Calling QGIS3 from R

Calling QGIS3 from R

If you can’t tell from my last post, I’m trying to delineate watersheds for about 650 lakes. It’s not possible (er, I am unwilling) to do this by hand, so I need a way to do this programmatically. R is my weapon of choice, but unfortunately the watershed delineation tools are based in GRASS and SAGA. It’s possible to run GRASS and SAGA directly (via rgrass7, and RSAGA), but QGIS makes installing these a bit easier, plus the user interface makes trying out the algorithms interactively relatively easy. RQGIS only supports QGIS2, and RQGIS3 has a stark warning for those on UNIX-based OSes (me!) that RQGIS3 will crash your terminal.

Running QGIS3 processing algorithms in Python isn’t bad. On Linux this is especially straightforward, since the QGIS libraries are installed to the system python3. On MacOS, you have to set a few environment variables. I’ll use the "native:filedownloader" algorithm as an example, since it only has two arguments. To find the algorithm identifier, you’ll need to hover over the name in the Processing toolbox (in the GUI):

The QGIS Processing Toolbox

...does anybody else feel that there could be a better place to put this?

To get a handle on how the input parameters look to Python, you’ll have to run the algorithm once in the GUI. The window might get hidden when the command ends, but it should still be there behind the main window! At the top there should be a repl of a Python dictionary with the input parameters that were used:

Algorithm input parameters

There’s pretty much zero rhyme or reason to the parameter names, so you’ll probably have to do this for every processing function you need.

Now we need some Python code to (1) load QGIS, (2) load the processing plugin, and (3) call the algorithm with the right parameters. Running the algorithm is very readable:

Processing.runAlgorithm(
  "native:filedownloader", 
  {'OUTPUT': 'response.json', 'URL': 'http://httpbin.org/get}
)

…but it takes at least 12 lines to set up and tear down the Processing object:

# --- call_download_ex.py ---

import os
import sys

# add additional directories that QGIS needs to find Python plugins
# this is different for all the OSes
if sys.platform == "darwin":
  qgis_app = os.path.abspath(os.path.join(sys.executable, "..", "..", "..", ".."))
  # for QT
  os.environ['QT_QPA_PLATFORM_PLUGIN_PATH'] = f'{qgis_app}/Contents/PlugIns'
  # for Processing
  sys.path.append(f'{qgis_app}/Contents/Resources/python/plugins')
  
elif sys.platform in ('linux', 'linux2'):
  # for Processing
  # may be different on different flavours of linux
  sys.path.append('/usr/share/qgis/python/plugins')

# load QGIS modules
from qgis.core import QgsApplication, QgsProcessingFeedback
from qgis.analysis import QgsNativeAlgorithms 

# make an "application" (False for no UI)
qgs = QgsApplication([], False)
QgsApplication.initQgis()

# add the native routines to the processing registry
QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())

# load the processing library
from processing.core.Processing import Processing
Processing.initialize()

# run the algorithm
result = Processing.runAlgorithm(
  "native:filedownloader", 
  {'OUTPUT': 'response.json', 'URL': 'http://httpbin.org/get'}
)

# exit QGIS
QgsApplication.exitQgis()

# print result to stdout
print(result)

This is mostly inspired by this GIS stackexchange post, where you might be able to find more detail about your particular platform. In particular, I have no way to test Windows, but I imagine the pattern is similar.

The next hurdle is calling the right python3. In Linux, you should be able to use the system python3. On Mac, you have to use the python3 that was shipped with QGIS, which on my computer is "/Applications/QGIS3.10.app/Contents/MacOS/bin/python3". This is a bit specific, so I’ve been using the following functions to detect the QGIS Python installation:

find_qgis <- function() {
  if (Sys.info()["sysname"] == "Darwin") {
    app_dirs  <- list.files("/Applications", "^QGIS3\\.", full.names = TRUE)
    if (length(app_dirs) == 0) {
      rlang::abort("Can't find QGIS3* in '/Applications'")
    }
    
    message(glue::glue("Using `.qgis = '{app_dirs[1]}'`"))
    app_dirs[1]
  }
}

find_qgis_python <- function(.qgis = find_qgis()) {
  if (Sys.info()["sysname"] == "Darwin") {
    file.path(.qgis, "Contents/MacOS/bin/python3")
  } else {
    "python3"
  }
}

find_qgis_python()
## Using `.qgis = '/Applications/QGIS3.10.app'`
## [1] "/Applications/QGIS3.10.app/Contents/MacOS/bin/python3"

