Trouble using gdal utilities in python (no output files created)

Question

I am trying to use gdal utilities like gdal_polygonize in python to polygonize my raster image but I'm having trouble understanding the commands. What I mean is in some places the command is of the given form say:

gdal.Polygonize( srcband, None, dst_layer, -1, [], callback=None )

I found it here

And the command is also called in python with os.system().

I read and tried to implement commands from Gdal_polygonize: How to filter pixels above a given value (elevation)?

gdal_polygonize.py result.tif result.shp

and python gdal_polygonize error

os.system('gdal_polygonize.py ' + filename + ' -f "ESRI Shapefile" ' + shapefile)

but no shapefile is created although there was no error while running the code.

I am stuck and having great trouble learning to use gdal with python as I don't understand the need or difference between the two types of commands. Please address my question.

What is the proper syntax for using (lets say) gdal_calc.py when called with os.system() and also when used as gdal_calc() ?

P.s I use python 2.7 on win 8.1 platform.

EDIT: With gdal.polygonize() I get the output shapefile, but when I try to view it in qgis, I get nothing but white screen. And in the case where I try to call utilities with os.system(), I don't get an output file only. Please tell me what is going wrong.

Answer 1

In your first example you have a simple script, where all modules are imported, parameters are created and your function is executed with these parameters. Your vector data are written to dst_layer.

There are few ways to execute your script. You can do this in command line (if you have enviromental variables for python with gdal):

$ python script_name.py

You also can execute your scripts and commands in Python shell. os.system() Executes the command (a string) in a subshell. More about that. So in your example, there is a string inside os.system(). You can execute all commands you want, they must be windows cmd commands.

In Python shell you also can execute files with execfile('path/to/file'), for example you can run your first script and new shapefile will be created.

I hope it will help.