# `warnings` — Controle de avisos

Código-fonte: Lib/warnings.py

As mensagens de aviso são normalmente emitidas em situações em que é útil alertar o usuário sobre alguma condição em um programa, onde essa condição (normalmente) não garante o levantamento de uma exceção e o encerramento do programa. Por exemplo, pode-se querer emitir um aviso quando um programa usa um módulo obsoleto.

Os programadores Python emitem avisos chamando a função `warn()` definida neste módulo. (Os programadores C usam `PyErr_WarnEx()`; veja Manipulando Exceções para detalhes).

Mensagens de aviso são normalmente escritas no `sys.stderr`, mas sua disposição pode ser alterada de forma flexível, desde ignorar todos os avisos até transformá-los em exceções. A disposição dos avisos pode variar de acordo com categoria de aviso, o texto da mensagem de aviso e o local de origem onde ela é emitida. As repetições de um aviso específico para o mesmo local de origem são normalmente suprimidas.

Existem duas etapas no controle de avisos: primeiro, cada vez que um aviso é emitido, é feita uma determinação se uma mensagem deve ser emitida ou não; a seguir, se uma mensagem deve ser emitida, ela é formatada e impressa usando um gancho configurável pelo usuário.

A determinação de emitir ou não uma mensagem de aviso é controlada pelo filtro de aviso, que é uma sequência de regras e ações correspondentes. As regras podem ser adicionadas ao filtro chamando `filterwarnings()` e redefinidas para seu estado padrão chamando `resetwarnings()`.

A exibição de mensagens de aviso é feita chamando `showwarning()`, que pode ser substituída; a implementação padrão desta função formata a mensagem chamando `formatwarning()`, que também está disponível para uso por implementações personalizadas.

Ver também

`logging.captureWarnings()` permite que você manipule todos os avisos com a infraestrutura de registro padrão.

## Categorias de avisos

Existem várias exceções embutidas que representam categorias de aviso. Essa categorização é útil para filtrar grupos de avisos.

Embora sejam tecnicamente exceções embutidas, elas são documentadas aqui, porque conceitualmente pertencem ao mecanismo de avisos.

O código do usuário pode definir categorias de aviso adicionais criando uma subclasse de uma das categorias de aviso padrão. Uma categoria de aviso deve ser sempre uma subclasse da classe `Warning`.

As seguintes classes de categorias de avisos estão definidas atualmente:

Classe | Descrição
--- | ---
`Warning` | Esta é a classe base de todas as classes de categoria de aviso. É uma subclasse de `Exception`.
`UserWarning` | A categoria padrão para `warn()`.
`DeprecationWarning` | Categoria base para avisos sobre recursos descontinuados quando esses avisos são destinados a outros desenvolvedores Python (ignorado por padrão, a menos que acionado por código em `__main__`).
`SyntaxWarning` | Categoria base para avisos sobre recursos sintáticos duvidosos.
`RuntimeWarning` | Categoria base para avisos sobre recursos duvidosos de tempo de execução.
`FutureWarning` | Categoria base para avisos sobre recursos descontinuados quando esses avisos se destinam a usuários finais de aplicações escritas em Python.
`PendingDeprecationWarning` | Categoria base para avisos sobre recursos que serão descontinuados no futuro (ignorados por padrão).
`ImportWarning` | Categoria base para avisos acionados durante o processo de importação de um módulo (ignorado por padrão).
`UnicodeWarning` | Categoria base para avisos relacionados a Unicode.
`BytesWarning` | Categoria base para avisos relacionados a `bytes` e `bytearray`.
`ResourceWarning` | Base category for warnings related to resource usage (ignored by default).

Alterado na versão 3.7: Anteriormente, `DeprecationWarning` e `FutureWarning` eram diferenciadas com base em se um recurso estava sendo removido completamente ou mudando seu comportamento. Elas agora são diferenciadas com base em seu público-alvo e na maneira como são tratadas pelos filtros de avisos padrão.

## O filtro de avisos

O filtro de avisos controla se os avisos são ignorados, exibidos ou transformados em erros (levantando uma exceção).

