QGIS python script : zoom only to pre-defined scales

gistfile1.py

# -*- coding: utf-8 -*-

# Import the PyQt and QGIS libraries
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtWebKit import *
from qgis.core import *

class forcedScale():
    
    # pre-defined scales
    predefinedScales = [
        5000,
        10000,
        25000,
        50000,
        100000,
        250000,
        500000
    ]
    
    # mapCanvas
    mc = None
    
    # avoid loop
    replayed = False
    
    def __init__( self, *predefinedScales ):
        '''
        Initialize class instance
        '''
        mc = iface.mapCanvas()
        self.mc = mc        
        self.mc.scaleChanged.connect(self.setScale)
        
        if predefinedScales:
            self.predefinedScales = predefinedScales

    def setScale( self, scale ):
        if self.replayed:
            return
        
        print "initial scale: %s" % scale
        
        targetScale = min(
            self.predefinedScales, 
            key=lambda x:abs(x-scale)
        )
        if targetScale == scale:
            return
        
        self.replayed = True
        print "zoom to %s" % targetScale
        self.mc.zoomScale( targetScale )
        self.replayed = False

# You can instanciate this class this way
# fs = forcedScale()
# which will use the scales defined in the property predefinedScales of the class,
# or you can provide a list of scales like this
# fs = forcedScale( 25000, 50000, 100000 )

Here is a different way:

def forcedScale(*scales):
    def setScale(scale ):
        iface.mapCanvas().scaleChanged.disconnect(setScale)

        print "initial scale: %s" % scale

        targetScale = min(
            scales, 
            key=lambda x:abs(x-scale)
        )
        if targetScale == scale:
            return

        print "zoom to %s" % targetScale
        iface.mapCanvas().zoomScale( targetScale )
        iface.mapCanvas().scaleChanged.connect(setScale)

    # pre-defined scales
    predefinedScales = [
        5000,
        10000,
        25000,
        50000,
        100000,
        250000,
        500000
    ]

    # avoid loop

    iface.mapCanvas().scaleChanged.connect(setScale)

    if not scales:
        scales = predefinedScales