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.

While these are technicallybuilt-in exceptions, they aredocumented here, because conceptually they belong to the warnings mechanism.

User code can define additional warning categories by subclassing one of thestandard warning categories. A warning category must always be a subclass oftheWarning class.

The following warnings category classes are currently defined:

The Warnings Filter¶

The warnings filter controls whether warnings are ignored, displayed, or turnedinto errors (raising an exception).

Conceptually, the warnings filter maintains an ordered list of filterspecifications; any specific warning is matched against each filterspecification in the list in turn until a match is found; the filter determinesthe disposition of the match. Each entry is a tuple of the form (action,message,category,module,lineno), where:

• action is one of the following strings:

  Value	Disposition
  "default"	print the first occurrence of matchingwarnings for each location (module +line number) where the warning is issued
  "error"	turn matching warnings into exceptions
  "ignore"	never print matching warnings
  "always"	always print matching warnings
  "module"	print the first occurrence of matchingwarnings for each module where the warningis issued (regardless of line number)
  "once"	print only the first occurrence of matchingwarnings, regardless of location

• message is a string containing a regular expression that the start ofthe warning message must match, case-insensitively. In-W andPYTHONWARNINGS,message is a literal string that the start of thewarning message must contain (case-insensitively), ignoring any whitespace atthe start or end ofmessage.

• category is a class (a subclass ofWarning) of which the warningcategory must be a subclass in order to match.

• module is a string containing a regular expression that the start of thefully qualified module name must match, case-sensitively. In-W andPYTHONWARNINGS,module is a literal string that thefully qualified module name must be equal to (case-sensitively), ignoring anywhitespace at the start or end ofmodule.

• lineno is an integer that the line number where the warning occurred mustmatch, or0 to match all line numbers.

Since theWarning class is derived from the built-inExceptionclass, to turn a warning into an error we simply raisecategory(message).

If a warning is reported and doesn't match any registered filter then the"default" action is applied (hence its name).

action:message:category:module:line

default# Show all warnings (even those ignored by default)ignore# Ignore all warningserror# Convert all warnings to errorserror::ResourceWarning# Treat ResourceWarning messages as errorsdefault::DeprecationWarning# Show DeprecationWarning messagesignore,default:::mymodule# Only report warnings triggered by "mymodule"error:::mymodule# Convert warnings to errors in "mymodule"

default::DeprecationWarning:__main__ignore::DeprecationWarningignore::PendingDeprecationWarningignore::ImportWarningignore::ResourceWarning

importsysifnotsys.warnoptions:importwarningswarnings.simplefilter("ignore")

importsysifnotsys.warnoptions:importos,warningswarnings.simplefilter("default")# Change the filter in this processos.environ["PYTHONWARNINGS"]="default"# Also affect subprocesses

importwarningswarnings.filterwarnings("default",category=DeprecationWarning,module=user_ns.get("__name__"))

Temporarily Suppressing Warnings¶

If you are using code that you know will raise a warning, such as a deprecatedfunction, but do not want to see the warning (even when warnings have beenexplicitly configured via the command line), then it is possible to suppressthe warning using thecatch_warnings context manager:

importwarningsdeffxn():warnings.warn("deprecated",DeprecationWarning)withwarnings.catch_warnings():warnings.simplefilter("ignore")fxn()

While within the context manager all warnings will simply be ignored. Thisallows you to use known-deprecated code without having to see the warning whilenot suppressing the warning for other code that might not be aware of its useof deprecated code. Note: this can only be guaranteed in a single-threadedapplication. If two or more threads use thecatch_warnings contextmanager at the same time, the behavior is undefined.

測試警告¶

To test warnings raised by code, use thecatch_warnings contextmanager. With it you can temporarily mutate the warnings filter to facilitateyour testing. For instance, do the following to capture all raised warnings tocheck:

importwarningsdeffxn():warnings.warn("deprecated",DeprecationWarning)withwarnings.catch_warnings(record=True)asw:# Cause all warnings to always be triggered.warnings.simplefilter("always")# Trigger a warning.fxn()# Verify some thingsassertlen(w)==1assertissubclass(w[-1].category,DeprecationWarning)assert"deprecated"instr(w[-1].message)

One can also cause all warnings to be exceptions by usingerror instead ofalways. One thing to be aware of is that if a warning has already beenraised because of aonce/default rule, then no matter what filters areset the warning will not be seen again unless the warnings registry related tothe warning has been cleared.