Conceitualmente, o filtro de avisos mantém uma lista ordenada de especificações de filtro; qualquer aviso específico é comparado com cada especificação de filtro na lista, por sua vez, até que uma correspondência seja encontrada; o filtro determina a disposição da correspondência. Cada entrada é uma tupla no formato (action, message, category, module, lineno), sendo:

- action é uma das seguintes strings:

  Valor | Disposição
  --- | ---
  `"default"` | exibe a primeira ocorrência de avisos correspondentes para cada local (módulo + número da linha) onde o aviso é emitido
  `"error"` | transforma avisos correspondentes em exceções
  `"ignore"` | nunca exibe avisos correspondentes
  `"always"` | sempre exibe avisos correspondentes
  `"module"` | exibe a primeira ocorrência de avisos correspondentes para cada módulo onde o aviso é emitido (independentemente do número da linha)
  `"once"` | exibe apenas a primeira ocorrência de avisos correspondentes, independentemente da localização

- message is a string containing a regular expression that the start of the warning message must match, case-insensitively. In `-W` and `PYTHONWARNINGS`, message is a literal string that the start of the warning message must contain (case-insensitively), ignoring any whitespace at the start or end of message.

- category é uma classe (uma subclasse de `Warning`) da qual a categoria de aviso deve ser uma subclasse para corresponder.

- module is a string containing a regular expression that the start of the fully qualified module name must match, case-sensitively. In `-W` and `PYTHONWARNINGS`, module is a literal string that the fully qualified module name must be equal to (case-sensitively), ignoring any whitespace at the start or end of module.

- lineno é um número inteiro que deve corresponder ao número da linha onde ocorreu o aviso, ou `0` para corresponder a todos os números de linha.

Como a classe `Warning` é derivada da classe embutida `Exception`, para transformar um aviso em um erro, simplesmente levantamos `category(message)`.

Se um aviso for relatado e não corresponder a nenhum filtro registrado, a ação “padrão” será aplicada (daí seu nome).

### Describing Warning Filters

The warnings filter is initialized by `-W` options passed to the Python interpreter command line and the `PYTHONWARNINGS` environment variable. The interpreter saves the arguments for all supplied entries without interpretation in `sys.warnoptions`; the `warnings` 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 O filtro de avisos. When listing multiple filters on a single line (as for `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"

### Filtro de avisos padrão

By default, Python installs several warning filters, which can be overridden by the `-W` command-line option, the `PYTHONWARNINGS` environment variable and calls to `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 a debug build, the list of default warning filters is empty.

Alterado na versão 3.2: `DeprecationWarning` is now ignored by default in addition to `PendingDeprecationWarning`.

Alterado na versão 3.7: `DeprecationWarning` is once again shown by default when triggered directly by code in `__main__`.

Alterado na versão 3.7: `BytesWarning` no longer appears in the default filter list and is instead configured via `sys.warnoptions` when `-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` 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` 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__"))

## Temporarily Suppressing Warnings

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` 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` context manager at the same time, the behavior is undefined.

## Testing Warnings

To test warnings raised by code, use the `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()` 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` 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` (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` module does this).

In less ideal cases, applications can be checked for use of deprecated interfaces by passing `-Wd` 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` (e.g. `-W error`). See the `-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; it defaults to `UserWarning`. Alternatively, message can be a `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. 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`.

  Alterado na versão 3.6: 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()`, 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` or message may be a `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`.

  Alterado na versão 3.6: 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()` 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()` 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. 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. The meaning of the function parameters is as for `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()`, including that of the `-W` command line options and calls to `simplefilter()`.

## Available Context Managers

class warnings.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 filter and the `showwarning()` function. If the record argument is `False` (the default) the context manager returns `None` on entry. If record is `True`, a list is returned that is progressively populated with objects as seen by a custom `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()`.

  The module argument takes a module that will be used instead of the module returned when you import `warnings` whose filter will be protected. This argument exists primarily for testing the `warnings` module itself.

  If the action argument is not `None`, the remaining arguments are passed to `simplefilter()` as if it were called immediately on entering the context.

  Nota

  The `catch_warnings` manager works by replacing and then later restoring the module’s `showwarning()` function and internal list of filter specifications. This means the context manager is modifying global state and therefore is not thread-safe.

  Alterado na versão 3.11: Added the action, category, lineno, and append parameters.