Stop

The Stop helper lets you stop executing a notebook cell silently inside a Mercury App. It is useful for early exits like validation, missing inputs, or conditional flows (for example: stop until the user uploads a file).

Usage

Call mr.Stop() to stop execution immediately.

Basic Example

Code

import mercury as mr

x = 0
if x == 0:
    mr.Stop()

print("This line will not run")

Typical Pattern: Stop until a widget has a value

This is a common pattern in Mercury Apps: render a widget, and stop execution until the user provides input.

Code

import mercury as mr

uploader = mr.UploadFile(label="Upload CSV")

if uploader.value is None:
    mr.Stop()

print("File uploaded:", uploader.name)

Validation Guard

Use mr.Stop() to prevent running expensive code when inputs are invalid.

Code

import mercury as mr

threshold = mr.NumberInput(label="Threshold", value=0.5, min=0.0, max=1.0, step=0.05)

if threshold.value < 0.2:
    # too small — stop here
    mr.Stop()

print("Running with threshold:", threshold.value)

API

mr.Stop()

Stops execution of the current notebook cell.

• It raises an internal exception used only for flow control.
• It does not show a traceback.
• It does not display an error message.

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Notes

• mr.Stop() is intended for control flow, not for error handling.
• If you need to show a message to the user, display it before calling mr.Stop().

Note

mr.Stop() stops only the current execution flow. When the user changes a widget, Mercury may re-run the notebook, and execution can continue once your condition is satisfied.