I use the processx package to call external programs because it works more predictably on multiple platforms, passes interrupts on to the subprocess, doesn’t require shell-quoting arguments, and allows a great deal more flexibility. For example, calling python3 call_download_ex.py would look like this:

processx::run(
  find_qgis_python(),
  args = "call_download_ex.py",
  echo = TRUE, echo_cmd = TRUE
)
## Using `.qgis = '/Applications/QGIS3.10.app'`
## Running /Applications/QGIS3.10.app/Contents/MacOS/bin/python3 \
##   call_download_ex.py
## {'OUTPUT': 'response.json'}
## $status
## [1] 0
## 
## $stdout
## [1] "{'OUTPUT': 'response.json'}\n"
## 
## $stderr
## [1] ""
## 
## $timeout
## [1] FALSE

That works great for one use case, but really, we need a way to specify the algorithms and specify the parameters from R. I’ll use command line arguments for the input and JSON for the output, since processx::run() helpfully hands us the stdout string. In Python, you can get the command-line arguments from sys.argv, which can contain JSON no problem (particularly since processx::run() doesn’t require shell quoting). My generalized Python file to call one QGIS algorithm looks like this:

# -----  call_qgis.py -----

import os
import sys
import json

qgis_algorithm  = sys.argv[1]
qgis_params = json.loads(sys.argv[2])

if sys.platform == "darwin":
  qgis_app = os.path.abspath(os.path.join(sys.executable, "..", "..", "..", ".."))
  # for QT
  os.environ['QT_QPA_PLATFORM_PLUGIN_PATH'] = f'{qgis_app}/Contents/PlugIns'
  # for Processing
  sys.path.append(f'{qgis_app}/Contents/Resources/python/plugins')

elif sys.platform in ('linux', 'linux2'):
  # for Processing
  # may be different on different flavours of linux
  sys.path.append('/usr/share/qgis/python/plugins')

from qgis.core import QgsApplication
from qgis.analysis import QgsNativeAlgorithms 

qgs = QgsApplication([], False)
QgsApplication.initQgis()
QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())

from processing.core.Processing import Processing
Processing.initialize()

result = Processing.runAlgorithm(qgis_algorithm, qgis_params)

QgsApplication.exitQgis()

print(json.dumps(result))

Now, we can call any processing algorithm in QGIS:

processx::run(
  find_qgis_python(),
  args = c(
    "call_qgis.py",
    "native:filedownloader", 
    '{"OUTPUT": "response.json", "URL": "http://httpbin.org/get"}'
  ),
  echo = TRUE, echo_cmd = TRUE
)
## Using `.qgis = '/Applications/QGIS3.10.app'`
## Running /Applications/QGIS3.10.app/Contents/MacOS/bin/python3 \
##   call_qgis.py 'native:filedownloader' \
##   '{"OUTPUT": "response.json", "URL": "http://httpbin.org/get"}'
## {"OUTPUT": "response.json"}
## $status
## [1] 0
## 
## $stdout
## [1] "{\"OUTPUT\": \"response.json\"}\n"
## 
## $stderr
## [1] ""
## 
## $timeout
## [1] FALSE

Cool! It’s still not very R-like though, but because the input and output are JSON, we can wrap this in an R function. Mine looks like this (note the use of rlang::list2(), which is a quick and dirty way to support tidy eval for ...).

call_qgis <- function(.algorithm, ..., 
                      .python = find_qgis_python()) {
  params <- rlang::list2(...)

  result <- processx::run(
    .python, 
    args = c(
      "call_qgis.py", 
      .algorithm,
      jsonlite::toJSON(params, null = "null", auto_unbox = TRUE)
    ), 
    echo = TRUE, echo_cmd = TRUE
  )
  
  jsonlite::fromJSON(result$stdout)
}

result <- call_qgis(
  "native:filedownloader", 
  URL = "https://httpbin.org/get", 
  OUTPUT = "response.json"
)
## Using `.qgis = '/Applications/QGIS3.10.app'`
## Running /Applications/QGIS3.10.app/Contents/MacOS/bin/python3 \
##   call_qgis.py 'native:filedownloader' \
##   '{"URL":"https://httpbin.org/get","OUTPUT":"response.json"}'
## {"OUTPUT": "response.json"}
result$OUTPUT
## [1] "response.json"

