class_docstr: A parabolic GeometryEntity.
  
  A parabola is declared with a point, that is called 'focus', and
  a line, that is called 'directrix'.
  Only vertical or horizontal parabolas are currently supported.
  
  Parameters
  ==========
  
  focus : Point
      Default value is Point(0, 0)
  directrix : Line
  
  Attributes
  ==========
  
  focus
  directrix
  axis of symmetry
  focal length
  p parameter
  vertex
  eccentricity
  
  Raises
  ======
  ValueError
      When `focus` is not a two dimensional point.
      When `focus` is a point of directrix.
  NotImplementedError
      When `directrix` is neither horizontal nor vertical.
  
  Examples
  ========
  
  >>> from sympy import Parabola, Point, Line
  >>> p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7,8)))
  >>> p1.focus
  Point2D(0, 0)
  >>> p1.directrix
  Line2D(Point2D(5, 8), Point2D(7, 8))
docstr: The focal length of the parabola.
  
  Returns
  =======
  
  focal_lenght : number or symbolic expression
  
  Notes
  =====
  
  The distance between the vertex and the focus
  (or the vertex and directrix), measured along the axis
  of symmetry, is the "focal length".
  
  See Also
  ========
  
  https://en.wikipedia.org/wiki/Parabola
  
  Examples
  ========
  
  >>> from sympy import Parabola, Point, Line
  >>> p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7, 8)))
  >>> p1.focal_length
  4
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code: def focal_length(self):
      """The focal length of the parabola.
  
      Returns
      =======
  
      focal_lenght : number or symbolic expression
  
      Notes
      =====
  
      The distance between the vertex and the focus
      (or the vertex and directrix), measured along the axis
      of symmetry, is the "focal length".
  
      See Also
      ========
  
      https://en.wikipedia.org/wiki/Parabola
  
      Examples
      ========
  
      >>> from sympy import Parabola, Point, Line
      >>> p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7, 8)))
      >>> p1.focal_length
      4
  
      """
      distance = self.directrix.distance(self.focus)
      focal_length = distance / 2
  
      return focal_length
  
imports: from sympy.core import S
  from sympy.core.sorting import ordered
  from sympy.core.symbol import _symbol, symbols
  from sympy.geometry.entity import GeometryEntity, GeometrySet
  from sympy.geometry.point import Point, Point2D
  from sympy.geometry.line import Line, Line2D, Ray2D, Segment2D, LinearEntity3D
  from sympy.geometry.ellipse import Ellipse
  from sympy.functions import sign
  from sympy.simplify import simplify
  from sympy.solvers.solvers import solve
label_desc: Use image node_id 7 for calling the Parabola obj's underlying member method code with example usage: obj.focal_length() and returns: focal_length