如何在QGIS / Python中循环使用.format和变量

Question

I've got the following code, but I can't quite get it to work:

import glob
import os
import processing # for QGIS functionality

os.chdir("/home/mydir/vector")

columnid = ['SAMP300','SAMP100','SAMP30', 'SAMP10']

for x in glob.glob("prefix*.shp"):
    for y in columnid:
        for z in [1, 2, 3]:
            output_alg0='"/home/mydir/vector/samples/{x}_{y}_{z}.shp"'.format(x=x.rstrip('.shp'), y=y, z=z)
            output_0='processing.runalg("qgis:randompointsinsidepolygonsvariable","/home/mydir/vector/{x}",0,"{y}",1000,output_alg0)'.format(x=x, y=y)

The script reads through a directory of .shp files, and uses variables to name the outputs. output_alg0 creates the output file name used in the next step. The file name is based on the original file, and two variables within the loop. output_0 is the actual QGIS algorithm that is run, which references each .shp file in the loop, passes the columnid variable and some fixed parameters, and references output_alg0 for naming the output .shp file.

If I append print in front of the two commands within the loop, I get the output that I'm expecting (ie, the {x},{y}, and {z} variables are correctly populated. Furthermore, the script gives no error when executed, but no output is produced.

Here's an example of the output by appending print and parentheses to the two lines within the loop:

output_alg0="/home/mydir/vector/samples/prefix_SAMP10_3.shp"
output_0=processing.runalg("qgis:randompointsinsidepolygonsvariable","/home/mydir/vector/prefix.shp",0,"SAMP10",1000,output_alg0)

I can copy and paste both lines exactly as they appear above into the QGIS Python Console and the commands are executed as expected (ie, random point file is generated based on the input shapefile, and an output shapefile is created as specified).

I think it has something to do with how I'm using .format and/or perhaps how I'm using single and/or double quotations, and/or some sort of Python/QGIS interaction that I don't quite understand.

EDIT : Instead of using the two output_ prefixes, I also tried out this one-liner version, and the script executes (without error), but no output is created.

import glob
import os
import processing # for QGIS functionality

os.chdir("/home/mydir/vector")

columnid = ['SAMP300','SAMP100','SAMP30', 'SAMP10']

for x in glob.glob("prefix*.shp"):
    for y in columnid:
        for z in [1, 2, 3]:
           'processing.runalg("qgis:randompointsinsidepolygonsvariable","/home/mydir/vector/{w}",0,"{y}",1000,"/home/mydir/vectorsamples/{x}_{y}_{z}.shp")'.format(w=x, x=x.rstrip('.shp'), y=y, z=z)

Answer 1

Currently your loop is generating the variables how you want them to and assigning them to output_alg0 and output_0 the way you want, but you don't ever DO anything with them after that.

I've asked for your expected output in a comment. If you add that we can probably figure out how to get you there. My GUESS is that you're trying to execute output_0 (which currently is just a string that looks like a python command). In which case you'd have to use exec(output_0) but be wary of using exec in general. It's a scary command.

EDIT:

If I were you, I'd do something more like:

# inside your loops::
output_alg0='"/home/mydir/vector/samples/{x}_{y}_{z}.shp"'.format(x=x.rstrip('.shp'), y=y, z=z)
output_0 = processing.runalg("qgis:randompointsinsidepolygonsvariable", "/home/mydir/vector/{x}".format(x=x), 0, y, 1000, output_alg0)

That way you're running the script directly rather than writing a bit of code then executing it.

Answer 2

Changed my EDIT -ed line written as:

'processing.runalg("qgis:randompointsinsidepolygonsvariable","/home/mydir/vector/{w}",0,"{y}",1000,"/home/mydir/vectorsamples/{x}_{y}_{z}.shp")'.format(w=x, x=x.rstrip('.shp'), y=y, z=z)

to this, which wraps exec with quotations marks around the line above:

exec('processing.runalg("qgis:randompointsinsidepolygonsvariable","/home/mydir/vector/{w}",0,"{y}",1000,"/home/mydir/vectorsamples/{x}_{y}_{z}.shp")'.format(w=x, x=x.rstrip('.shp'), y=y, z=z))

The above modification to the script is running and iterates through the list of .shp files within the specified directory; however, I don't think this is the most kosher answer possible. I'll hold off accepting my own answer if something better and/or more QGIS-friendly comes along.

PS-thanks to @Adam Smith for pointing out exec