Python Gotchas in the GDAL and OGR Python Bindings

This page lists aspects of GDAL's and OGR's Python bindings that may catch Python programmers by surprise. If you find something new, feel free to open a pull request adding it to the list. Consider discussing it on the gdal-dev mailing list first, to make sure you fully understand the issue and that others agree that it is unexpected, "non-Pythonic", or something that would catch many Python programmers by surprise. Be sure to reference email threads, GitHub tickets, and other sources of additional information.

This list is not the place to report bugs. If you believe something is a bug, please open a ticket and report the problem to gdal-dev. Then consider listing it here if it is something related to Python specifically. Do not list it here if it relates to GDAL or OGR generally, and not the Python bindings specifically.

Not all items listed here are bugs. Some of these are just how GDAL and OGR work and cannot be fixed easily without breaking existing code. If you don't like how something works and think it should be changed, feel free to discuss it on gdal-dev and see what can be done.

Gotchas that are by design... or per history

These are unexpected behaviors that are not considered by the GDAL and OGR teams to be bugs and are unlikely to be changed due to effort required, or whose fixing might affect backward compatibility, etc.

Python bindings do not raise exceptions unless you explicitly call UseExceptions()

By default, the GDAL and OGR Python bindings do not raise exceptions when errors occur. Instead they return an error value such as None and write an error message to sys.stdout. For example, when you try to open a non-existing dataset with GDAL:

>>> from osgeo import gdal
>>> gdal.Open('C:\\foo.img')
ERROR 4: 'C:\foo.img does not exist in the file system,
and is not recognized as a supported dataset name.

>>>

In Python, it is traditional to report errors by raising exceptions. You can enable this behavior in GDAL and OGR by calling the UseExceptions() function:

>>> from osgeo import gdal
>>> gdal.UseExceptions()    # Enable exceptions
>>> gdal.open('C:\\foo.img')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: 'C:\foo.img' does not exist in the file system,
and is not recognized as a supported dataset name.

>>>

警告

It is planned that exceptions will be enabled by default in GDAL 4.0. Code that does not want exceptions to be raised in a future version of GDAL should explicitly disable them with gdal.DontUseExceptions().

Python crashes or throws an exception if you use an object after deleting a related object

Consider this example:

>>> from osgeo import gdal
>>> dataset = gdal.Open('C:\\RandomData.img')
>>> band = dataset.GetRasterBand(1)
>>> print(band.Checksum())
31212

In this example, band has a relationship with dataset that requires dataset to remain allocated in order for band to work. If we delete dataset and then try to use band, Python will throw a confusing exception:

>>> from osgeo import gdal
>>> dataset = gdal.Open('C:\\RandomData.img')
>>> band = dataset.GetRasterBand(1)
>>> del dataset           # This will cause the Python garbage collector to deallocate dataset
>>> band.Checksum()       # This will throw an exception because the band's dataset is gone
# TypeError: in method 'Band_Checksum', argument 1 of type 'GDALRasterBandShadow *'

注釈

In GDAL 3.7 and earlier, using a band after the dataset has been destroyed will cause a crash instead of an exception.

This problem can manifest itself in subtle ways. For example, it can occur if you try to instantiate a temporary dataset instance within a single line of code:

>>> from osgeo import gdal
>>> print(gdal.Open('C:\\RandomData.img').GetRasterBand(1).Checksum())
# TypeError: in method 'Band_Checksum', argument 1 of type 'GDALRasterBandShadow *'

In this example, the dataset instance was no longer needed after the call to GetRasterBand() so Python deallocated it before calling Checksum().

>>> from osgeo import gdal
>>>
>>> def load_band(data_filename):
>>>     dataset = gdal.Open(data_filename)
>>>     return dataset.GetRasterBand(1)
>>>
>>> band = load_band('c:\\RandomData.img')
>>> print(band.Checksum())
# TypeError: in method 'Band_Checksum', argument 1 of type 'GDALRasterBandShadow *'

This example is the same case as above but it looks different. The dataset object is only available in the load_band function and will be deleted right after leaving the function.

The problem is not restricted to GDAL band and dataset objects and happens in other areas where objects have relationships with each other. The issue occurs because deleting an object in Python causes not only the C++ object behind it to be deallocated, but also other objects for which that C++ object maintains ownership (e.g., a Dataset owning a Band, a Feature owning a Geometry.) If the Python object associated with one of these child objects retains a reference to that object, Python will crash when the object is accessed. In common cases such as the Band/Dataset relationship above, the GDAL bindings invalidate references to objects that no longer exist so that an exception is thrown instead of a crash, but the work is not complete.

Unfortunately there is no complete list of such relationships, so you have to watch for it yourself.

Python crashes if you add a new field to an OGR layer when features deriving from this layer definition are still active

For example:

>>> feature = lyr.GetNextFeature()
>>> field_defn = ogr.FieldDefn("foo", ogr.OFTString)
>>> lyr.CreateField(field_defn)                       # now, existing features deriving from this layer are invalid
>>> feature.DumpReadable()                            # segfault
< Python crashes >

For more information, please see #3552.

