Set predefined scales in PyQGIS for standalone application

Question

I am trying to figure out how to set the zoom scales to match those meant for OpenStreetMaps and Google Maps, etc., as I am using OSM for a basemap. I've looked all over the internet and can't seem to find any support for this particular goal.

I have an XML document (scales.xml) with all of the scales needed, but I just can't figure out how to implement it:

<qgsScales version="1.0">
    <scale value="1:591657528"/>
    <scale value="1:295828764"/>
    <scale value="1:147914382"/>
    <scale value="1:73957191"/>
    <scale value="1:36978595"/>
    <scale value="1:18489298"/>
    <scale value="1:9244649"/>
    <scale value="1:4622324"/>
    <scale value="1:2311162"/>
    <scale value="1:1155581"/>
    <scale value="1:577791"/>
    <scale value="1:288895"/>
    <scale value="1:144448"/>
    <scale value="1:72224"/>
    <scale value="1:36112"/>
    <scale value="1:18056"/>
    <scale value="1:9028"/>
    <scale value="1:4514"/>
    <scale value="1:2257"/>
    <scale value="1:1128"/>
    <scale value="1:564"/>
    <scale value="1:282"/>
    <scale value="1:141"/>
    <scale value="1:71"/>
</qgsScales>

I've tried reworking some of the code from thr QuickMapServices plugin, but I get no results from it:

def _load_scales_list(self):
    scales_filename = os.path.join(self.plugin_dir, 'scales.xml')
    scales_list = []

    xml_root = ET.parse(scales_filename).getroot()
    for scale_el in xml_root.findall('scale'):
        scales_list.append(scale_el.get('value'))
    return scales_list

def scales_list(self):
    if not self._scales_list:
        self._scales_list = self._load_scales_list()
    return self._scales_list

def set_nearest_scale(self):
    #get current scale
    curr_scale = self.mapCanvas().scale()
    #find nearest
    nearest_scale = sys.maxint
    for scale_str in self.scales_list:
        scale = scale_str.split(':')[1]
        scale_int = int(scale)
        if abs(scale_int-curr_scale) < abs(nearest_scale - curr_scale):
            nearest_scale = scale_int

    #set new scale
    if nearest_scale != sys.maxint:
        self.mapCanvas().zoomScale(nearest_scale)

This code neither throws an error nor does anything at all. Any ideas as to how I can get this working in a standalone window?

Answer 1

You should place the definition of the list of predefined scales in your init-method.

self.predefinedScale = [...]

Then connect the scaleChanged event to the function actually setting the proper scale.

self.mapCanvas.scaleChanged.connect(self.zoomToScale)

Define this function as class method, not as local function of some other method, so that it will be executed in the global context.

def zoomToScale(self)
    self.mapCanvas.scaleChanged.disconnect(self.zoomToScale)
    scale = self.mapCanvas.scale()
    targetScale = min(self.predefinedScales, key=lambda x: abs(x - scale))
    self.mapCanvas.zoomScale(targetScale)
    self.mapCanvas.scaleChanged.connect(self.zoomToScale)

Answer 2

you can define this scale in predefined scale for qgis

USING Python :

def forcedScale(scales):
    def zoomToScale(scale):
        self.iface.mapCanvas().scaleChanged.disconnect(zoomToScale)
        targetScale = min(scales, key=lambda x:abs(x-scale))
        self.iface.mapCanvas().zoomScale( targetScale )
        self.iface.mapCanvas().scaleChanged.connect(zoomToScale)

    predefinedScales = [
        591657528,
        295828764,
        147914382,
        73957191,
        36978595,
        18489298,
        9244649,
        4622324,
        2311162,
        1155581,
        577791,
        288895,
        144448,
        72224,
        36112,
        18056,
        9028,
        4514,
        2257,
        1128,
        564,
        282,
        141,
        71
    ]

    self.iface.mapCanvas().scaleChanged.connect(zoomToScale)
    scales = predefinedScales