Add the Variable class with an example

docs/source/changelog.rst

@@ -59,6 +59,7 @@ Mobjects, Scenes, and Animations
 #. Most :code:`get_` and :code:`set_` methods have been removed in favor of instance attributes and properties
 #. The :code:`Container` class has been made into an AbstractBaseClass, i.e. in cannot be instantiated.  Instead, use one of its children classes
 #. The ``TextMobject`` and ``TexMobject`` objects have been deprecated, due to their confusing names, in favour of ``Tex`` and ``MathTex``. You can still, however, continue to use ``TextMobject`` and ``TexMobject``, albeit with Deprecation Warnings constantly reminding you to switch.
+#. Add a :code:`Variable` class for displaying text that continuously updates to reflect the value of a python variable.
 
 
 

example_scenes/basic.py

@@ -178,4 +178,44 @@ def construct(self):
         self.wait()
 
 
-# See old_projects folder for many, many more
+class VariableExample(Scene):
+    def construct(self):
+        var = 0.5
+        on_screen_var = Variable(var, Text("var"), num_decimal_places=3)
+
+        # You can also change the colours for the label and value
+        on_screen_var.label.set_color(RED)
+        on_screen_var.value.set_color(GREEN)
+
+        self.play(Write(on_screen_var))
+        # The above line will just display the variable with
+        # its initial value on the screen. If you also wish to
+        # update it, you can do so by accessing the `tracker` attribute
+        self.wait()
+        var_tracker = on_screen_var.tracker
+        var = 10.5
+        self.play(var_tracker.set_value, var)
+        self.wait()
+
+        int_var = 0
+        on_screen_int_var = Variable(int_var, Text("int_var"), var_type=Integer).next_to(
+            on_screen_var, DOWN
+        )
+        on_screen_int_var.label.set_color(RED)
+        on_screen_int_var.value.set_color(GREEN)
+
+        self.play(Write(on_screen_int_var))
+        self.wait()
+        var_tracker = on_screen_int_var.tracker
+        var = 10.5
+        self.play(var_tracker.set_value, var)
+        self.wait()
+
+        # If you wish to have a somewhat more complicated label for your
+        # variable with subscripts, superscripts, etc. the default class
+        # for the label is MathTex
+        subscript_label_var = 10
+        on_screen_subscript_var = Variable(subscript_label_var, "{a}_{i}").next_to(on_screen_int_var, DOWN)
+        self.play(Write(on_screen_subscript_var))
+        self.wait()
+

manim/mobject/numbers.py

@@ -1,9 +1,11 @@
-__all__ = ["DecimalNumber", "Integer"]
+__all__ = ["DecimalNumber", "Integer", "Variable"]
 
 
 from ..constants import *
-from ..mobject.svg.tex_mobject import SingleStringMathTex
-from ..mobject.types.vectorized_mobject import VMobject
+from ..mobject.svg.tex_mobject import MathTex, SingleStringMathTex, Tex
+from ..mobject.svg.text_mobject import Text
+from ..mobject.types.vectorized_mobject import VDict, VMobject
+from ..mobject.value_tracker import ValueTracker
 
 
 class DecimalNumber(VMobject):
@@ -143,3 +145,71 @@ class Integer(DecimalNumber):
 
     def get_value(self):
         return int(np.round(super().get_value()))
+
+
+class Variable(VMobject):
+    """A class for displaying text that continuously updates to reflect the value of a python variable.
+
+    Automatically adds the text for the label and the value when instantiated and added to the screen.
+
+    Parameters
+    ----------
+    var : Union[:class:`int`, :class:`float`]
+        The python variable you need to keep track of and display.
+    label : Union[:class:`str`, :class:`~.Tex`, :class:`~.MathTex`, :class:`Text`]
+        The label for your variable, for example `x = ...`. To use math mode, for e.g.
+        subscripts, superscripts, etc. simply pass in a raw string.
+    var_type : Union[:class:`DecimalNumber`, :class:`Integer`], optional
+        The class used for displaying the number. Defaults to :class:`DecimalNumber`.
+    num_decimal_places : :class:`int`, optional
+        The number of decimal places to display in your variable. Defaults to 2.
+        If `var_type` is an :class:`Integer`, this parameter is ignored.
+    kwargs : Any
+            Other arguments to be passed to `~.Mobject`.
+
+    Attributes
+    ----------
+    label : Union[:class:`str`, :class:`~.Tex`, :class:`~.MathTex`, :class:`Text`]
+        The label for your variable, for example `x = ...`.
+    tracker : :class:`~.ValueTracker`
+        Useful in updating the value of your variable on-screen.
+    value : Union[:class:`DecimalNumber`, :class:`Integer`]
+        The tex for the value of your variable.
+
+    Examples
+    --------
+    Normal usage::
+        # DecimalNumber type
+        var = 0.5
+        on_screen_var = Variable(var, "var", num_decimal_places=3)
+        # Integer type
+        int_var = 0
+        on_screen_int_var = Variable(int_var, "int_var", var_type=Integer)
+        # Using math mode for the label
+        on_screen_int_var = Variable(int_var, "${a}_{i}$", var_type=Integer)
+
+    """
+
+    def __init__(
+        self, var, label, var_type=DecimalNumber, num_decimal_places=2, **kwargs
+    ):
+
+        self.label = MathTex(label) if isinstance(label,str) else label
+        equals = MathTex("=").next_to(self.label, RIGHT)
+        self.label.add(equals)
+
+        self.tracker = ValueTracker(var)
+
+        if var_type == DecimalNumber:
+            self.value = DecimalNumber(
+                self.tracker.get_value(), num_decimal_places=num_decimal_places
+            )
+        elif var_type == Integer:
+            self.value = Integer(self.tracker.get_value())
+
+        self.value.add_updater(lambda v: v.set_value(self.tracker.get_value())).next_to(
+            self.label, RIGHT
+        )
+
+        VMobject.__init__(self, **kwargs)
+        self.add(self.label, self.value)