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# Core Events

The following events are defined by xonsh itself. For more information about events, see the events tutorial.

on_chdir | on_command_not_found | on_envvar_change
on_envvar_new | on_exit | on_import_post_create_module
on_import_post_exec_module | on_import_post_find_spec | on_import_pre_create_module
on_import_pre_exec_module | on_import_pre_find_spec | on_lscolors_change
on_post_cmdloop | on_post_init | on_post_prompt
on_post_rc | on_postcommand | on_pre_cmdloop
on_pre_prompt | on_pre_prompt_format | on_pre_rc
on_pre_spec_run_ls | on_precommand | on_ptk_create
on_transform_command | on_xonfig_info_requested | on_xontribs_loaded

## Listing

### `on_chdir(olddir: str, newdir: str) -> None`

Fires when the current directory is changed for any reason.

    @events.on_chdir
    def _source_env_xsh(olddir, newdir):
        '''Source `env.xsh` file if it's exist in the new directory.'''
        env_file = @.imp.pathlib.Path(newdir) / 'env.xsh'
        if env_file.exists():
            print(f'Source {env_file}')
            source @(env_file)

### `on_command_not_found(cmd: list[str]) -> list[str] | tuple[str, ...] | None`

Fires if a command is not found (only in interactive sessions).

Parameters:

- `cmd`: The command that was attempted

Returns:

- `list[str]` or `tuple[str, ...]`: A replacement command to execute instead. The first valid replacement from any handler will be used.

- `None`: To let the error be raised normally

Note: If the replacement command also fails, the original error is shown.

Example:

    @events.on_command_not_found
    def _vim_to_vi(cmd, **kwargs):
        '''If vim not found let's try to use vi.'''
        if cmd[0] == 'vim':
            return ['vi'] + cmd[1:]

### `on_envvar_change(name: str, oldvalue: Any, newvalue: Any) -> None`

Fires after an environment variable is changed. Note: Setting envvars inside the handler might cause a recursion until the limit.

    @events.on_envvar_change
    def _on_env_path_change(name, oldvalue, newvalue):
        '''Keep `/tmp/bin` on top of PATH.'''
        if name == 'PATH' and (newvalue == [] or newvalue[0] != '/tmp/bin'):
            $PATH = ['/tmp/bin'] + [v for v in newvalue if v != '/tmp/bin']

### `on_envvar_new(name: str, value: Any) -> None`

Fires after a new environment variable is created. Note: Setting envvars inside the handler might cause a recursion until the limit.

### `on_exit(exit_code : int) -> None`

Fired after all commands have been executed, before tear-down occurs.

NOTE: All the caveats of the `atexit` module also apply to this event.

### `on_import_post_create_module(module: Module, spec: ModuleSpec) -> None`

Fires after a module is created by its loader but before the loader returns it. The parameters here are the module object itself and the spec object. See importlib for more details.

### `on_import_post_exec_module`

on_import_post_create_module(module: Module) -> None

Fires after a module is executed by its loader but before the loader returns it. The only parameter is the module itself. See importlib for more details.

### `on_import_post_find_spec(spec, fullname, path, target) -> None`

Fires after all import find_spec() calls have been executed. The parameters here the spec and the arguments importlib.abc.MetaPathFinder.find_spec(). Namely,

`spec`:

  A ModuleSpec object if the spec was found, or None if it was not.

`fullname`:

  The full name of the module to import.

`path`:

  None if a top-level import, otherwise the `__path__` of the parent package.

`target`:

  Target module used to make a better guess about the package spec.

### `on_import_pre_create_module(spec: ModuleSpec) -> None`

Fires right before a module is created by its loader. The only parameter is the spec object. See importlib for more details.

### `on_import_pre_exec_module(module: Module) -> None`

Fires right before a module is executed by its loader. The only parameter is the module itself. See importlib for more details.

### `on_import_pre_find_spec(fullname: str, path: str, target: module or None) -> None`

Fires before any import find_spec() calls have been executed. The parameters here are the same as importlib.abc.MetaPathFinder.find_spec(). Namely,

`fullname`:

  The full name of the module to import.

`path`:

  None if a top-level import, otherwise the `__path__` of the parent package.

`target`:

  Target module used to make a better guess about the package spec.

### `on_lscolors_change(key: str, oldvalue: Any, newvalue: Any) -> None`

Fires after a value in LS_COLORS changes, when a new key is added (oldvalue is None) or when an existing key is deleted (newvalue is None). LS_COLORS values must be (ANSI color) strings, None is unambiguous. Does not fire when the whole environment variable changes (see on_envvar_change). Does not fire for each value when LS_COLORS is first instantiated. Normal usage is to arm the event handler, then read (not modify) all existing values.

### `on_post_cmdloop() -> None`

Fired just after the command loop finishes, if it is.

NOTE: All the caveats of the `atexit` module also apply to this event.

### `on_post_init() -> None`

Fired after all initialization is finished and we’re ready to do work.

NOTE: This is fired before the wizard is automatically started.

### `on_post_prompt() -> None`

Fires just after the prompt returns

### `on_post_rc() -> None`

Fired just after rc files are loaded, if they are.