There’s a few things you should be aware of with this approach. First, you might have to restart R to get it to work the first time (I don’t know why). Second, some processing algorithms pop up a user interface. You obviously don’t want that in a script, and I don’t know how to avoid it (a good example is putting a bad URL in as the URL parameter above). You can somewhat get around this by passing timeout to processx::run(), which will kill a process that hasn’t responded in a certain amount of time. Next, getting a feedback log is possible, but is more complex. Finally, while I think that QGIS processing respects the working directory, there’s a very real chance you will have to use fs::path_abs() to force a complete filename.

Now let’s try something spatial, since that’s what we’re in this for. I’m interested in the watershed tools from GRASS7, but I’ll use "gdal:cliprasterbymasklayer" as an example. The parameters I scraped from the QGIS GUI were these:

{ 
  'ALPHA_BAND' : False, 
  'CROP_TO_CUTLINE' : True,
  'DATA_TYPE' : 0,
  'EXTRA' : '',
  'INPUT' : 'dem_filled.tif', 
  'KEEP_RESOLUTION' : False, 
  'MASK' : 'lakes.shp',
  'MULTITHREADING' : False, 
  'NODATA' : None, 
  'OPTIONS' : '', 
  'OUTPUT' : 'TEMPORARY_OUTPUT', 
  'SET_RESOLUTION' : False, 
  'SOURCE_CRS' : None, 
  'TARGET_CRS' : None, 
  'X_RESOLUTION' : None, 
  'Y_RESOLUTION' : None
}

The only three we need are INPUT, MASK, and OUTPUT, all of which are vector or raster layers. Be aware that occasionally, processing algorithms return a Python object here instead of a filename. This isn’t the case GDAL, SAGA, and GRASS, and more than likely you can do the other stuff in R much more efficiently. You can use TEMPORARY_OUTPUT, as a filename output and the actual name of the tempfile will get returned in the output.

Using our call_qgis() wrapper, this call would look like this:

result <- call_qgis(
  "gdal:cliprasterbymasklayer",
  
  "INPUT" = "dem_filled.tif", 
  "MASK" = "lakes.shp",
  "OUTPUT" = "TEMPORARY_OUTPUT", 
  
  "ALPHA_BAND" = FALSE, 
  "CROP_TO_CUTLINE" = TRUE,
  "DATA_TYPE" = 0,
  "EXTRA" = "",
  "KEEP_RESOLUTION" = FALSE, 
  "MULTITHREADING" = FALSE, 
  "NODATA" = NULL, 
  "OPTIONS" = "", 
  "SET_RESOLUTION" = FALSE, 
  "SOURCE_CRS" = NULL, 
  "TARGET_CRS" = NULL, 
  "X_RESOLUTION" = NULL, 
  "Y_RESOLUTION" = NULL
)
## Using `.qgis = '/Applications/QGIS3.10.app'`
## Running /Applications/QGIS3.10.app/Contents/MacOS/bin/python3 \
##   call_qgis.py 'gdal:cliprasterbymasklayer' \
##   '{"INPUT":"dem_filled.tif","MASK":"lakes.shp","OUTPUT":"TEMPORARY_OUTPUT","ALPHA_BAND":false,"CROP_TO_CUTLINE":true,"DATA_TYPE":0,"EXTRA":"","KEEP_RESOLUTION":false,"MULTITHREADING":false,"NODATA":null,"OPTIONS":"","SET_RESOLUTION":false,"SOURCE_CRS":null,"TARGET_CRS":null,"X_RESOLUTION":null,"Y_RESOLUTION":null}'
## {"OUTPUT": "/private/var/folders/bq/2rcjstv90nx1_wrt8d3gqw6m0000gn/T/processing_56ce6c257e724c36afedc0d73b28b9d6/0c9640b16c6842cc81a9bad730755843/OUTPUT.tif"}
plot(stars::read_stars(result$OUTPUT))

This is sufficient to get you access to some of the harder-to-come-by algorithms in SAGA and GRASS, although there’s certainly more functionality if you can jump to Python (particularly if you’re on Linux, where this is slightly easier).

Avatar
Dewey Dunnington
Geoscientist, Programmer, Educator