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