Layers with attribute filters (SetAttributeFilter()) will only return filtered features when using GetNextFeature()

If you read the documentation for SetAttributeFilter() carefully you will see the caveat about OGR_L_GetNextFeature(). This means that if you use GetFeature(), instead of GetNextFeature(), then you can still access and work with features from the layer that are not covered by the filter. GetFeatureCount() will respect the filter and show the correct number of features filtered. However, working with GetFeatureCount() in a loop can lead to some subtle confusion. Iterating over the Layer object or using GetNextFeature() should be the default method for accessing features:

>>> lyr = inDataSource.GetLayer()
>>> lyr.SetAttributeFilter("PIN = '0000200001'")      # this is a unique filter for only one record
>>> for i in range( 0, lyr.GetFeatureCount() ):
...    feat = lyr.GetFeature( i )
...    print(feat)                                    # this will print one feat, but it's the first feat in the Layer and not the filtered feat
...

Certain objects contain a Destroy() method, but you should never use it

You may come across examples that call the Destroy() method. This tutorial even gives specific advice on page 12 about when to call Destroy.

Calling Destroy forces the underlying native object to be destroyed. This is typically unnecessary because these objects are automatically destroyed during garbage collection when no references to the Python object remain.

In most situations, it is not necessary to force the object to be destroyed at a specific point in time. However, because the contents of gdal.Dataset and ogr.DataSource objects are only guaranteed to be written to disk when the backing native object is destroyed, it may be necessary to explicitly destroy these objects. In these cases, a context manager (with block) is often a good solution, e.g.:

from osgeo import ogr
with ogr.GetDriverByName("ESRI Shapefile").CreateDataSource("/tmp/test.shp") as ds:
    lyr = ds.CreateLayer("test")
    feat = ogr.Feature(lyr.GetLayerDefn())
    feat.SetGeometry(ogr.CreateGeometryFromWkt('POINT (1 2)')
    lyr.CreateFeature(feat)
# contents of ds are written to disk

If this is not possible, for example if the object needs to be destroyed within a function, then the Close() method may be called.

注釈

Context managers and the Close() method are available beginning in GDAL 3.8. In earlier versions, Destroy() can be used for ogr.DataSource objects, or garbage collection may be forced by destroying reference using del or setting variables to None.

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 use a context manager or call Close().

Exceptions raised in custom error handlers do not get caught

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 catch warnings as well as errors, something like this won't work:

from osgeo import gdal

def error_handler(err_level, err_no, err_msg):
    if err_level >= 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:

from osgeo import gdal

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()
    gdal.PushErrorHandler(err.handler)
    gdal.UseExceptions()  # Exceptions will get raised on anything >= gdal.CE_Failure

    assert err.err_level == gdal.CE_None, 'the error level starts at 0'

    try:
        # Demonstrate handling of a warning message
        try:
            gdal.Error(gdal.CE_Warning, 8675309, 'Test warning message')
        except Exception:
            raise AssertionError('Operation raised an exception, this should not happen')
        else:
            assert err.err_level == gdal.CE_Warning, (
                'The handler error level should now be at warning')
            print('Handled error: level={}, no={}, msg={}'.format(
                err.err_level, err.err_no, err.err_msg))

        # Demonstrate handling of an error message
        try:
            gdal.Error(gdal.CE_Failure, 42, 'Test error message')
        except Exception as e:
            assert err.err_level == gdal.CE_Failure, (
                'The handler error level should now be at failure')
            assert err.err_msg == e.args[0], 'raised exception should contain the message'
            print('Handled warning: level={}, no={}, msg={}'.format(
                err.err_level, err.err_no, err.err_msg))
        else:
            raise AssertionError('Error message was not raised, this should not happen')

    finally:
        gdal.PopErrorHandler()

Gotchas that result from bugs or behaviors of other software

Python crashes in GDAL functions when you upgrade or downgrade numpy

Much of GDAL's Python bindings are implemented in C++. Much of the core of numpy is implemented in C. The C++ part of GDAL's Python bindings interacts with the C part of numpy through numpy's ABI (application binary interface). This requires GDAL's Python bindings to be compiled using numpy header files that define numpy C data structures. Those data structures sometimes change between numpy versions. When this happens, the new version of numpy is not be compatible at the binary level with the old version, and the GDAL Python bindings must be recompiled before they will work with the new version of numpy. And when they are recompiled, they probably won't work with the old version.

If you obtained a precompiled version of GDAL's Python bindings, such as the Windows packages from http://gisinternals.com/sdk.php be sure you look up what version of numpy was used to compile them, and install that version of numpy on your machine.

Python bindings cannot be used successfully from ArcGIS in-process geoprocessing tools (ArcGIS 9.3 and later)

ArcGIS allows the creation of custom, Python-based geoprocessing tools. Until ArcGIS 10, there was no easy way to read raster data into memory. GDAL provides such a mechanism.

Starting with ArcGIS 9.3, geoprocessing tools can either run in the ArcGIS process itself (ArcCatalog.exe or ArcMap.exe) or run in a separate python.exe worker process. Unfortunately ArcGIS contains a bug in how it runs in-process tools. Thus, if you use GDAL from an in-process tool, it will run fine the first time but after that it may fail with TypeError exceptions until you restart the ArcGIS process. For example, band.ReadAsArray() fails with:

TypeError: in method 'BandRasterIONumpy', argument 1 of type 'GDALRasterBandShadow *'

This is a bug in ArcGIS. Please see #3672 for complete details and advice on workarounds.