Convert from EPSG 3857 to EPSG 4326 QGIS2.18

• You could use QgsCoordinateReferenceSystem class (as described in this post):

  from qgis.core import QgsCoordinateReferenceSystem, QgsCoordinateTransform, QgsPoint
  
  old_crs = QgsCoordinateReferenceSystem(3857)
  new_crs = QgsCoordinateReferenceSystem(4326)
  tr = QgsCoordinateTransform(old_crs, new_crs)
  transform_points = tr.transform(QgsPoint(5062443.00656, -952576.546977))
  
  print transform_points
  >>> (45.4767,-8.52551)
  

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• Or use the osgeo.osr.SpatialReference class:

  from osgeo import osr
  
  old_crs = osr.SpatialReference() 
  old_crs.ImportFromEPSG(3857) 
  new_crs = osr.SpatialReference() 
  new_crs.ImportFromEPSG(4326) 
  transform = osr.CoordinateTransformation(old_crs,new_crs) 
  transform_points = transform.TransformPoint(5062443.00656, -952576.546977)
  
  print transform_points[0], transform_points[1] 
  >>> 45.4766992778 -8.5255050757
  

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Notice how in both cases, the x and y coordinates returned are 45.4766992778 and -8.5255050757 respectively which differ slightly from those in your question.

To convert with Qgis API you need to 2 class QgsCoordinateReferenceSystem and QgsCoordianteTransform here them in action :

#example EPSG 3857
src_crs = QgsCoordinateReferenceSystem(3857)
dest_crs = QgsCoordinateReferenceSystem(4326)
xform = QgsCoordinateTransform(src_crs, dest_crs)
x = 5062443.00656
y = -952576.546977
point = QgsPoint(x, y)
#convert here to EPSG 4326
pt_reproj = xform.transform(point)
#where x2 =41.3387949319,
#where y2 = -8.55714071443
#but mine return QgsPoint(45.4766992777683,-8.52550507570475)
return pt_reproj.x(),pt_reproj.y() #return in EPSG 4326