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Callbacks with no output and pattern MATCH not allowing dash to run #2944
Description
Using a callback with no output in Dash 2.17.1 and using a MATCH pattern on the input from a data table prevents dash from loading.
Describe your context
Create a simple single file app with a data table and a pattern matching callback with no output.
This works:
@callback(
Input({'type': 'data-table', 'index': ALL}, 'active_cell'),
State('url', 'pathname'),
)
def print_table_id_and_column(active_cell, pathname):
print("hello")
This however causes the app to not load at all. The only indication of an issue is in the browser console. screenshot provided below:
@callback(
Input({'type': 'data-table', 'index': MATCH}, 'active_cell'),
State('url', 'pathname'),
)
def print_table_id_and_column(active_cell, pathname):
print("hello")
Environment:
pip list | grep dash
dash 2.17.1
dash-bootstrap-components 1.5.0
dash-bootstrap-templates 1.1.2
dash-core-components 2.0.0
dash-extensions 1.0.14
dash-html-components 2.0.0
dash-iconify 0.1.2
dash_mantine_components 0.14.4
dash-table 5.0.0
Describe the bug
Uncaught error TypeError: Cannot read properties of undefined (reading 'id') prevents dash from running.
Expected behavior
I expect that using a callback with no output works with both ALL and MATCH pattern matching.
Screenshots
Reacted by Rafael Bernardelli and Ole Jørgen Brønner
Activity
[-][BUG] Callbacks with no output and pattern MATCH not allowing dash to run[/-][+]Callbacks with no output and pattern MATCH not allowing dash to run[/+]aGitForEveryone commentedon Aug 30, 2024
This is the same issue as in #2462. Dash requires that if you use MATCH in the input, it should also appear in the output. The referenced issue also requested this restriction to be removed now that we can have callbacks with duplicate outputs. No idea, where on the roadmap this issue is.
BSd3v commentedon Aug 19, 2025
Been running into this issue as well
AnnMarieW commentedon Nov 25, 2025
Closing as a duplicate of #2462. It's best to consolidate into one issue.