# Panel

> A dockable, resizable side panel. A native Pyzed component for Python desktop GUIs, with a live preview, code examples and its API.

Source: https://albakore.github.io/pyzed/library/panel/

:::tip[In plain words]
A side area of the window, like the file list on the left, that you can make wider or narrower.
:::

A dockable panel with an optional header. `side` decides which edge gets the border line; with a numeric `width` (`height` for the bottom) that edge can be dragged to resize it, and `on_resize` reports the new size.

`ui.panel` · Category: [Workspace](https://albakore.github.io/pyzed/library/#workspace) · [Full reference →](https://albakore.github.io/pyzed/reference/ui/#panel)

**Code:**

```python
pz.row([
    ui.panel(
        pz.column([ui.list_item("Inbox", on_open, icon="inbox", selected=True, inset=True),
                   ui.list_item("Archive", on_open, icon="archive", inset=True)]),
        header=ui.panel_header("Mail", [ui.icon_button("plus", on_new, size="xs")], icon="mail"),
        width=220, side="left",
    ),
    pz.container(ui.muted("Content"), center=True, width="fill", height="fill"),
], height="fill")
```

## When to use

- When you build your own layout instead of `ui.workspace`: sidebars, inspectors, outlines.

## Usage

```python
import pyzed as pz
from pyzed import ui

pz.row([
    ui.panel(
        pz.column([ui.list_item("Inbox", on_open, icon="inbox", selected=True, inset=True),
                   ui.list_item("Archive", on_open, icon="archive", inset=True)]),
        header=ui.panel_header("Mail", [ui.icon_button("plus", on_new, size="xs")], icon="mail"),
        width=220, side="left",
    ),
    pz.container(ui.muted("Content"), center=True, width="fill", height="fill"),
], height="fill")
```

## Variants

:::note[About these captures]
Each capture is recorded by pyzed itself, off-screen at 2x, in One Dark or One Light to match this site's theme, and cropped to the component so it shows at its real size. The pointer hovers, presses or types into it; a component that does not react to the pointer cycles through a few themes. With reduced motion you see the first frame. Open **Code** under a capture to see the snippet that draws it; free names such as `on_save` stand for your own callbacks.
:::

### Side

```python
pz.row([ui.panel(ui.list_item("Inbox", icon="inbox", inset=True), header=ui.panel_header("Mail"),
                 width=160, side="left"), pz.container(width="fill")], height="fill")
```

```python
pz.row([pz.container(width="fill"), ui.panel(ui.muted("Properties"), header=ui.panel_header("Inspector"),
                                          width=160, side="right")], height="fill")
```

```python
pz.column([pz.container(height="fill"), ui.panel(ui.terminal("$ ls"), header=ui.panel_header("Terminal"),
                                             height=100, side="bottom")], width="fill")
```

## Events

Panel has **1 event of its own**, listed below. Like every widget it also has the **18 common events** (`click`, `double_click`, `right_click`, `mouse_enter`, `drag`, `key_down`, `resize`…), attached with `.on(...)`: see [Events](https://albakore.github.io/pyzed/guides/events/) and [Events of every component](https://albakore.github.io/pyzed/library/events/).

| Event | Handler gets | When |
| --- | --- | --- |
| `on_resize` | `(size)` | Called with the new size when a drag ends. |

**Handler gets** is what Pyzed passes to your callable: `()` means nothing. Any handler can also be a message (a value that is not callable), which goes to `update()`.

### Handling the events

The same events written three ways. **Imperative**: methods of the app change `self`. **Declarative**: the handlers produce messages and one `update()` decides. **Functional**: a pure function draws an immutable state and asks for changes with `set_state(...)`. See [Imperative, declarative and functional](https://albakore.github.io/pyzed/guides/code-styles/).

**Imperative:**

```python
class Mail(pz.App):
    def __init__(self):
        self.width = 220

    def view(self):
        return pz.row([
            ui.panel(ui.muted("Inbox"), header=ui.panel_header("Mail"), width=self.width,
                     on_resize=lambda size: setattr(self, "width", size)),   # when the drag ends
            main_view(),
        ], height="fill")
```

**Declarative:**

```python
from dataclasses import dataclass

@dataclass
class Width:
    size: float

class Mail(pz.App):
    width = 220

    def update(self, message):
        match message:
            case Width(size):
                self.width = size

    def view(self):
        return pz.row([ui.panel(ui.muted("Inbox"), width=self.width, on_resize=Width), main_view()],
                      height="fill")
```

**Functional:**

```python
from dataclasses import dataclass

@dataclass(frozen=True)
class State:
    width: float = 220

def view(state, set_state):
    return pz.row([ui.panel(ui.muted("Inbox"), width=state.width, on_resize=lambda s: set_state(width=s)),
                   main_view()], height="fill")
```

## Properties and API

### `ui.panel`

```python
panel(
    content,
    *,
    header=None,
    width=None,
    height=None,
    side='left',
    resizable=True,
    on_resize=None,
    min_size=120,
    theme=None,
)
```

A dockable panel. `side` decides which edge gets the border line; with a numeric `width` (`height` for the bottom) that border can be dragged to resize it.

| Property | Default | Description |
| --- | --- | --- |
| `content` | *required* | Content: a `Widget`, a string or a number (converted to text). |
| `header` | `None` | Header (`ui.panel_header`). |
| `width` | `None` | Width: pixels, `"fill"`, `"shrink"` or `"fill:N"` (proportion). |
| `height` | `None` | Height: pixels, `"fill"`, `"shrink"` or `"fill:N"` (proportion). |
| `side` | `"left"` | `"left"`, `"right"` or `"bottom"`. |
| `resizable` | `True` | `False` keeps the size fixed. |
| `min_size` | `120` | Smallest size when resizing. |
| `theme` | `None` | Theme to use instead of the current one (`pyzed.theme.current()`). |

Events: `on_resize` ([events](#events)).

[Full reference →](https://albakore.github.io/pyzed/reference/ui/#panel)

## Related

- [Panel header](https://albakore.github.io/pyzed/library/panel-header/): Icon, title and actions on top of a dock.
- [Workspace](https://albakore.github.io/pyzed/library/workspace/): The layout of an editor-style window: title bar, docks, center, status bar.
- [Split](https://albakore.github.io/pyzed/library/split/): Two panes with a draggable divider.