Error boundary

In plain words

A safety net for one part of a window: if that part breaks, a small error box shows there and the rest of the app keeps working.

ui.error_boundary(build) returns build() or, when it raises, an error card in its place, so one broken part of the view (a sidebar, a panel) never takes the whole window down. The card shows the error, the file and line it comes from and a Copy error button; on_details adds a Details button and fallback replaces the card with your own widget. on_error(info) is told about the error each time the part fails to draw. ui.ErrorInfo describes an error (its type, message, traceback and origin) and ui.error_card is the card on its own.

ui.error_boundary · Category: Feedback · Full reference →

Preview

Code

def totals():
    values = []
    return ui.label(f"Average: {sum(values) / len(values)}")

pz.row([
    ui.card(ui.muted("This panel keeps working."), title="Files", width=200),
    pz.container(ui.error_boundary(totals, title="Totals", on_details=on_details), width=380),
], spacing=12, align="center")

New

This component was added recently. Tell us how it works for you in the issue tracker.

When to use

• Around each panel of a big window, so a bug in one leaves the others working.
• Around parts that draw data you don’t control: a plugin, a file the user opened, a reply from a server.
• Open an Error dialog from on_details to show the whole traceback.

Usage

import pyzed as pz
from pyzed import ui

class Report(pz.App):
    def __init__(self):
        self.error = None  # the ui.ErrorInfo shown in the dialog

    def totals(self):
        ...  # may raise

    def show(self, info):
        self.error = info

    def close(self):
        self.error = None

    def view(self):
        page = pz.row([
            self.files(),
            ui.error_boundary(self.totals, title="Totals", on_details=self.show),
        ])
        dialog = ui.error_dialog(self.error, self.close) if self.error else None
        return ui.dialog_layer(page, dialog, on_dismiss=self.close)

Variants

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.

Boundary

While build() works the boundary returns its widget untouched; only a part that raises is replaced.

No error

Code

ui.error_boundary(lambda: ui.label("Average: 4.2"))

Default card

Code

def totals():
    values = []
    return ui.label(f"Average: {sum(values) / len(values)}")

pz.container(ui.error_boundary(totals), width=360)

title="Totals"

Code

def totals():
    values = []
    return ui.label(f"Average: {sum(values) / len(values)}")

pz.container(ui.error_boundary(totals, title="Totals"), width=360)

on_details=

Code

def totals():
    values = []
    return ui.label(f"Average: {sum(values) / len(values)}")

pz.container(ui.error_boundary(totals, title="Totals", on_details=on_details),
             width=360)

fallback=

Code

def totals():
    values = []
    return ui.label(f"Average: {sum(values) / len(values)}")

ui.error_boundary(totals, fallback=lambda info: ui.callout(
    "Totals are not available", info.summary, tint="warning"))

Error card

ui.error_card is the card on its own, for errors you catch yourself.

ui.error_card(info)

Code

info = ui.ErrorInfo("KeyError", "'email'", origin=("app/forms.py", 42), where="Sign-up form")
pz.container(ui.error_card(info), width=360)

title=, actions=

Code

info = ui.ErrorInfo("KeyError", "'email'", origin=("app/forms.py", 42), where="Sign-up form")
pz.container(ui.error_card(info, title="The form could not be loaded",
                           actions=[ui.button("Retry", on_retry, icon="rotate-ccw",
                                              size="sm", variant="ghost")]),
             width=360)

No origin

Code

pz.container(ui.error_card(ui.ErrorInfo("TimeoutError", "the server took too long")),
             width=360)

Events

Error boundary has 2 events 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 and Events of every component.

Event	Of	Handler gets	When
on_error	ui.error_boundary	(info)	Receives the ui.ErrorInfo.
on_details	ui.error_boundary, ui.error_card	(info)	Adds a Details button; receives the ui.ErrorInfo (open a ui.error_dialog).

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.

