Exception API

Exception definition

All the exceptions that can be thrown by the underlying Tango C++ API are available in the PyTango python module. Hence a user can catch one of the following exceptions:

When an exception is caught, the sys.exc_info() function returns a tuple of three values that give information about the exception that is currently being handled. The values returned are (type, value, traceback). Since most functions don’t need access to the traceback, the best solution is to use something like exctype, value = sys.exc_info()[:2] to extract only the exception type and value. If one of the Tango exceptions is caught, the exctype will be class name of the exception (DevFailed, .. etc) and the value a tuple of dictionary objects all of which containing the following kind of key-value pairs:

  • reason: a string describing the error type (more readable than the associated error code)

  • desc: a string describing in plain text the reason of the error.

  • origin: a string giving the name of the (C++ API) method which thrown the exception

  • severity: one of the strings WARN, ERR, PANIC giving severity level of the error.

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import tango

#  How to protect the script from exceptions raised by the Tango
try:
    # Get proxy on a non existing device should throw an exception
    device = tango.DeviceProxy("non/existing/device")
except DevFailed as df:
    print("Failed to create proxy to non/existing/device:\n%s" % df)

Throwing exception in a device server

The C++ tango::Except class with its most important methods have been wrapped to Python. Therefore, in a Python device server, you have the following methods to throw, re-throw or print a Tango::DevFailed exception :

  • throw_exception() which is a static method

  • re_throw_exception() which is also a static method

  • print_exception() which is also a static method

The following code is an example of a command method requesting a command on a sub-device and re-throwing the exception in case of:

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try:
    dev.command_inout("SubDevCommand")
except tango.DevFailed as df:
    tango.Except.re_throw_exception(df,
        "MyClass_CommandFailed",
        "Sub device command SubdevCommand failed",
        "Command()")
line 2

Send the command to the sub device in a try/catch block

line 4-6

Re-throw the exception and add a new level of information in the exception stack

Exception API

class tango.Except(*args, **kwargs)

A containner for the static methods:

  • throw_exception

  • re_throw_exception

  • print_exception

  • compare_exception

class tango.DevError(*args, **kwargs)

Structure describing any error resulting from a command execution, or an attribute query, with following members:

  • reason : (str) reason

  • severity : (ErrSeverity) error severty (WARN, ERR, PANIC)

  • desc : (str) error description

  • origin : (str) Tango server method in which the error happened

exception tango.DevFailed(*args, **kwargs)
exception tango.ConnectionFailed(*args, **kwargs)

This exception is thrown when a problem occurs during the connection establishment between the application and the device. The API is stateless. This means that DeviceProxy constructors filter most of the exception except for cases described in the following table.

The desc DevError structure field allows a user to get more precise information. These informations are :

DB_DeviceNotDefined

The name of the device not defined in the database

API_CommandFailed

The device and command name

API_CantConnectToDevice

The device name

API_CorbaException

The name of the CORBA exception, its reason, its locality, its completed flag and its minor code

API_CantConnectToDatabase

The database server host and its port number

API_DeviceNotExported

The device name

exception tango.CommunicationFailed(*args, **kwargs)

This exception is thrown when a communication problem is detected during the communication between the client application and the device server. It is a two levels Tango::DevError structure. In case of time-out, the DevError structures fields are:

Level

Reason

Desc

Severity

0

API_CorbaException

CORBA exception fields translated into a string

ERR

1

API_DeviceTimedOut

String with time-out value and device name

ERR

For all other communication errors, the DevError structures fields are:

Level

Reason

Desc

Severity

0

API_CorbaException

CORBA exception fields translated into a string

ERR

1

API_CommunicationFailed

String with device, method, command/attribute name

ERR

exception tango.WrongNameSyntax(*args, **kwargs)

This exception has only one level of Tango::DevError structure. The possible value for the reason field are :

API_UnsupportedProtocol

This error occurs when trying to build a DeviceProxy or an AttributeProxy instance for a device with an unsupported protocol. Refer to the appendix on device naming syntax to get the list of supported database modifier

API_UnsupportedDBaseModifier

This error occurs when trying to build a DeviceProxy or an AttributeProxy instance for a device/attribute with a database modifier unsupported. Refer to the appendix on device naming syntax to get the list of supported database modifier

API_WrongDeviceNameSyntax

This error occurs for all the other error in device name syntax. It is thrown by the DeviceProxy class constructor.

