### 导航 - [索引](../genindex.xhtml "总目录") - [模块](../py-modindex.xhtml "Python 模块索引") | - [下一页](dataclasses.xhtml "dataclasses --- 数据类") | - [上一页](__main__.xhtml "__main__ --- 顶层脚本环境") | -  - [Python](https://www.python.org/) » - zh\_CN 3.7.3 [文档](../index.xhtml) » - [Python 标准库](index.xhtml) » - [Python运行时服务](python.xhtml) » - $('.inline-search').show(0); | # [`warnings`](#module-warnings "warnings: Issue warning messages and control their disposition.") --- Warning control **源代码：** [Lib/warnings.py](https://github.com/python/cpython/tree/3.7/Lib/warnings.py) \[https://github.com/python/cpython/tree/3.7/Lib/warnings.py\] - - - - - - 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 [`warn()`](#warnings.warn "warnings.warn") function defined in this module. (C programmers use [`PyErr_WarnEx()`](../c-api/exceptions.xhtml#c.PyErr_WarnEx "PyErr_WarnEx"); see [异常处理](../c-api/exceptions.xhtml#exceptionhandling) for details). Warning messages are normally written to `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 [`filterwarnings()`](#warnings.filterwarnings "warnings.filterwarnings") and reset to its default state by calling [`resetwarnings()`](#warnings.resetwarnings "warnings.resetwarnings"). The printing of warning messages is done by calling [`showwarning()`](#warnings.showwarning "warnings.showwarning"), which may be overridden; the default implementation of this function formats the message by calling [`formatwarning()`](#warnings.formatwarning "warnings.formatwarning"), which is also available for use by custom implementations. 参见 [`logging.captureWarnings()`](logging.xhtml#logging.captureWarnings "logging.captureWarnings") allows you to handle all warnings with the standard logging infrastructure. ## 警告类别 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 technically [built-in exceptions](exceptions.xhtml#warning-categories-as-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 [`Warning`](exceptions.xhtml#Warning "Warning") class. The following warnings category classes are currently defined: Class 描述 [`Warning`](exceptions.xhtml#Warning "Warning") This is the base class of all warning category classes. It is a subclass of [`Exception`](exceptions.xhtml#Exception "Exception"). [`UserWarning`](exceptions.xhtml#UserWarning "UserWarning") The default category for [`warn()`](#warnings.warn "warnings.warn"). [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning") Base category for warnings about deprecated features when those warnings are intended for other Python developers (ignored by default, unless triggered by code in `__main__`). [`SyntaxWarning`](exceptions.xhtml#SyntaxWarning "SyntaxWarning") Base category for warnings about dubious syntactic features. [`RuntimeWarning`](exceptions.xhtml#RuntimeWarning "RuntimeWarning") Base category for warnings about dubious runtime features. [`FutureWarning`](exceptions.xhtml#FutureWarning "FutureWarning") Base category for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python. [`PendingDeprecationWarning`](exceptions.xhtml#PendingDeprecationWarning "PendingDeprecationWarning") Base category for warnings about features that will be deprecated in the future (ignored by default). [`ImportWarning`](exceptions.xhtml#ImportWarning "ImportWarning") Base category for warnings triggered during the process of importing a module (ignored by default). [`UnicodeWarning`](exceptions.xhtml#UnicodeWarning "UnicodeWarning") Base category for warnings related to Unicode. [`BytesWarning`](exceptions.xhtml#BytesWarning "BytesWarning") Base category for warnings related to [`bytes`](stdtypes.xhtml#bytes "bytes") and [`bytearray`](stdtypes.xhtml#bytearray "bytearray"). [`ResourceWarning`](exceptions.xhtml#ResourceWarning "ResourceWarning") Base category for warnings related to resource usage. 在 3.7 版更改: Previously [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning") and [`FutureWarning`](exceptions.xhtml#FutureWarning "FutureWarning") were distinguished based on whether a feature was being removed entirely or changing its behaviour. They are now distinguished based on their intended audience and the way they're handled by the default warnings filters. ## 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 filter determines the 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: 值 处置 `"default"` 为发出警告的每个位置（模块+行号）打印第一个匹配警告 `"error"` 将匹配警告转换为异常 `"ignore"` 从不打印匹配的警告 `"always"` 总是打印匹配的警告 `"module"` 为发出警告的每个模块打印第一次匹配警告（无论行号如何） `"once"` 无论位置如何，仅打印第一次出现的匹配警告 - *message* is a string containing a regular expression that the start of the warning message must match. The expression is compiled to always be case-insensitive. - *category* is a class (a subclass of [`Warning`](exceptions.xhtml#Warning "Warning")) of which the warning category must be a subclass in order to match. - *module* is a string containing a regular expression that the module name must match. The expression is compiled to be case-sensitive. - *lineno* is an integer that the line number where the warning occurred must match, or `0` to match all line numbers. Since