Once the context manager exits, the warnings filter is restored to its statewhen the context was entered. This prevents tests from changing the warningsfilter in unexpected ways between tests and leading to indeterminate testresults. Theshowwarning() function in the module is also restored toits original value. Note: this can only be guaranteed in a single-threadedapplication. If two or more threads use thecatch_warnings contextmanager at the same time, the behavior is undefined.

When testing multiple operations that raise the same kind of warning, itis important to test them in a manner that confirms each operation is raisinga new warning (e.g. set warnings to be raised as exceptions and check theoperations raise exceptions, check that the length of the warning listcontinues to increase after each operation, or else delete the previousentries from the warnings list before each new operation).

Updating Code For New Versions of Dependencies¶

Warning categories that are primarily of interest to Python developers (ratherthan end users of applications written in Python) are ignored by default.

Notably, this "ignored by default" list includesDeprecationWarning(for every module except__main__), which means developers should make sureto test their code with typically ignored warnings made visible in order toreceive timely notifications of future breaking API changes (whether in thestandard library or third party packages).

In the ideal case, the code will have a suitable test suite, and the test runnerwill take care of implicitly enabling all warnings when running tests(the test runner provided by theunittest module does this).

In less ideal cases, applications can be checked for use of deprecatedinterfaces by passing-Wd to the Python interpreter (this isshorthand for-Wdefault) or settingPYTHONWARNINGS=default inthe environment. This enables default handling for all warnings, including thosethat are ignored by default. To change what action is taken for encounteredwarnings you can change what argument is passed to-W (e.g.-Werror). See the-W flag for more details on what ispossible.

Available Functions¶

warnings.warn(message,category=None,stacklevel=1,source=None,*,skip_file_prefixes=())¶

Issue a warning, or maybe ignore it or raise an exception. Thecategoryargument, if given, must be awarning category class; itdefaults toUserWarning. Alternatively,message can be aWarning instance,in which casecategory will be ignored andmessage.__class__ will be used.In this case, the message text will bestr(message). This function raises anexception if the particular warning issued is changed into an error by thewarnings filter. Thestacklevel argument can be used by wrapperfunctions written in Python, like this:

defdeprecated_api(message):warnings.warn(message,DeprecationWarning,stacklevel=2)

This makes the warning refer todeprecated_api's caller, rather than tothe source ofdeprecated_api itself (since the latter would defeat thepurpose of the warning message).

Theskip_file_prefixes keyword argument can be used to indicate whichstack frames are ignored when counting stack levels. This can be useful whenyou want the warning to always appear at call sites outside of a packagewhen a constantstacklevel does not fit all call paths or is otherwisechallenging to maintain. If supplied, it must be a tuple of strings. Whenprefixes are supplied, stacklevel is implicitly overridden to bemax(2,stacklevel). To cause a warning to be attributed to the caller fromoutside of the current package you might write:

# example/lower.py_warn_skips=(os.path.dirname(__file__),)defone_way(r_luxury_yacht=None,t_wobbler_mangrove=None):ifr_luxury_yacht:warnings.warn("Please migrate to t_wobbler_mangrove=.",skip_file_prefixes=_warn_skips)# example/higher.pyfrom.importlowerdefanother_way(**kw):lower.one_way(**kw)

This makes the warning refer to both theexample.lower.one_way() andpackage.higher.another_way() call sites only from calling code livingoutside ofexample package.

source, if supplied, is the destroyed object which emitted aResourceWarning.

在 3.6 版的變更:新增source 參數。

在 3.12 版的變更:新增skip_file_prefixes。

warnings.warn_explicit(message,category,filename,lineno,module=None,registry=None,module_globals=None,source=None)¶

This is a low-level interface to the functionality ofwarn(), passing inexplicitly the message, category, filename and line number, and optionally themodule name and the registry (which should be the__warningregistry__dictionary of the module). The module name defaults to the filename with.py stripped; if no registry is passed, the warning is never suppressed.message must be a string andcategory a subclass ofWarning ormessage may be aWarning instance, in which casecategory will beignored.

module_globals, if supplied, should be the global namespace in use by the codefor which the warning is issued. (This argument is used to support displayingsource for modules found in zipfiles or other non-filesystem importsources).

source, if supplied, is the destroyed object which emitted aResourceWarning.