Imperative

class Report(pz.App):
    def __init__(self):
        self.values = []
        self.error = None                # the ui.ErrorInfo shown in the dialog

    def totals(self):
        return ui.label(f"Average: {sum(self.values) / len(self.values)}")   # raises on []

    def view(self):
        page = ui.error_boundary(self.totals, title="Totals",
                                 on_error=lambda info: print(info.message),         # every failure
                                 on_details=lambda info: setattr(self, "error", info))  # Details
        dialog = ui.error_dialog(self.error, lambda: setattr(self, "error", None)) if self.error else None
        return ui.dialog_layer(page, dialog)

Declarative

from dataclasses import dataclass


@dataclass
class ShowError:
    info: object


class Report(pz.App):
    error = None

    def totals(self):
        return ui.label(f"Average: {sum([]) / len([])}")

    def update(self, message):
        match message:
            case ShowError(info):
                self.error = info
            case "close":
                self.error = None

    def view(self):
        page = ui.error_boundary(self.totals, title="Totals", on_details=ShowError)
        return ui.dialog_layer(page, ui.error_dialog(self.error, "close") if self.error else None)

Functional

from dataclasses import dataclass


@dataclass(frozen=True)
class State:
    values: tuple = ()
    failures: int = 0


def totals(values):
    return ui.label(f"Average: {sum(values) / len(values)}")


def view(state, set_state):
    return pz.column([
        ui.error_boundary(lambda: totals(state.values), title="Totals",
                          on_error=lambda info: set_state(failures=state.failures + 1)),
        ui.button("Add 10", lambda: set_state(values=state.values + (10,))),
    ], spacing=8, width=380)

Tips

Tip

The boundary runs build() on every redraw, so on_error is called again each time the part fails. To report an error once, compare it with the last one: two ErrorInfo with the same type, message, traceback and origin are equal (time is left out). ErrorInfo.from_exception(err) describes any exception you catch yourself, and info.report() gives the text to paste in a bug report.

Properties and API

ui.error_boundary

error_boundary(
    build,
    *,
    on_error=None,
    title=None,
    where='',
    on_details=None,
    fallback=None,
    theme=None,
)

Draws build(), or an error card when it raises, so one part of the view that fails never takes the whole window down (like React’s error boundaries). on_error(info) is told about the error each time the part is drawn and fails, so report it once (compare info.traceback).

Property	Default	Description
build	required	A function that returns the part’s widget.
title	None	Heading of the card (and the error’s where).
where	""	What the part is, for the report.
fallback	None	Your own widget for the error: fallback(info).
theme	None	Theme override.

Events: on_error, on_details (events).

Full reference →

ui.error_card

error_card(info, *, title=None, on_details=None, actions=(), theme=None)

The card ui.error_boundary draws in place of what failed: the error, where it comes from and a Copy error button (and Details with on_details).

Property	Default	Description
info	required	A ui.ErrorInfo.
title	None	Heading.
actions	()	More buttons after them.
theme	None	Theme override.

Events: on_details (events).

Full reference →

ui.ErrorInfo

@dataclass
class ErrorInfo(kind, message, traceback='', origin=None, where='', time=0.0)

An error as ui.error_card and ui.error_dialog show it: kind (the exception’s type), message, the whole traceback, the origin (path, line) it comes from (the last frame outside the standard library, installed packages and Pyzed), where it happened and time. ErrorInfo.from_exception(err, where=...) makes one (prefer= picks which frames may be the origin); ErrorInfo.current() describes the exception being handled. summary is Type: message, location is file.py:12 and report() is everything as text for a bug report.

Property	Default	Description
kind	required	The exception’s type name.
message	required	Its message.
traceback	""	The whole traceback, as Python prints it.
origin	None	(path, line) of the code it comes from, or None.
where	""	What was running.
time	0.0	When it happened (time.time()).

Full reference →

Related

Error dialogWhat went wrong, where, and the traceback, with Copy and Go to source.

CalloutAn inline notice with a title, text and actions.

ConsoleA dev-tools style log with levels, filter, copy and clear.