Renaming field in attribute table via PyQGIS?

How can I rename a field in PyQGIS?

I tried:

for field in layer.pendingFields():
    if field.name() == 'old':
        layer.startEditing()
        oldname = field.name()
        field.setName('new')
        newname = field.name()
        print(u'Old name: {}, New name: {}'.format(oldname,newname))
        layer.commitChanges()

This code print 'Old name: old, New name: new', but in attribute table is field name still 'old'.