why a background job can use current_app after app.app_context() but still cannot touch request.

Two contexts means two different errors

Called with nothing pushed, the proxies fail differently — and the wording tells you which context is missing:

Proxy	Error
current_app	RuntimeError: Working outside of application context.
g	RuntimeError: Working outside of application context.
request	RuntimeError: Working outside of **request** context.

So with app.app_context(): fixes the first two and not the third. A background task or a CLI command can reach configuration and the database that way, but there is no request to look at — if it needs request data, that data has to be passed in as an argument.

g is per-request, not global, despite the name. Verified: a view that sets g.val followed by a second request reports NOT SET. It is scratch space for one request — use the config or a module-level object for anything that must outlive it.

Example

python

import datetime
 
@app.context_processor
def inject_globals():
    return {
        "site_name": "PythonCentralHub",
        "year": datetime.datetime.now().year,
    }

Now every template can use:

html

<footer>
  <p>&copy; {{ year }} {{ site_name }}</p>
</footer>

Best practice

Keep context processors small.

Don’t put heavy database queries in them.

If you need navigation loaded from DB, consider caching.