Changes between Version 16 and Version 17 of PythonGotchas

Timestamp:

Oct 1, 2014, 7:28:17 PM (10 years ago)

Author:

Mike Taves

Comment:

gotcha for saving and closing GDAL datasets / OGR datasources

PythonGotchas

@@ -171 +171 @@
 shouldn't call Destroy() at all.
 }}}
+
+
+=== Saving and closing datasets/datasources ===
+
+To save and close GDAL raster datasets or OGR vector datasources, the object needs to be dereferenced, such as setting it to {{{None}}}, a different value, or deleting the object. If there are more than one copies of the dataset or datasource object, then each copy needs to be dereferenced.
+
+For example, creating and saving a raster dataset:
+{{{
+#!python
+>>> from osgeo import gdal
+>>> driver = gdal.GetDriverByName('GTiff')
+>>> dst_ds = driver.Create('new.tif', 10, 15)
+>>> band = dst_ds.GetRasterBand(1)
+>>> arr = band.ReadAsArray()  # raster values are all zero
+>>> arr[2, 4:] = 50  # modify some data
+>>> band.WriteArray(arr)  # raster file still unmodified
+>>> band = None  # dereference band to avoid gotcha described previously
+>>> dst_ds = None  # save, close
+}}}
+The last dereference to the raster dataset writes the data modifications and closes the raster file. {{{WriteArray(arr)}}} does not write the array to disk, unless the GDAL block cache is full (typically 40 MB).
+
+With some drivers, raster datasets can be intermittently saved without closing using {{{FlushCache()}}}. Similarly, vector datasets can be saved using {{{SyncToDisk()}}}. However, neither of these methods guarantee that the data are written to disk, so the preferred method is to deallocate as shown above.
+
 
 === Exceptions raised in custom error handlers do not get caught ===