Manage layers

Remove all the layers from the QGIS canvas

project = QgsProject.instance()
project.removeAllMapLayers()
project.clear()
iface.mapCanvas().refresh()

Clone a layer

oldLayer = project.mapLayersByName("Old layer")[0]
newLayer = oldLayer.clone()
newLayer.setName('New layer')
project.addMapLayer(newLayer)

Rename a layer

mes_sommets.setName('New name')

Copy-paste a layer with all his files (tab, shp ...) and open them

If you have a layer named test in a directory named my layer, you can create a copy named new file in the same directory and open them in QGIS:

import os
import shutil
 
QgsProject.instance().removeAllMapLayers()
iface.mapCanvas().refresh()
 
directory = r'C:/Users/Georges/Downloads/my layer'
new_file = 'new file'
 
# REMOVE NEW LAYER IF EXISTS
files = os.listdir(directory)
for f in files:
    if f.startswith(new_file):
        try:
            os.remove(directory + '/' + f)
        except:
            pass
 
# COPY LAYER
files = os.listdir(directory)
for f in files:
    if not f.startswith(new_file):
        shutil.copyfile(directory + '/' + f, directory + '/' + new_file + '.' + f.split('.')[1])
print('Layer copied!')
 
test = QgsVectorLayer(directory + '/test.tab', 'test', 'ogr')
QgsProject.instance().addMapLayer(test)
 
new = QgsVectorLayer(directory + '/' + new_file + '.tab', 'new file', 'ogr')
QgsProject.instance().addMapLayer(new)

Change a layer's encoding

myLayer = QgsVectorLayer(r'C:/Users/Georges/Downloads/my layer/new_file.tab', new_file, 'ogr')
myLayer.setProviderEncoding('ISO-8859-1')

Create a group and put a layer into

directory = r'C:/Users/Georges/Downloads/my layer'
newfile = QgsVectorLayer(directory + '/new file.tab', 'newfile', 'ogr')
QgsProject.instance().addMapLayer(newfile)
 
groupName = 'Mon joli groupe'
root = QgsProject.instance().layerTreeRoot()
group = root.addGroup(groupName)
 
layer = QgsProject.instance().mapLayersByName('newfile')[0]
 
mylayer = root.findLayer(layer.id())
myClone = mylayer.clone()
parent = mylayer.parent()
 
group = root.findGroup('Mon joli groupe')
group.insertChildNode(0, myClone)
 
parent.removeChildNode(mylayer)

Hide the rasters bands

root = QgsProject.instance().layerTreeRoot()
for layer in QgsProject.instance().mapLayers().values():
    if layer.type() == QgsMapLayerType.RasterLayer:
        LayerNode = root.findLayer(layer.id())
        LayerNode.setExpanded(False)

Uncheck a layer

project = QgsProject.instance()
myLayer = project.mapLayersByName('My layer')[0]
project.layerTreeRoot().findLayer(myLayer.id()).setItemVisibilityCheckedParentRecursive(False)

Transform a layer projection

This is not a reprojection, which need a processing.run, here below it is just a transformation, which can be unefficient according your context.

project = QgsProject.instance()
 
myLayer = project.mapLayersByName("My layer")[0]
 
my_projection = QgsCoordinateReferenceSystem(4326)
myLayer.setCrs(my_projection)
my_transform = QgsCoordinateTransform(myLayer.crs(), iface.mapCanvas().mapSettings().destinationCrs(), project)
myLayerExtent = my_transform.transform(myLayer.extent())
 
iface.mapCanvas().setExtent(myLayerExtent)
iface.mapCanvas().refresh()

Manage the layers display order

firstLayer = project.mapLayersByName("First layer")[0]
secondLayer = project.mapLayersByName("Second layer")[0]
thirdLayer = project.mapLayersByName("Third layer")[0]
 
root = project.layerTreeRoot()
root.setHasCustomLayerOrder (True)
 
order = root.customLayerOrder()
order.insert(0, order.pop(order.index(firstLayer)))
order.insert(1, order.pop(order.index(secondLayer)))
order.insert(2, order.pop(order.index(thirdLayer)))
 
root.setCustomLayerOrder(order)

Another way cloning the layer:

myLayer = QgsVectorLayer(myPathLayer, "Buffer area", "ogr")
project.addMapLayer(peaks_buffer)
 
root = project.layerTreeRoot()
myBelowLayer = root.findLayer(myLayer.id())
myClone = myBelowLayer.clone()
parent = myBelowLayer.parent()
parent.insertChildNode(-1, myClone)
parent.removeChildNode(myBelowLayer)

Zoom on a layer

countries = QgsVectorLayer(myPath + 'simple_countries/simple_countries.shp', 'Countries', 'ogr')
extent_countries = countries.extent()
iface.mapCanvas().setExtent(extent_countries)
iface.mapCanvas().refresh()