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# Sub-Typer Callback Override
When creating a **Typer** app you can define a callback function, it always executes and defines the *CLI arguments* and *CLI options* that go before a command.
When adding a Typer app inside of another, the sub-Typer can also have its own callback.
It can handle any *CLI parameters* that go before its own commands and execute any extra code:
{* docs_src/subcommands/callback_override/tutorial001.py hl[9,10,11] *}
In this case it doesn't define any *CLI parameters*, it just writes a message.
Check it:
<div class="termy">
```console
$ python main.py users create Camila
// Notice the first message is not created by the command function but by the callback
Running a users command
Creating user: Camila
```
</div>
## Add a callback on creation
It's also possible to add a callback when creating the `typer.Typer()` app that will be added to another Typer app:
{* docs_src/subcommands/callback_override/tutorial002.py hl[6,7,10] *}
This achieves exactly the same as above, it's just another place to add the callback.
Check it:
<div class="termy">
```console
$ python main.py users create Camila
Running a users command
Creating user: Camila
```
</div>
## Overriding the callback on creation
If a callback was added when creating the `typer.Typer()` app, it's possible to override it with a new one using `@app.callback()`.
This is the same information you saw on the section about [Commands - Typer Callback](../commands/callback.md){.internal-link target=_blank}, and it applies the same for sub-Typer apps:
{* docs_src/subcommands/callback_override/tutorial003.py hl[6,7,10,14,15,16] *}
Here we had defined a callback when creating the `typer.Typer()` sub-app, but then we override it with a new callback with the function `user_callback()`.
As `@app.callback()` takes precedence over `typer.Typer(callback=some_function)`, now our CLI app will use this new callback.
Check it:
<div class="termy">
```console
$ python main.py users create Camila
// Notice the message from the new callback
Callback override, running users command
Creating user: Camila
```
</div>
## Overriding the callback when adding a sub-Typer
Lastly, you can override the callback defined anywhere else when adding a sub-Typer with `app.add_typer()` using the `callback` parameter.
This has the highest priority:
{* docs_src/subcommands/callback_override/tutorial004.py hl[13,14,17] *}
Notice that the precedence goes to `app.add_typer()` and is not affected by the order of execution. There's another callback defined below, but the one from `app.add_typer()` wins.
Now when you use the CLI program it will use the new callback function `callback_for_add_typer()`.
Check it:
<div class="termy">
```console
$ python users create Camila
// Notice the message from the callback added in add_typer()
I have the high land! Running users command
Creating user: Camila
```
</div>
|
