Changes between Version 11 and Version 12 of PythonGotchas

Timestamp:

Aug 8, 2013, 4:36:11 PM (11 years ago)

Author:

lpinner

Comment:

Added gotcha re. exceptions and custom python error handlers

PythonGotchas

@@ -157 +157 @@
 }}}
 
+=== Exceptions raised in custom error handlers do not get caught ===
+If using GDAL 1.10+ the python bindings allow you to specify a python callable as an error handler (#4993). However, these error handlers appear to be called in a separate thread and any exceptions raised do not propagate back to the main thread (#5186). 
+
+So if you want to [http://gis.stackexchange.com/questions/43404/how-to-detect-a-gdal-ogr-warning/68042 catch warnings as well as errors], something like this won't work:
+{{{
+ef error_handler(err_level, err_no, err_msg):
+
+    if err_class >= gdal.CE_Warning:
+        raise RuntimeError(err_level, err_no, err_msg) #this exception does not propagate back to main thread!
+
+if __name__=='__main__':
+
+    #Test custom error handler
+    gdal.PushErrorHandler(error_handler)
+    gdal.Error(gdal.CE_Warning,2,'test warning message')
+    gdal.PopErrorHandler()
+}}}
+
+But you can do something like this instead:
+{{{
+
+class GdalErrorHandler(object):
+    def __init__(self):
+        self.err_level=gdal.CE_None
+        self.err_no=0
+        self.err_msg=''
+
+    def handler(self, err_level, err_no, err_msg):
+        self.err_level=err_level
+        self.err_no=err_no
+        self.err_msg=err_msg
+
+if __name__=='__main__':
+
+    err=GdalErrorHandler()
+    handler=err.handler # Note don't pass class method directly or python segfaults
+                        # due to a reference counting bug 
+                        # http://trac.osgeo.org/gdal/ticket/5186#comment:4
+
+    gdal.PushErrorHandler(handler)
+    gdal.UseExceptions() #Exceptions will get raised on anything >= gdal.CE_Failure
+
+    try:
+        gdal.Error(gdal.CE_Warning,1,'Test warning message')
+    except Exception as e:
+        print 'Operation raised an exception'
+        raise
+    else:
+        if err.err_level >= gdal.CE_Warning:
+            print 'Operation raised an warning'
+            raise RuntimeError(err.err_level, err.err_no, err.err_msg)
+    finally:
+        gdal.PopErrorHandler()
+}}}
+
 = Gotchas fixed in GDAL 1.8.0 =