# Labels & Control Flow

## Label Statement

Label statements allow the given name to be assigned to a program point. They exist solely to be called or jumped to, either from Ren'Py script, Python functions, or from screens.

    label sample1:
        "Here is 'sample1' label."

    label sample2(a="default"):
        "Here is 'sample2' label."
        "a = [a]"

A label statement may have a block associated with it. In that case, control enters the block whenever the label statement is reached, and proceeds with the statement after the label statement whenever the end of the block is reached: the following code, when jumping to the "origin" label, produces the "a, b, c" sequence.

    label origin:
    "a"
    label hasblock:
        "b"
    "c"
    return

There are two kinds of labels: global and local labels. Global labels live in one global namespace shared across all project files and thus should have unique names per game. A local label on the other hand refer to a global label, so several local labels in the game can have the same name, provided they are related to different global labels. To declare a local label, prefix its name with a period `.`, and put it under a global label which it will belong to. For example:

    label global_label:
        "Under a global label.."
    label .local_label:
        "..resides a local one."
        jump .another_local
    label .another_local:
        "And another !"
        jump .local_label

Local labels can be referenced directly inside the same global label they are declared in, or by their full name, consisting of global and local name parts:

    label another_global:
        "Now lets jump inside a local label located somewhere else."
        jump global_label.local_name

The label statement may take an optional list of parameters. These parameters are processed as described in PEP 570, with two exceptions:

First, the values of default parameters are evaluated at call time.

Second, the variables are scoped dynamically, rather than lexically. This means that when a variable gets its value from a label parameter, it will be reverted (to the previous value of the variable if it had one, or to the absence of the variable otherwise) when a return statement is reached. It also means that given a statement using a certain variable, that variable may or may not get its value from a label parameter depending on how the statement was reached ; that is not possible in pure Python code.

    default a = 3

    label start:
        menu:
            "Call":
                call label_with_params(5)
            "Jump":
                jump label_without_params
        jump start

    label label_with_params(a):
    label label_without_params:
        e "a = [a]" # displays 5 or 3 depending on what path was taken
        return

It doesn't generally make sense to have a label with parameters be reached by a jump or a previous statement. For an example of labels with parameters, see the call statement.

## Jump Statement

The jump statement is used to transfer control to the given label.

If the `expression` keyword is present, the expression following it is evaluated, and the string so computed is used as the label name of the statement to jump to. If the `expression` keyword is not present, the label name of the statement to jump to must be explicitly given.

A local label name can be passed, either with `expression` or without, and either with the global label prepended ("global_label.local_label"), or starting with a dot (".local_label").

Unlike call, jump does not push the next statement onto a stack. As a result, there's no way to return to where you've jumped from.

    label loop_start:

        e "Oh no! It looks like we're trapped in an infinite loop."

        jump loop_start

## Call Statement

The call statement is used to transfer control to the given label. It also pushes the next statement onto the call stack, allowing the return statement to return control to the statement following the call.

If the `expression` keyword is present, the expression following it is evaluated, and the resulting string is used as the name of the label to call. If the `expression` keyword is not present, the name of the label to call must be explicitly given.

A local label name can be passed, either with `expression` or without, and either with the global label prepended ("global_label.local_label"), or starting with a dot (".local_label").

If the optional `from` clause is present, it has the effect of including a label statement with the given name as the statement immediately following the call statement. An explicit label helps to ensure that saved games with return stacks can return to the proper place when loaded on a changed script.

The call statement may take arguments, which are processed as described in PEP 448.

When using a call expression with an arguments list, the `pass` keyword must be inserted between the expression and the arguments list. Otherwise, the arguments list will be parsed as part of the expression, not as part of the call.

    label start:

        e "First, we will call a subroutine."

        call subroutine

        call subroutine(2)

        call expression "sub" + "routine" pass (count=3)

        return

    # ...

    label subroutine(count=1):

        e "I came here [count] time(s)."
        e "Next, we will return from the subroutine."

        return

Warning

Publishing a game without `from` clauses for each `call` statement is dangerous, if you intend to publish updates of the game later on. If no such clauses are added, and if you edit the file containing the `call` instruction, there is a potential risk for saves made inside the called label to become broken.

Using the "Add from clauses to calls" option when building a game's distribution can solve that issue.

## Return Statement

The `return` statement pops the top statement off of the call stack, and transfers control to it. If the call stack is empty, the return statement restarts Ren'Py, returning control to the main menu.

If the optional expression is given to return, it is evaluated, and it's result is stored in the `_return` variable. This variable is dynamically scoped to each context.

## Special Labels

The following labels are used by Ren'Py:

`start`

  By default, Ren'Py jumps to this label when the game starts.

`quit`

  If it exists, this label is called in a new context when the user quits the game.

`after_load`

  If it exists, this label is called when a game is loaded. It can be use to fix data when the game is updated. If data is changed by this label, `renpy.block_rollback()` should be called to prevent those changes from being reverted if the player rolls back past the load point.

`splashscreen`

  If it exists, this label is called when the game is first run, before showing the main menu. Please see Adding a Splashscreen.

`before_main_menu`

  If it exists, this label is called before the main menu. It is used in rare cases to set up the main menu, for example by starting a movie playing in the background.

`main_menu`

  If it exists, this label is called instead of the main menu. If it returns, Ren'Py will start the game at the `start` label. For example, the following will immediately start the game without displaying the main menu.

      label main_menu:
          return

`after_warp`

  If it is existed, this label is called after a warp but before the warped-to statement executes. Please see Warping to a line.

`hide_windows`

  If it exists, this label is called when the player hides the windows with the right mouse button or the H key. If this returns true, the hide is cancelled (it's assumed the hide has occurred). Otherwise, the hide continues.

Ren'Py also uses the following labels to show some of the special screens:

- `main_menu_screen`

- `load_screen`

- `save_screen`

- `preferences_screen`

- `joystick_preferences_screen`

## Labels & Control Flow Functions

## Contexts

Contexts are used internally by Ren'Py to manage the changeable and saveable state of the game. Contexts include:

- the currently running Ren'Py statement,

- the call stack, as described above, and the names and former values of dynamic variables created by `renpy.dynamic()`,

- the images currently being shown (and informations about them like their attributes, the transforms applied to them and so on),

- the screens being shown, and the variables inside them,

- the audio that is playing or queued.

Most of the time there is only one context at play, and only one instance of each of these elements exists. This changes when entering the main or game game menus; everything above can be changed, and will be restored when leaving the menu context. Some of these changes are automatic, like the screens layer being cleared when entering a context.

Ren'Py also creates new contexts as part of Replay and when `hiding the interface`.

The creation of screen language has considerably lessened the need for creating contexts.

Rollback is only enabled in the base context (meaning, when there is only one context), and only the base context is saved, which is why the game menu uses a context.