Class Variable

Lib/tkinter/__init__.py:377–566

Class to define value holders for e.g. buttons. Subclasses StringVar, IntVar, DoubleVar, BooleanVar are specializations that constrain the type of the value returned from get().

class Variable:
    """Class to define value holders for e.g. buttons.

    Subclasses StringVar, IntVar, DoubleVar, BooleanVar are specializations
    that constrain the type of the value returned from get()."""
    _default = ""
    _tk = None
    _tclCommands = None

    def __init__(self, master=None, value=None, name=None):
        """Construct a variable

        MASTER can be given as master widget.
        VALUE is an optional value (defaults to "")
        NAME is an optional Tcl name (defaults to PY_VARnum).

        If NAME matches an existing variable and VALUE is omitted
        then the existing value is retained.
        """
        # check for type of NAME parameter to override weird error message
        # raised from Modules/_tkinter.c:SetVar like:
        # TypeError: setvar() takes exactly 3 arguments (2 given)
        if name is not None and not isinstance(name, str):
            raise TypeError("name must be a string")
        global _varnum
        if master is None:
            master = _get_default_root('create variable')
        self._root = master._root()
        self._tk = master.tk
        if name:
            self._name = name
        else:
            self._name = 'PY_VAR' + repr(_varnum)
            _varnum += 1
        if value is not None:
            self.initialize(value)
        elif not self._tk.getboolean(self._tk.call("info", "exists", self._name)):
            self.initialize(self._default)

    def __del__(self):
        """Unset the variable in Tcl."""
        if self._tk is None:
            return
        if self._tk.getboolean(self._tk.call("info", "exists", self._name)):
            self._tk.globalunsetvar(self._name)
        if self._tclCommands is not None:
            for name in self._tclCommands:
                self._tk.deletecommand(name)
            self._tclCommands = None

    def __str__(self):
        """Return the name of the variable in Tcl."""
        return self._name

    def set(self, value):
        """Set the variable to VALUE."""
        return self._tk.globalsetvar(self._name, value)