\input{libwarnings}

\section{\module{warnings} ---

Warning control}

\declaremodule{standard}{warnings}

\modulesynopsis{Issue warning messages and control their disposition.}

\index{warnings}

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

\code{PyErr_Warn()}).

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}

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}{code}{Class}{Description}

\lineii{Warning}{This is the base class of all warning category

classes. It itself a subclass of 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}

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{\code{"error"}}{turn matching warnings into exceptions}

\lineii{\code{"ignore"}}{never print matching warnings}

\lineii{\code{"always"}}{always print matching warnings}

\lineii{\code{"default"}}{print the first occurrence of matching

warnings for each location where the warning is issued}

\lineii{\code{"module"}}{print the first occurrence of matching

warnings for each module where the warning is issued}

\lineii{\code{"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 \samp{-W} options passed to the

Python interpreter command line. The interpreter saves the arguments

for all \samp{-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}

\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}

\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}{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}}}}}

Insert an entry into the list of warnings filters (at the front).

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 \samp{-W}

command line options.

\end{funcdesc}