warnings — 警告控制 ¶

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 , 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 类。

The following warnings category classes are currently defined:

警告过滤 ¶

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 , 模块 , lineno ), where:

• action is one of the following strings:

  值	Disposition
  "default"	print the first occurrence of matching warnings 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 matching warnings for each module where the warning is issued (regardless of line number)
  "once"	print only the first occurrence of matching warnings, regardless of location

• 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 ) of which the warning category must be a subclass in order to match.

• 模块 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.

由于 Warning class is derived from the built-in 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).

action:message:category:module:line
									

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"
									

default::DeprecationWarning:__main__
ignore::DeprecationWarning
ignore::PendingDeprecationWarning
ignore::ImportWarning
ignore::ResourceWarning
									

import sys
if not sys.warnoptions:
    import warnings
    warnings.simplefilter("ignore")
									

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
									

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

临时抑制警告 ¶

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 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.

测试警告 ¶

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 而不是 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).

为新版本的依赖更新代码 ¶

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 (如 -W error ). See the -W flag for more details on what is possible.

可用函数 ¶

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 。 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 .

3.6 版改变： 添加 source 参数。

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 可以是 Warning instance, in which case category 将被忽略。

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 .

3.6 版改变： Add the source 参数。

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 ，其默认为 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 模块 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() .

可用上下文管理器 ¶

class warnings. catch_warnings ( * , record=False , module=None ) ¶

A context manager that copies and, upon exit, restores the warnings filter and the showwarning() function. If the record 自变量为 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() .

模块 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.

注意

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.