class_docstr: Create a new variable as a key-value pair. All variables are stored
  in ``Variable.variables``. Duplicate keys or values are not allowed,
  unless the ``replace`` parameter is ``True``.
  
  Parameters
  ----------
  name : str
      Variable key.
  expr : str
      Variable value.
  replace : bool, optional
      Replace the current key-value pair if it already exists.
  
  Attributes
  ----------
  variables : dict
      Class variable for storing all known variables
  
  Examples
  --------
  
  >>> Variable('rrunder', 'rr_3_20 <= 0.9')
  >>> Variable('hc', 'higher_close')
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code: def __init__(self, name, expr, replace=False):
      # code
      self.name = name
      self.expr = expr
      # add key with expression
      Variable.variables[name] = self
  
imports: from alphapy.alias import get_alias
  from alphapy.frame import Frame
  from alphapy.frame import frame_name
  from alphapy.globals import BSEP, LOFF, ROFF, USEP
  from alphapy.utilities import valid_name
  import builtins
  from collections import OrderedDict
  from importlib import import_module
  import logging
  import numpy
  import pandas
  import parser
  import re
  import sys
label_desc: Use image node_id 2 to create a new Variable object from inherited base classes: object with example: obj = Variable(name, expr, replace)