This is different from other exceptions that are typically raised by the system when something goes wrong, such as invalid inputs to a method call. This is an exception you, the author of a Python SOP, or Python Object, or Python LOP, etc., raise in the node’s implementation script to set messages/warnings/errors on the node.

Errors, warnings, and messages are part of a node’s user interface. They show up in the network editor, parameter editor, and info window.

• Errors signal to the node’s user that the node has encountered an error. For example, invalid inputs or parameter values.

• Warnings signal that the node was able to cook, but there was a problem the user might want to check. For example, a missing texture.

See Writing Python SOPs for more information.

If you're writing a Python SOP, you might use the content of an external file (referenced in a parameter on the node), even though the node can work without the file:

import os.path

thisnode = hou.pwd()
filepath = thisnode.parm("file").evalAsString()
content = hou.readFile()
if content:
    # Use the file contents
    # ...

# Continue script operation
# ...

# At the end of the script, raise any warning that might have
# come up during the script
if not content:
    raise NodeWarning("The texture file is missing or empty")