Logging, Warnings, and Exceptions¶
sunpy makes use of a logging system to deal with messages (see Logging system). This provides the users and
developers the ability to decide which messages to show, to capture them, and to optionally also send
them to a file. The logger will log all messages issued directly to it but also warnings issued
warnings.warn as well as exceptions.
The logger is configured as soon as sunpy is imported. You can access it by importing it explicitly:
from sunpy import log
Messages can be issued directly to it with the following levels and in the following way:
log.debug("Detailed information, typically of interest only when diagnosing problems.") log.info("A message conveying information about the current task, and confirming that things are working as expected.") log.warning("An indication that something unexpected happened, or indicative of some problem in the near future (e.g. disk space low). The software is still working as expected.") log.error("Due to a more serious problem, the software has not been able to perform some function but the task is still continuing.") log.critical("A serious error, indicating that the program itself may be unable to continue running. A real error may soon by issued and the task will fail.")
The difference between logging a warning/error/critical compared to issuing a Python warning or raising an exception are subtle but important.
warnings.warn in library code if the issue is avoidable and the user code should be
modified to eliminate the warning.
log.warning() if there is likely nothing the user can do about the situation, but the event
should still be noted. An example of this might be if the input data are all zeros. This may be unavoidable or
even by design but you may want to let the user know.
True exceptions (not
log.error()) should be raised only when there is no way for the function to proceed.
Regardless of the type (log message or warning or exception) messages should be one or two complete sentences that fully describe the issue and end with a period.
sunpy warnings are provided by the
sunpy.util module. The primary warning which
should be used is
sunpy.util.exceptions.SunpyUserWarning. For deprecation use
These three warning types have corresponding functions to raise them:
>>> from sunpy.util.exceptions import warn_user >>> from sunpy.util.exceptions import warn_deprecated >>> from sunpy.util.exceptions import warn_metadata
These warning functions must be used to interact correctly with the logging system. A warning can be issued in the following way:
>>> from sunpy.util.exceptions import warn_user >>> warn_user("You have been warned about something you did not do correctly.")
See the section above for a discussion about the distinction between
log.warn() and raising a warning.
Raising errors causes the program to halt. Likely the primary error that a sunpy developer will want to use is
ValueError: should be raised if the input is not what was expected and could not be used. Functions should not return anything (like None) in that case or issue a warning.
Exceptions are raised simply with:
>>> raise ValueError("The following error occurred.")
For more information on exceptions see the Built-in Exceptions documentation.