Center things

If you have ever needed to center something in a web page, you will be glad to know it is much easier in Textual.

This article discusses a few different ways in which things can be centered, and the differences between them.

Aligning widgets

The align rule will center a widget relative to one or both edges. This rule is applied to a container, and will impact how the container's children are arranged. Let's see this in practice with a trivial app containing a Static widget:

from textual.app import App, ComposeResult
from textual.widgets import Static


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static("Hello, World!")


if __name__ == "__main__":
    app = CenterApp()
    app.run()

Here's the output:

The container of the widget is the screen, which has the align: center middle; rule applied. The center part tells Textual to align in the horizontal direction, and middle tells Textual to align in the vertical direction.

The output may surprise you. The text appears to be aligned in the middle (i.e. vertical edge), but left aligned on the horizontal. This isn't a bug — I promise. Let's make a small change to reveal what is happening here. In the next example, we will add a background and a border to our text:

Tip

Adding a border is a very good way of visualizing layout issues, if something isn't behaving as you would expect.

from textual.app import App, ComposeResult
from textual.widgets import Static


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }

    #hello {
        background: blue 50%;
        border: wide white;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static("Hello, World!", id="hello")


if __name__ == "__main__":
    app = CenterApp()
    app.run()

The static widget will now have a blue background and white border:

Note the static widget is as wide as the screen. Since the widget is as wide as its container, there is no room for it to move in the horizontal direction.

Info

The align rule applies to widgets, not the text.

In order to see the center alignment, we will have to make the widget smaller than the width of the screen. Let's set the width of the Static widget to auto, which will make the widget just wide enough to fit the content:

from textual.app import App, ComposeResult
from textual.widgets import Static


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }

    #hello {
        background: blue 50%;
        border: wide white;
        width: auto;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static("Hello, World!", id="hello")


if __name__ == "__main__":
    app = CenterApp()
    app.run()

If you run this now, you should see the widget is aligned on both axis:

Aligning text

In addition to aligning widgets, you may also want to align text. In order to demonstrate the difference, lets update the example with some longer text. We will also set the width of the widget to something smaller, to force the text to wrap.

from textual.app import App, ComposeResult
from textual.widgets import Static

QUOTE = "Could not find you in Seattle and no terminal is in operation at your classified address."


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }

    #hello {
        background: blue 50%;
        border: wide white;
        width: 40;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static(QUOTE, id="hello")


if __name__ == "__main__":
    app = CenterApp()
    app.run()

Here's what it looks like with longer text:

Note how the widget is centered, but the text within it is flushed to the left edge. Left aligned text is the default, but you can also center the text with the text-align rule. Let's center align the longer text by setting this rule:

from textual.app import App, ComposeResult
from textual.widgets import Static

QUOTE = "Could not find you in Seattle and no terminal is in operation at your classified address."


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }

    #hello {
        background: blue 50%;
        border: wide white;
        width: 40;
        text-align: center;
    }
    """

    def compose(self) -> ComposeResult:
        yield Static(QUOTE, id="hello")


if __name__ == "__main__":
    app = CenterApp()
    app.run()

If you run this, you will see that each line of text is individually centered:

You can also use text-align to right align text or justify the text (align to both edges).

Aligning content

There is one last rule that can help us center things. The content-align rule aligns content within a widget. It treats the text as a rectangular region and positions it relative to the space inside a widget's border.

In order to see why we might need this rule, we need to make the Static widget larger than required to fit the text. Let's set the height of the Static widget to 9 to give the content room to move:

from textual.app import App, ComposeResult
from textual.widgets import Static

QUOTE = "Could not find you in Seattle and no terminal is in operation at your classified address."


class CenterApp(App):
    """How to center things."""

    CSS = """
    Screen {
        align: center middle;
    }

    #hello {
        background: blue 50%;
        border: wide white;
        width: 40;
        height: 9;