API_WrongAttributeNameSyntax

This error occurs for all the other error in attribute name syntax. It is thrown by the AttributeProxy class constructor.

API_WrongWildcardUsage

This error occurs if there is a bad usage of the wildcard character

exception tango.NonDbDevice(*args, **kwargs)

This exception has only one level of Tango::DevError structure. The reason field is set to API_NonDatabaseDevice. This exception is thrown by the API when using the DeviceProxy or AttributeProxy class database access for non-database device.

exception tango.WrongData(*args, **kwargs)

This exception has only one level of Tango::DevError structure. The possible value for the reason field are :

API_EmptyDbDatum

This error occurs when trying to extract data from an empty DbDatum object

API_IncompatibleArgumentType

This error occurs when trying to extract data with a type different than the type used to send the data

API_EmptyDeviceAttribute

This error occurs when trying to extract data from an empty DeviceAttribute object

API_IncompatibleAttrArgumentType

This error occurs when trying to extract attribute data with a type different than the type used to send the data

API_EmptyDeviceData

This error occurs when trying to extract data from an empty DeviceData object

API_IncompatibleCmdArgumentType

This error occurs when trying to extract command data with a type different than the type used to send the data

exception tango.NonSupportedFeature(*args, **kwargs)

This exception is thrown by the API layer when a request to a feature implemented in Tango device interface release n is requested for a device implementing Tango device interface n-x. There is one possible value for the reason field which is API_UnsupportedFeature.

exception tango.AsynCall(*args, **kwargs)

This exception is thrown by the API layer when a the asynchronous model id badly used. This exception has only one level of Tango::DevError structure. The possible value for the reason field are :

API_BadAsynPollId

This error occurs when using an asynchronous request identifier which is not valid any more.

API_BadAsyn

This error occurs when trying to fire callback when no callback has been previously registered

API_BadAsynReqType

This error occurs when trying to get result of an asynchronous request with an asynchronous request identifier returned by a non-coherent asynchronous request (For instance, using the asynchronous request identifier returned by a command_inout_asynch() method with a read_attribute_reply() attribute).

exception tango.AsynReplyNotArrived(*args, **kwargs)

This exception is thrown by the API layer when:

  • a request to get asynchronous reply is made and the reply is not yet arrived

  • a blocking wait with timeout for asynchronous reply is made and the timeout expired.

There is one possible value for the reason field which is API_AsynReplyNotArrived.

exception tango.EventSystemFailed(*args, **kwargs)

This exception is thrown by the API layer when subscribing or unsubscribing from an event failed. This exception has only one level of Tango::DevError structure. The possible value for the reason field are :

API_NotificationServiceFailed

This error occurs when the subscribe_event() method failed trying to access the CORBA notification service

API_EventNotFound

This error occurs when you are using an incorrect event_id in the unsubscribe_event() method

API_InvalidArgs

This error occurs when NULL pointers are passed to the subscribe or unsubscribe event methods

API_MethodArgument

This error occurs when trying to subscribe to an event which has already been subsribed to

API_DSFailedRegisteringEvent

This error means that the device server to which the device belongs to failed when it tries to register the event. Most likely, it means that there is no event property defined

API_EventNotFound

Occurs when using a wrong event identifier in the unsubscribe_event method

exception tango.DeviceUnlocked(*args, **kwargs)

This exception is thrown by the API layer when a device locked by the process has been unlocked by an admin client. This exception has two levels of Tango::DevError structure. There is only possible value for the reason field which is

API_DeviceUnlocked

The device has been unlocked by another client (administration client)

The first level is the message reported by the Tango kernel from the server side. The second layer is added by the client API layer with informations on which API call generates the exception and device name.

exception tango.NotAllowed(*args, **kwargs)
exception tango.NamedDevFailedList(*args, **kwargs)

This exception is only thrown by the DeviceProxy::write_attributes() method. In this case, it is necessary to have a new class of exception to transfer the error stack for several attribute(s) which failed during the writing. Therefore, this exception class contains for each attributes which failed :

  • The name of the attribute

  • Its index in the vector passed as argumen tof the write_attributes() method

  • The error stack