# General Translation Python SDKs: Detecting the request locale
URL: https://generaltranslation.com/en-US/docs/python/guides/detecting-locale.mdx
Docs index: https://generaltranslation.com/llms.txt
Description: How to resolve each request's locale automatically or with your own logic in General Translation for Python.

Every [`t`](/docs/python/reference/functions/t) call translates into the current request's locale. [`initialize_gt`](/docs/python/reference/functions/initialize-gt) registers locale detection for you; you can rely on the default or provide your own callback.

## Use the default detection [#default]

By default the SDK reads the `Accept-Language` header, parses its quality values, and picks the best match from your configured locales, falling back to the default locale. Flask does this in a `before_request` hook; FastAPI does it in HTTP middleware. No extra setup is required.

Read the resolved locale anywhere in a request with [`get_locale`](/docs/python/reference/functions/get-locale).

```python
from gt_flask import get_locale  # or: from gt_fastapi import get_locale

get_locale()
```

## Provide custom detection [#custom]

To honor an explicit choice — a query parameter, cookie, or path prefix — pass a [`get_locale`](/docs/python/reference/functions/get-locale) callback to [`initialize_gt`](/docs/python/reference/functions/initialize-gt). A custom callback fully replaces `Accept-Language` detection; it is not consulted when you supply your own.

<Tabs items={['Flask', 'FastAPI']}>
  <Tab value="Flask">
    ```python
    def detect_locale(request):
        return request.args.get("lang") or request.cookies.get("locale") or "en"

    initialize_gt(app, default_locale="en", locales=["es", "fr"], get_locale=detect_locale)
    ```
  </Tab>

  <Tab value="FastAPI">
    ```python
    def detect_locale(request):
        return request.query_params.get("lang") or request.cookies.get("locale") or "en"

    initialize_gt(app, default_locale="en", locales=["es", "fr"], get_locale=detect_locale)
    ```
  </Tab>
</Tabs>

The value you return is stored as-is for the request. If your callback returns a falsy value such as `None`, the locale resolves to the configured default locale (not `Accept-Language`), so return an explicit fallback if you want different behavior.

*Note: Unlike the built-in `Accept-Language` detection, a custom callback's return value is not re-validated against your configured `locales`. Return a supported locale, or resolve unsupported values yourself.*

## Next steps

- /docs/python/guides/translating-strings
- /docs/python/guides/configuring
- /docs/python/guides/declaring-variables
- /docs/python/guides/storing-translations

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