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I'm looking for some Python + QGIS for the following:

import processing
for i in range (1,20):
    processing.runalg("qgis:zonalstatistics", "/folder/input/raster.tif",%i,
    "/folder/input/poly.shp","Var_%i",True, "/folder/output/zonalpoly_%i.shp"

The above BASH like example doesn't work, but if I substitute an integer in place of %i, the script runs correctly, so I know that the syntax is correct.

I'm a bit fuzzy on how to properly pass a variable into a string using Python and/or how to run a QGIS plugin from the built-in Python interpreter for QGIS.

Also, qgis:zonalstatistics calculates several summary statistics (mean, stddev, maximum, etc.) with no apparent means of specifying one or a few of these.

Is there a way of specifying just one, or a few?

It seems like a complete waste of processing time to calculate each if I'm only interested in one or a few measures.

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1 Answer 1

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1) string formatting:

It is a not a problem of QGIS or processing but a simple problem of string formatting in Python, look at Python String Format, for example.

print  "Var_{}".format(3)
Var_3
# or old
print "Var_%s" % (1) 
Var_1

Script example 1:

for i in range(1,20):
    processing.runalg("qgis:zonalstatistics", "/folder/input/raster.tif",i,"/folder/input/poly.shp","Var_{0}".format(i),True, "/folder/output/zonalpoly_{0}.shp".format(i))

Script example 2

for i in range(1,20):
     processing.runalg("qgis:zonalstatistics","/folder/input/raster.tif",i,"/folder/input/poly.shp", "Var_%s" %i ,True, "/folder/output/zonalpoly_%s.shp"%i

2) But QGIS actually has also zonal statistics methods built into the API with Python bindings (look at QGIS: How to calculate raster statistics for polygons?)

3) For the other question, no solution, look at Processing-Help: ZONAL STATISTICS:

processing.runalg("qgis:zonalstatistics", input_raster, band_number, input_vector, column_prefix, load_flag, output_layer)
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  • Thanks! I've managed to get the first example running, although it was missing a right bracket at the end. Example two also seems to be missing a bracket. Unfortunately, running example two gets me a TypeError: unsupported operand type(s) for %: 'dict' and 'int'
    – Prophet60091
    Apr 5, 2014 at 0:32
  • Option 2) isn't really an option as I believe the built-in API version of zonal statistics doesn't allow for selecting an individual band within a file. If it does, it doesn't seem obvious to me how it's implemented. Good to know though.
    – Prophet60091
    Apr 5, 2014 at 0:36
  • Also missing a 0 in "Var_{}". Should be "Var_{0}" in the first example. Example 1 definitely works. Let me know if you can figure out the issue for Example 2.
    – Prophet60091
    Apr 5, 2014 at 0:54
  • Var_{0}in Python 2.6, Var_{}in Python 2.7
    – gene
    Apr 6, 2014 at 19:56

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