pyqgis - How do I fix "name 'iface' is not defined"?

i try to export a map but at the end of my process I always get the following message: "name 'iface' is not defined".

What is wrong with my script?

I am working with QGIS 2.18.12

from PyQt4.QtGui import QColor, QImage, QPainter
from qgis.core import QgsExpression, QgsStatisticalSummary, QgsSymbolV2,   QgsDataDefined, QgsRuleBasedRendererV2, QgsMarkerSymbolV2
from PyQt4.QtCore import Qsize


...

map_settings = iface.mapCanvas().mapSettings()
c = QgsComposition(map_settings)
c.setPaperSize(400,160)
c.setPrintResolution(dpi)

...