the [`Warning`](exceptions.xhtml#Warning "Warning") class is derived from the built-in [`Exception`](exceptions.xhtml#Exception "Exception")class, to turn a warning into an error we simply raise `category(message)`. If a warning is reported and doesn't match any registered filter then the "default" action is applied (hence its name). ### Describing Warning Filters The warnings filter is initialized by [`-W`](../using/cmdline.xhtml#cmdoption-w) options passed to the Python interpreter command line and the [`PYTHONWARNINGS`](../using/cmdline.xhtml#envvar-PYTHONWARNINGS) environment variable. The interpreter saves the arguments for all supplied entries without interpretation in `sys.warnoptions`; the [`warnings`](#module-warnings "warnings: Issue warning messages and control their disposition.") module parses these when it is first imported (invalid options are ignored, after printing a message to `sys.stderr`). Individual warnings filters are specified as a sequence of fields separated by colons: ``` action:message:category:module:line ``` The meaning of each of these fields is as described in [The Warnings Filter](#warning-filter). When listing multiple filters on a single line (as for [`PYTHONWARNINGS`](../using/cmdline.xhtml#envvar-PYTHONWARNINGS)), the individual filters are separated by commas,and the filters listed later take precedence over those listed before them (as they're applied left-to-right, and the most recently applied filters take precedence over earlier ones). Commonly used warning filters apply to either all warnings, warnings in a particular category, or warnings raised by particular modules or packages. Some examples: ``` default # Show all warnings (even those ignored by default) ignore # Ignore all warnings error # Convert all warnings to errors error::ResourceWarning # Treat ResourceWarning messages as errors default::DeprecationWarning # Show DeprecationWarning messages ignore,default:::mymodule # Only report warnings triggered by "mymodule" error:::mymodule[.*] # Convert warnings to errors in "mymodule" # and any subpackages of "mymodule" ``` ### 默认警告过滤器 By default, Python installs several warning filters, which can be overridden by the [`-W`](../using/cmdline.xhtml#cmdoption-w) command-line option, the [`PYTHONWARNINGS`](../using/cmdline.xhtml#envvar-PYTHONWARNINGS) environment variable and calls to [`filterwarnings()`](#warnings.filterwarnings "warnings.filterwarnings"). In regular release builds, the default warning filter has the following entries (in order of precedence): ``` default::DeprecationWarning:__main__ ignore::DeprecationWarning ignore::PendingDeprecationWarning ignore::ImportWarning ignore::ResourceWarning ``` In debug builds, the list of default warning filters is empty. 在 3.2 版更改: [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning") is now ignored by default in addition to [`PendingDeprecationWarning`](exceptions.xhtml#PendingDeprecationWarning "PendingDeprecationWarning"). 在 3.7 版更改: [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning") is once again shown by default when triggered directly by code in `__main__`. 在 3.7 版更改: [`BytesWarning`](exceptions.xhtml#BytesWarning "BytesWarning") no longer appears in the default filter list and is instead configured via [`sys.warnoptions`](sys.xhtml#sys.warnoptions "sys.warnoptions") when [`-b`](../using/cmdline.xhtml#cmdoption-b) is specified twice. ### Overriding the default filter Developers of applications written in Python may wish to hide *all* Python level warnings from their users by default, and only display them when running tests or otherwise working on the application. The [`sys.warnoptions`](sys.xhtml#sys.warnoptions "sys.warnoptions") attribute used to pass filter configurations to the interpreter can be used as a marker to indicate whether or not warnings should be disabled: ``` import sys if not sys.warnoptions: import warnings warnings.simplefilter("ignore") ``` Developers of test runners for Python code are advised to instead ensure that *all* warnings are displayed by default for the code under test, using code like: ``` import sys if not sys.warnoptions: import os, warnings warnings.simplefilter("default") # Change the filter in this process os.environ["PYTHONWARNINGS"] = "default" # Also affect subprocesses ``` Finally, developers of interactive shells that run user code in a namespace other than `__main__` are advised to ensure that [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning")messages are made visible by default, using code like the following (where `user_ns` is the module used to execute code entered interactively): ``` import warnings warnings.filterwarnings("default", category=DeprecationWarning, module=user_ns.get("__name__")) ``` ## 暂时禁止警告 If you are using code that you know will raise a warning, such as a deprecated function, but do not want to see the warning (even when warnings have been explicitly configured via the command line), then it is possible to suppress the warning using the [`catch_warnings`](#warnings.catch_warnings "warnings.catch_warnings") context manager: ``` import warnings def fxn(): warnings.warn("deprecated", DeprecationWarning) with warnings.catch_warnings(): warnings.simplefilter("ignore") fxn() ``` While within the context manager all warnings will simply be ignored. This allows you to use known-deprecated code without having to see the warning while not suppressing the warning for other code that might not be aware of its use of deprecated code. Note: this can only be guaranteed in a single-threaded application. If two or more threads use the [`catch_warnings`](#warnings.catch_warnings "warnings.catch_warnings") context manager at the same time, the behavior is undefined. ## 测试警告 To test warnings raised by code, use the [`catch_warnings`](#warnings.catch_warnings "warnings.catch_warnings") context manager. With it you can temporarily mutate the warnings filter to facilitate your testing. For instance, do the following to capture all raised warnings to check: ``` import warnings def fxn(): warnings.warn("deprecated", DeprecationWarning) with warnings.catch_warnings(record=True) as w: # Cause all warnings to always be triggered. warnings.simplefilter("always") # Trigger a warning. fxn() # Verify some things assert len(w) == 1 assert issubclass(w[-1].category, DeprecationWarning) assert "deprecated" in str(w[-1].message) ``` One can also cause all warnings to be exceptions by using `error` instead of `always`. One thing to be aware of is that if a warning has already been raised because of a `once`/`default` rule, then no matter what filters are set the warning will not be seen again unless the warnings registry related to the warning has been cleared. Once the context manager exits, the warnings filter is restored to its state when the context was entered. This prevents tests from changing the warnings filter in unexpected ways between tests and leading to indeterminate test results. The [`showwarning()`](#warnings.showwarning "warnings.showwarning") function in the module is also restored to its original value. Note: this can only be guaranteed in a single-threaded application. If two or more threads use the [`catch_warnings`](#warnings.catch_warnings "warnings.catch_warnings") context manager at the same time, the behavior is undefined. When testing multiple operations that raise the same kind of warning, it is important to test them in a manner that confirms each operation is raising a new warning (e.g. set warnings to be raised as exceptions and check the operations raise exceptions, check that the length of the warning list continues to increase after each operation, or else delete the previous entries 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 (rather than end users of applications written in Python) are ignored by default. Notably, this "ignored by default" list includes [`DeprecationWarning`](exceptions.xhtml#DeprecationWarning "DeprecationWarning")(for every module except `__main__`), which means developers should make sure to test their code with typically ignored warnings made visible in order to receive timely notifications of future breaking API changes (whether in the standard library or third party packages). In the ideal case, the code will have a suitable test suite, and the test runner will take care of implicitly enabling all warnings when running tests (the test runner provided by the [`unittest`](unittest.xhtml#module-unittest "unittest: Unit testing framework for Python.") module does this). In less ideal cases, applications can be checked for use of deprecated interfaces by passing [`-Wd`](../using/cmdline.xhtml#cmdoption-w) to the Python interpreter (this is shorthand for `-W default`) or setting `PYTHONWARNINGS=default` in the environment. This enables default handling for all warnings, including those that are ignored by default. To change what action is taken for encountered warnings you can change what argument is passed to [`-W`](../using/cmdline.xhtml#cmdoption-w) (e.g. `-W error`). See the [`-W`](../using/cmdline.xhtml#cmdoption-w) flag for more details on what is possible. ## Available Functions `warnings.``warn`(*message*, *category=None*, *stacklevel=1*, *source=None*)Issue a warning, or maybe ignore it or raise an exception. The *category*argument, if given, must be a warning category class (see above); it defaults to [`UserWarning`](exceptions.xhtml#UserWarning "UserWarning"). Alternatively *message* can be a [`Warning`](exceptions.xhtml#Warning "Warning") instance, in which case *category* will be ignored and `message.__class__` will be used. In this case the message text will be `str(message)`. This function raises an exception if the particular warning issued is changed into an error by the warnings filter see above. The *stacklevel* argument can be used by wrapper functions written in Python, like this: ``` def deprecation(message): warnings.warn(message, DeprecationWarning, stacklevel=2) ``` This makes the warning refer to `deprecation()`'s caller, rather than to the source of `deprecation()` itself (since the latter would defeat the purpose of the warning message). *source*, if supplied, is the destroyed object which emitted a [`ResourceWarning`](exceptions.xhtml#ResourceWarning "ResourceWarning"). 