QGIS 3 + Processing + Python API example

qgis_processing_api_example.py

import os
import pathlib
import sys
import typing

# The exact values of these paths depend on your QGIS installation
QGIS_ROOT = pathlib.Path("/opt/qgis-git")
QGIS_PATH = QGIS_ROOT / "share/qgis/python"
QGIS_PLUGIN_PATH = QGIS_PATH / "plugins"

# Add qgis to the PATH
sys.path.insert(1, QGIS_PATH.as_posix())
sys.path.insert(2, QGIS_PLUGIN_PATH.as_posix())

# We can now import QGIS modules
import qgis.core
import qgis.gui
import processing


# Initialize QGIS application
qgis.core.QgsApplication.setPrefixPath(QGIS_ROOT.as_posix(), True)
app = qgis.core.QgsApplication(
    [], False
)  # setting the second argument to False disables the GUI
app.initQgis()

# Initialize processing
from processing.core.Processing import Processing

Processing.initialize()


def get_raster_layer(
    path: typing.Union[str, pathlib.Path], name: str
) -> qgis.core.QgsRasterLayer:
    path = pathlib.Path(path).expanduser().resolve()
    layer = qgis.core.QgsRasterLayer(path.as_posix(), name)
    if not layer.isValid():
        raise ValueError(f"Could not load layer: {path} - {name}")
    return layer


# get a raster layer
b1 = get_raster_layer("~/LC08/LC80440342016259LGN00_B1.TIF", "b1")

# Run r.univar the raster layer
out_file = pathlib.Path("/tmp/out.txt")
r_univar_parameters = {"map": b1, "output": out_file.as_posix()}
result = processing.run("grass7:r.univar", parameters=r_univar_parameters)

# show output
print(result)
print(out_file.read_text())

app.exitQgis()