\section{\module{warnings} --- Warning control} \declaremodule{standard}{warnings} \modulesynopsis{Issue warning messages and control their disposition.} \index{warnings} \versionadded{2.1} Warning messages are typically issued in situations where it is useful to alert the user of some condition in a program, where that condition (normally) doesn't warrant raising an exception and terminating the program. For example, one might want to issue a warning when a program uses an obsolete module. Python programmers issue warnings by calling the \function{warn()} function defined in this module. (C programmers use \cfunction{PyErr_Warn()}; see the \citetitle[../api/exceptionHandling.html]{Python/C API Reference Manual} for details). Warning messages are normally written to \code{sys.stderr}, but their disposition can be changed flexibly, from ignoring all warnings to turning them into exceptions. The disposition of warnings can vary based on the warning category (see below), the text of the warning message, and the source location where it is issued. Repetitions of a particular warning for the same source location are typically suppressed. There are two stages in warning control: first, each time a warning is issued, a determination is made whether a message should be issued or not; next, if a message is to be issued, it is formatted and printed using a user-settable hook. The determination whether to issue a warning message is controlled by the warning filter, which is a sequence of matching rules and actions. Rules can be added to the filter by calling \function{filterwarnings()} and reset to its default state by calling \function{resetwarnings()}. The printing of warning messages is done by calling \function{showwarning()}, which may be overidden; the default implementation of this function formats the message by calling \function{formatwarning()}, which is also available for use by custom implementations. \subsection{Warning Categories \label{warning-categories}} There are a number of built-in exceptions that represent warning categories. This categorization is useful to be able to filter out groups of warnings. The following warnings category classes are currently defined: \begin{tableii}{l|l}{exception}{Class}{Description} \lineii{Warning}{This is the base class of all warning category classes. It is a subclass of \exception{Exception}.} \lineii{UserWarning}{The default category for \function{warn()}.} \lineii{DeprecationWarning}{Base category for warnings about deprecated features.} \lineii{SyntaxWarning}{Base category for warnings about dubious syntactic features.} \lineii{RuntimeWarning}{Base category for warnings about dubious runtime features.} \end{tableii} While these are technically built-in exceptions, they are documented here, because conceptually they belong to the warnings mechanism. User code can define additional warning categories by subclassing one of the standard warning categories. A warning category must always be a subclass of the \exception{Warning} class. \subsection{The Warnings Filter \label{warning-filter}} The warnings filter controls whether warnings are ignored, displayed, or turned into errors (raising an exception). Conceptually, the warnings filter maintains an ordered list of filter specifications; any specific warning is matched against each filter specification in the list in turn until a match is found; the match determines the disposition of the match. Each entry is a tuple of the form (\var{action}, \var{message}, \var{category}, \var{module}, \var{lineno}), where: \begin{itemize} \item \var{action} is one of the following strings: \begin{tableii}{l|l}{code}{Value}{Disposition} \lineii{"error"}{turn matching warnings into exceptions} \lineii{"ignore"}{never print matching warnings} \lineii{"always"}{always print matching warnings} \lineii{"default"}{print the first occurrence of matching warnings for each location where the warning is issued} \lineii{"module"}{print the first occurrence of matching warnings for each module where the warning is issued} \lineii{"once"}{print only the first occurrence of matching warnings, regardless of location} \end{tableii} \item \var{message} is a compiled regular expression that the warning message must match (the match is case-insensitive) \item \var{category} is a class (a subclass of \exception{Warning}) of which the warning category must be a subclass in order to match \item \var{module} is a compiled regular expression that the module name must match \item \var{lineno} is an integer that the line number where the warning occurred must match, or \code{0} to match all line numbers \end{itemize} Since the \exception{Warning} class is derived from the built-in \exception{Exception} class, to turn a warning into an error we simply raise \code{category(message)}. The warnings filter is initialized by \programopt{-W} options passed to the Python interpreter command line. The interpreter saves the arguments for all \programopt{-W} options without interpretation in \code{sys.warnoptions}; the \module{warnings} module parses these when it is first imported (invalid options are ignored, after printing a message to \code{sys.stderr}). \subsection{Available Functions \label{warning-functions}} \begin{funcdesc}{warn}{message\optional{, category\optional{, stacklevel}}} Issue a warning, or maybe ignore it or raise an exception. The \var{category} argument, if given, must be a warning category class (see above); it defaults to \exception{UserWarning}. This function raises an exception if the particular warning issued is changed into an error by the warnings filter see above. The \var{stacklevel} argument can be used by wrapper functions written in Python, like this: \begin{verbatim} def deprecation(message): warnings.warn(message, DeprecationWarning, stacklevel=2) \end{verbatim} This makes the warning refer to \function{deprecation()}'s caller, rather than to the source of \function{deprecation()} itself (since the latter would defeat the purpose of the warning message). \end{funcdesc} \begin{funcdesc}{warn_explicit}{message, category, filename, lineno\optional{, module\optional{, registry}}} This is a low-level interface to the functionality of \function{warn()}, passing in explicitly the message, category, filename and line number, and optionally the module name and the registry (which should be the \code{__warningregistry__} dictionary of the module). The module name defaults to the filename with \code{.py} stripped; if no registry is passed, the warning is never suppressed. \end{funcdesc} \begin{funcdesc}{showwarning}{message, category, filename, lineno\optional{, file}} Write a warning to a file. The default implementation calls \code{showwarning(\var{message}, \var{category}, \var{filename}, \var{lineno})} and writes the resulting string to \var{file}, which defaults to \code{sys.stderr}. You may replace this function with an alternative implementation by assigning to \code{warnings.showwarning}. \end{funcdesc} \begin{funcdesc}{formatwarning}{message, category, filename, lineno} Format a warning the standard way. This returns a string which may contain embedded newlines and ends in a newline. \end{funcdesc} \begin{funcdesc}{filterwarnings}{action\optional{, message\optional{, category\optional{, module\optional{, lineno\optional{, append}}}}}} Insert an entry into the list of warnings filters. The entry is inserted at the front by default; if \var{append} is true, it is inserted at the end. This checks the types of the arguments, compiles the message and module regular expressions, and inserts them as a tuple in front of the warnings filter. Entries inserted later override entries inserted earlier, if both match a particular warning. Omitted arguments default to a value that matches everything. \end{funcdesc} \begin{funcdesc}{resetwarnings}{} Reset the warnings filter. This discards the effect of all previous calls to \function{filterwarnings()}, including that of the \programopt{-W} command line options. \end{funcdesc}