254 lines
10 KiB
TeX
254 lines
10 KiB
TeX
\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 overridden; 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.}
|
|
|
|
\lineii{FutureWarning}{Base category for warnings about constructs
|
|
that will change semantically in the future.}
|
|
|
|
\lineii{PendingDeprecationWarning}{Base category for warnings about
|
|
features that will be deprecated in the future (ignored by default).}
|
|
|
|
\lineii{ImportWarning}{Base category for warnings triggered during the
|
|
process of importing a module (ignored by default).}
|
|
|
|
\lineii{UnicodeWarning}{Base category for warnings related to Unicode.}
|
|
|
|
\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 string containing a regular expression that
|
|
the warning message must match (the match is compiled to always be
|
|
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 string containing a regular expression that the module
|
|
name must match (the match is compiled to be case-sensitive)
|
|
|
|
\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}).
|
|
|
|
The warnings that are ignored by default may be enabled by passing
|
|
\programopt{-Wd} to the interpreter. This enables default handling
|
|
for all warnings, including those that are normally ignored by
|
|
default. This is particular useful for enabling ImportWarning when
|
|
debugging problems importing a developed package. ImportWarning can
|
|
also be enabled explicitly in Python code using:
|
|
|
|
\begin{verbatim}
|
|
warnings.simplefilter('default', ImportWarning)
|
|
\end{verbatim}
|
|
|
|
|
|
\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}. Alternatively
|
|
\var{message} can be a \exception{Warning} instance, in which case
|
|
\var{category} will be ignored and \code{message.__class__} will be used.
|
|
In this case the message text will be \code{str(message)}. 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\optional{,
|
|
module_globals}}}}
|
|
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.
|
|
\var{message} must be a string and \var{category} a subclass of
|
|
\exception{Warning} or \var{message} may be a \exception{Warning} instance,
|
|
in which case \var{category} will be ignored.
|
|
|
|
\var{module_globals}, if supplied, should be the global namespace in use
|
|
by the code for which the warning is issued. (This argument is used to
|
|
support displaying source for modules found in zipfiles or other
|
|
non-filesystem import sources, and was added in Python 2.5.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{showwarning}{message, category, filename,
|
|
lineno\optional{, file}}
|
|
Write a warning to a file. The default implementation calls
|
|
\code{formatwarning(\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 the
|
|
list of warnings filters. Entries closer to the front of the list
|
|
override entries later in the list, if both match a particular
|
|
warning. Omitted arguments default to a value that matches
|
|
everything.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{simplefilter}{action\optional{,
|
|
category\optional{,
|
|
lineno\optional{, append}}}}
|
|
Insert a simple entry into the list of warnings filters. The meaning
|
|
of the function parameters is as for \function{filterwarnings()}, but
|
|
regular expressions are not needed as the filter inserted always
|
|
matches any message in any module as long as the category and line
|
|
number match.
|
|
\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 and calls to
|
|
\function{simplefilter()}.
|
|
\end{funcdesc}
|