buffer on PyQgis

@author: sonny
"""
#Upload the shapfile to buffer in Qgis:

path_to_name_sampler_layer = r"D:\name_sampler.shp"

# The format is:
# vlayer = QgsVectorLayer(data_source, layer_name, provider_name)

vlayer = QgsVectorLayer(path_to_name_sampler_layer, "name_sampler", "ogr")
if not vlayer.isValid():
    print("Layer failed to load!")
else:
    QgsProject.instance().addMapLayer(vlayer)
    
#start buffering process

layerName = 'name_sampler' #input buffer layer, to add the name only
outFn = r"D:\name_sampler_buffer.shp" #directory and name of the output shapefile
bufferDist = 10 #input lenght parameter, in meters if your reference sistem is on WGS 84 

layer = QgsProject.instance().mapLayersByName('name_sampler')[0] #insert input layer name
layer = layer
fields = layer.fields ()
feats = layer.getFeatures ()

writer = QgsVectorFileWriter (outFn, 'UTF-8', fields, QgsWkbTypes.Polygon, layer.sourceCrs(),'ESRI shapefile') 
#typology of layer to write

for feat in feats:
    geom = feat.geometry ()
    buff = geom.buffer (bufferDist, 5) #insert a default bufferdistance
    feat.setGeometry (buff)
    writer.addFeature(feat)
    
iface.addVectorLayer(outFn, '', 'ogr')
del(writer)
print ("Buffer Done")

#The result is a buffer with a cartesian representation