class Backend(QObject):
fileCleared = Signal()
requestSaveAs = Signal()

def __init__(self):
    super().__init__()
    self.current_path = None  # None = unsaved new file

@Slot()
def new_file(self):
    self.current_path = None
    self.fileCleared.emit()  # Tell QML to clear the text

@Slot(str)
def save_file(self, text):
    if self.current_path is None:
        self.requestSaveAs.emit()
    else:
        with open(self.current_path, 'w') as f:
            f.write(text)

@Slot(str)
def open_file(self, path):
    self.current_path = path

qmlTextEdit {
id: textEdit
}

Connections {
target: backend
function onFileCleared() {
textEdit.clear()
// Don't touch textDocument.source at all
}
}
Why your attempts failed:

source = "" — Qt tries to resolve empty string as a URL, document loses context
source = undefined — QUrl binding is strict, doesn't accept undefined
new URL() — wrong argument count for QUrl constructor
textEdit.clear() — works but leaves source stale (which is fine!)

The key insight: textDocument.source is only meant for loading files. For a "New File" action, just track the file path in Python as None, call textEdit.clear() in QML, and never touch textDocument.source until the user actually opens or saves a real file. The stale source does not affect editing at all.

Edit: removed spam content

When the user creates a new file, I would like the QQuickTextDocument's source to be reset (it makes sense, right?).

@jayrickaby said in How to clear TextEdit's textDocument.source?:

Despite the QML ... (parent or ancestor of TextDocument): does not exist error, textDocument.source = Qt.resolvedUrl("") IS working how I need it to (even if it doing it in a broken way).
I hope proper support can be added

The described program structure isn't clear to me. Creating a new TextEdit should create a new TextDocument with an empty source. Is this program instead overwriting the text of an existing TextEdit?

The program below works as I expect with Qt 6.8.7, based on the documentation. When textDocument.modified == false, textDocument.source can be set to an empty string, but that results in a loading error.

import QtQuick
import QtQuick.Layouts
import QtQuick.Controls

Window {
    visible: true
    ColumnLayout {
        anchors.fill: parent
        TextInput {
            id: source
            focus: true
            Layout.fillWidth: true
            onEditingFinished: edit.textDocument.source = text
            Rectangle {
                border.width: 1
                border.color: "black"
                anchors.fill: parent
                z: -1
            }
        }
        CheckBox {
            id: cb
            text: "modified"
            checkState: edit.textDocument.modified ? Qt.Checked : Qt.Unchecked
            onClicked: if (Qt.Unchecked == checkState) edit.textDocument.modified = false;
        }
        Text {
            text: "source: " + edit.textDocument.source + "\nstatus: " + edit.textDocument.status
        }
        TextEdit {
            id: edit
            Layout.fillWidth: true
            Layout.fillHeight: true
            Rectangle {
                border.width: 1
                border.color: "black"
                anchors.fill: parent
                z: -1
            }
        }
    }
}

Did you set modified to false before change source's value as per the property documentation ? (Just an idea, I haven't used that type)

When you try to change the source when it is flagged as modified, a message is outputted console to notify why the source didnt change because of this. This isn't happening here as there is no message.

I tried to adapt that little script to print functions; I couldn't get it to work.

Despite the QML ... (parent or ancestor of TextDocument): does not exist error, textDocument.source = Qt.resolvedUrl("") IS working how I need it to (even if it doing it in a broken way).
I hope proper support can be added

The source property cannot be changed while the document's modified state is true. If the user has modified the document contents, you should prompt the user whether to save(), or else discard changes by setting modified = false before setting the source property to a different URL.

Is it worth checking/clearing the modified property? Otherwise you might check what is actually in source and what methods it has --- Googling javascript show all functions of an object gives something like https://stackoverflow.com/questions/2257993/how-to-display-all-methods-of-an-object

var methods = [];
for (var m in obj) {
    if (typeof obj[m] == "function") {
        methods.push(m);
    }
}
alert(methods.join(","));

or just print out each m regardless of type.

Also https://stackoverflow.com/questions/73830468/modify-a-qtextdocument-of-a-qml-textarea-element says

I read in the documentation that the QQuickTextDocument::textDocument() is read-only and that I can not modify its internal state

so that might be a problem?

Finally you might retry TextEdit.textDocument.source = new URL() with some parameter acceptable to to new URL()?

TextEdit.textDocument.source.clear()

This should actually work.
source is a QUrl, which has https://doc.qt.io/qt-6/qurl.html#clear method.