在 3.6 版的變更:新增source 參數。

warnings.showwarning(message,category,filename,lineno,file=None,line=None)¶

Write a warning to a file. The default implementation callsformatwarning(message,category,filename,lineno,line) and writes theresulting string tofile, which defaults tosys.stderr. You may replacethis function with any callable by assigning towarnings.showwarning.line is a line of source code to be included in the warningmessage; ifline is not supplied,showwarning() willtry to read the line specified byfilename andlineno.

warnings.formatwarning(message,category,filename,lineno,line=None)¶

Format a warning the standard way. This returns a string which may containembedded newlines and ends in a newline.line is a line of source code tobe included in the warning message; ifline is not supplied,formatwarning() will try to read the line specified byfilename andlineno.

warnings.filterwarnings(action,message='',category=Warning,module='',lineno=0,append=False)¶

Insert an entry into the list ofwarnings filter specifications. The entry is inserted at the front by default; ifappend is true, it is inserted at the end. This checks the types of thearguments, compiles themessage andmodule regular expressions, andinserts them as a tuple in the list of warnings filters. Entries closer tothe front of the list override entries later in the list, if both match aparticular warning. Omitted arguments default to a value that matcheseverything.

warnings.simplefilter(action,category=Warning,lineno=0,append=False)¶

Insert a simple entry into the list ofwarnings filter specifications. The meaning of the function parameters is as forfilterwarnings(), but regular expressions are not needed as the filterinserted always matches any message in any module as long as the category andline number match.

warnings.resetwarnings()¶

Reset the warnings filter. This discards the effect of all previous calls tofilterwarnings(), including that of the-W command line optionsand calls tosimplefilter().

@warnings.deprecated(msg,*,category=DeprecationWarning,stacklevel=1)¶

Decorator to indicate that a class, function or overload is deprecated.

When this decorator is applied to an object,deprecation warnings may be emitted at runtime when the object is used.static type checkerswill also generate a diagnostic on usage of the deprecated object.

Usage:

fromwarningsimportdeprecatedfromtypingimportoverload@deprecated("Use B instead")classA:pass@deprecated("Use g instead")deff():pass@overload@deprecated("int support is deprecated")defg(x:int)->int:...@overloaddefg(x:str)->int:...

The warning specified bycategory will be emitted at runtimeon use of deprecated objects. For functions, that happens on calls;for classes, on instantiation and on creation of subclasses.If thecategory isNone, no warning is emitted at runtime.Thestacklevel determines where thewarning is emitted. If it is1 (the default), the warningis emitted at the direct caller of the deprecated object; if itis higher, it is emitted further up the stack.Static type checker behavior is not affected by thecategoryandstacklevel arguments.

The deprecation message passed to the decorator is saved in the__deprecated__ attribute on the decorated object.If applied to an overload, the decoratormust be after the@overload decoratorfor the attribute to exist on the overload as returned bytyping.get_overloads().

在 3.13 版被加入:SeePEP 702.

Available Context Managers¶

classwarnings.catch_warnings(*,record=False,module=None,action=None,category=Warning,lineno=0,append=False)¶

A context manager that copies and, upon exit, restores the warnings filterand theshowwarning() function.If therecord argument isFalse (the default) the context managerreturnsNone on entry. Ifrecord isTrue, a list isreturned that is progressively populated with objects as seen by a customshowwarning() function (which also suppresses output tosys.stdout).Each object in the list has attributes with the same names as the arguments toshowwarning().

Themodule argument takes a module that will be used instead of themodule returned when you importwarnings whose filter will beprotected. This argument exists primarily for testing thewarningsmodule itself.

If theaction argument is notNone, the remaining arguments arepassed tosimplefilter() as if it were called immediately onentering the context.

SeeThe Warnings Filter for the meaning of thecategory andlinenoparameters.

備註

Thecatch_warnings manager works by replacing andthen later restoring the module'sshowwarning() function and internal list of filterspecifications. This means the context manager is modifyingglobal state and therefore is not thread-safe.

在 3.11 版的變更:新增action、category、lineno 和append 參數。