# Errors and exceptions

Source: https://libtmux.org/en/py/latest/topics/errors-and-exceptions/

> Handle command failures, inspect delivery status, and decide when a retry is safe.

A command can fail because tmux rejects it, or because the transport stops
before returning a reply. Inspect the reported error and delivery state before
retrying a mutation: tmux may already have received it.

For lookup failures caused by zero or multiple matches, see [Filtering and
queries](https://libtmux.org/en/py/latest/concepts/queries/#the-cardinality-contract-side-by-side).

## A failed command, as a value

Command failures raise [`LibTmuxException`](<https://libtmux.org/en/py/latest/reference/libtmux-exc-libtmuxexception/>). It can carry a
`subcommand`; its string representation includes the subcommand and stderr.

## Is it safe to retry?

Retry a mutation automatically only when you know it was not dispatched, or when
repeating it is safe for your operation. A timeout, cancellation, or dropped
connection can occur after tmux has acted. These APIs expose delivery
information:

Inspect the exit status after a subprocess completes. If the call was
interrupted, verify the resulting tmux state before repeating a mutation.

Treat unknown delivery as potentially executed.

For subprocess calls that complete normally, inspect the exit status. If a call
is interrupted or times out without a delivery state, check tmux's resulting
state before repeating a mutation.