在 3.6 版更改: Added *source* parameter. `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 of [`warn()`](#warnings.warn "warnings.warn"), passing in explicitly the message, category, filename and line number, and optionally the module 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 and *category* a subclass of [`Warning`](exceptions.xhtml#Warning "Warning") or *message* may be a [`Warning`](exceptions.xhtml#Warning "Warning") instance, in which case *category* will be ignored. *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). *source*, if supplied, is the destroyed object which emitted a [`ResourceWarning`](exceptions.xhtml#ResourceWarning "ResourceWarning"). 在 3.6 版更改: Add the *source* parameter. `warnings.``showwarning`(*message*, *category*, *filename*, *lineno*, *file=None*, *line=None*)Write a warning to a file. The default implementation calls `formatwarning(message, category, filename, lineno, line)` and writes the resulting string to *file*, which defaults to `sys.stderr`. You may replace this function with any callable by assigning to `warnings.showwarning`. *line* is a line of source code to be included in the warning message; if *line* is not supplied, [`showwarning()`](#warnings.showwarning "warnings.showwarning") will try to read the line specified by *filename* and *lineno*. `warnings.``formatwarning`(*message*, *category*, *filename*, *lineno*, *line=None*)Format a warning the standard way. This returns a string which may contain embedded newlines and ends in a newline. *line* is a line of source code to be included in the warning message; if *line* is not supplied, [`formatwarning()`](#warnings.formatwarning "warnings.formatwarning") will try to read the line specified by *filename* and *lineno*. `warnings.``filterwarnings`(*action*, *message=''*, *category=Warning*, *module=''*, *lineno=0*, *append=False*)Insert an entry into the list of [warnings filter specifications](#warning-filter). The entry is inserted at the front by default; if *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. `warnings.``simplefilter`(*action*, *category=Warning*, *lineno=0*, *append=False*)Insert a simple entry into the list of [warnings filter specifications](#warning-filter). The meaning of the function parameters is as for [`filterwarnings()`](#warnings.filterwarnings "warnings.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. `warnings.``resetwarnings`()Reset the warnings filter. This discards the effect of all previous calls to [`filterwarnings()`](#warnings.filterwarnings "warnings.filterwarnings"), including that of the [`-W`](../using/cmdline.xhtml#cmdoption-w) command line options and calls to [`simplefilter()`](#warnings.simplefilter "warnings.simplefilter"). ## Available Context Managers *class* `warnings.``catch_warnings`(*\**, *record=False*, *module=None*)A context manager that copies and, upon exit, restores the warnings filter and the [`showwarning()`](#warnings.showwarning "warnings.showwarning") function. If the *record* argument is [`False`](constants.xhtml#False "False") (the default) the context manager returns [`None`](constants.xhtml#None "None") on entry. If *record* is [`True`](constants.xhtml#True "True"), a list is returned that is progressively populated with objects as seen by a custom [`showwarning()`](#warnings.showwarning "warnings.showwarning") function (which also suppresses output to `sys.stdout`). Each object in the list has attributes with the same names as the arguments to [`showwarning()`](#warnings.showwarning "warnings.showwarning"). The *module* argument takes a module that will be used instead of the module returned when you import [`warnings`](#module-warnings "warnings: Issue warning messages and control their disposition.") whose filter will be protected. This argument exists primarily for testing the [`warnings`](#module-warnings "warnings: Issue warning messages and control their disposition.")module itself. 注解 The [`catch_warnings`](#warnings.catch_warnings "warnings.catch_warnings") manager works by replacing and then later restoring the module's [`showwarning()`](#warnings.showwarning "warnings.showwarning") function and internal list of filter specifications. This means the context manager is modifying global state and therefore is not thread-safe. ### 导航 - [索引](../genindex.xhtml "总目录") - [模块](../py-modindex.xhtml "Python 模块索引") | - [下一页](dataclasses.xhtml "dataclasses --- 数据类") | - [上一页](__main__.xhtml "__main__ --- 顶层脚本环境") | -  - [Python](https://www.python.org/) » - zh\_CN 3.7.3 [文档](../index.xhtml) » - [Python 标准库](index.xhtml) » - [Python运行时服务](python.xhtml) » - $('.inline-search').show(0); | © [版权所有](../copyright.xhtml) 2001-2019, Python Software Foundation. Python 软件基金会是一个非盈利组织。 [请捐助。](https://www.python.org/psf/donations/) 最后更新于 5月 21, 2019. [发现了问题](../bugs.xhtml)？ 使用[Sphinx](http://sphinx.pocoo.org/)1.8.4 创建。