### `on_postcommand(cmd: str, rtn: int, out: str or None, ts: list) -> None`

Fires just after a command is executed. The arguments are the same as history. This event only fires in interactive mode.

Parameters:

- `cmd`: The command that was executed (after transformation)

- `rtn`: The result of the command executed (`0` for success)

- `out`: If xonsh stores command output, this is the output

- `ts`: Timestamps, in the order of `[starting, ending]`

Example:

    @events.on_postcommand
    def _prompt_err_command_again(cmd, rtn, out, ts):
        '''If the result of executing the command is not zero, repeat the command on the next command prompt.'''
        if rtn != 0:
            $XONSH_PROMPT_NEXT_CMD = cmd.rstrip()

### `on_pre_cmdloop() -> None`

Fired just before the command loop is started, if it is.

### `on_pre_prompt() -> None`

Fires just before showing the prompt

### `on_pre_prompt_format() -> None`

Fires before the prompt will be formatted

### `on_pre_rc() -> None`

Fired just before rc files are loaded, if they are.

### `on_pre_spec_run_ls(spec: xonsh.built_ins.SubprocSpec) -> None`

Fires right before a SubprocSpec.run() is called for the ls command.

### `on_precommand(cmd: str) -> None`

Fires just before a command is executed.

### `on_ptk_create(prompter: PromptSession, history: PromptToolkitHistory, completer: PromptToolkitCompleter, bindings: KeyBindings) ->`

Fired after prompt toolkit has been initialized. Use this event in xonsh RC file.

    # ~/.xonshrc
    @events.on_ptk_create
    def _custom_keybindings(bindings, **kw):
        @bindings.add(@.imp.prompt_toolkit.keys.Keys.ControlW)
        def say_hi(event):
            event.current_buffer.insert_text('hi')

### `on_transform_command(cmd: str) -> str`

Fired to request xontribs to transform a command line. Return the transformed command, or the same command if no transformation occurs. Only done for interactive sessions.

This may be fired multiple times per command, with other transformers input or output, so design any handlers for this carefully.

    @events.on_transform_command
    def _pipe_prev_command(cmd, **kw):
        '''Pipe prev command e.g. `| grep 1` will be `<prev cmd> | grep 1`.'''
        if cmd and cmd.startswith('| ') and __xonsh__.history:
            return __xonsh__.history[-1].cmd.rstrip() + cmd.rstrip()
        return cmd

### `on_xonfig_info_requested() -> list[tuple[str, str]]`

Register a callable that will return extra info when `xonfig info` is called.

### `on_xontribs_loaded() -> None`

Fired after external xontribs with `entrypoints defined` are loaded.

## Event Categories

Additionally, there are a few categories of events whose names are part of the specification of the event. These events are fired if they exist, and are ignored otherwise. Here are their specifications.

### `on_pre_spec_run(spec: SubprocSpec) -> None`

This event fires whenever any command has its `SubprocSpec.run()` method called. This is fired prior to the run call executing anything at all. This receives the `SubprocSpec` object as `spec` that triggered the event, allowing the handler to modify the spec if needed. For example, if we wanted to intercept all specs, we could write:

    @events.on_pre_spec_run
    def print_when_ls(spec=None, **kwargs):
        print("Running a command")

### `on_pre_spec_run_<cmd-name>(spec: SubprocSpec) -> None`

This event fires whenever a command with a give name (`<cmd-name>`) has its `SubprocSpec.run()` method called. This is fired prior to the run call executing anything at all. This receives the `SubprocSpec` object as `spec` that triggered the event, allowing the handler to modify the spec if needed. For example, if we wanted to intercept an `ls` spec, we could write:

    @events.on_pre_spec_run_ls
    def print_when_ls(spec=None, **kwargs):
        print("Look at me list stuff!")

### `on_post_spec_run(spec: SubprocSpec) -> None`

This event fires whenever any command has its `SubprocSpec.run()` method called. This is fired after to the run call has executed everything except returning. This recieves the `SubprocSpec` object as `spec` that triggered the event, allowing the handler to modify the spec if needed. Note that because of the way process pipelines and specs work in xonsh, the command will have started running, but won’t necessarily have completed. This is because `SubprocSpec.run()` does not block. For example, if we wanted to get any spec after a command has started running, we could write:

    @events.on_post_spec_run
    def print_while_ls(spec=None, **kwargs):
        print("A command is running")

### `on_post_spec_run_<cmd-name>(spec: SubprocSpec) -> None`

This event fires whenever a command with a give name (`<cmd-name>`) has its `SubprocSpec.run()` method called. This is fired after to the run call has executed everything except returning. This recieves the `SubprocSpec` object as `spec` that triggered the event, allowing the handler to modify the spec if needed. Note that because of the way process pipelines and specs work in xonsh, the command will have started running, but won’t necessarily have completed. This is because `SubprocSpec.run()` does not block. For example, if we wanted to get an `ls` spec after ls has started running, we could write:

    @events.on_post_spec_run_ls
    def print_while_ls(spec=None, **kwargs):
        print("Mom! I'm listing!")