vstup = "vodne_toky_sjtsk"
vystup = "D:/PHD/výučba/Python/cvičenia/vrstvy_python/toky_buffer.shp"
velkost_buffra = 500

vrstvy = QgsProject.instance().mapLayersByName(vstup)
vrstva = vrstvy[0]
polia = vrstva.fields()
feats = vrstva.getFeatures()

zapis = QgsVectorFileWriter(vystup, "UTF-8", polia, QgsWkbTypes.Polygon,
vrstva.sourceCrs(), "ESRI shapefile")

for feat in feats:
    geom = feat.geometry()
    buff = geom.buffer(velkost_buffra, 20)
    feat.setGeometry(buff)
    zapis.addFeature(feat)

iface.addVectorLayer(vystup, "", "ogr")