warnings
— 警告控制
¶
源代码：
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()
function defined in this module. (C programmers use
PyErr_WarnEx()
；见
异常处理
了解细节)。
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
警告类别
, 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
警告过滤
, which is a sequence of matching rules and actions. Rules can be added to the filter by calling
filterwarnings()
and reset to its default state by calling
resetwarnings()
.
The printing of warning messages is done by calling
showwarning()
, which may be overridden; the default implementation of this function formats the message by calling
formatwarning()
, which is also available for use by custom implementations.
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:
|
类
|
描述
|
|
Warning
|
This is the base class of all warning category classes. It is a subclass of
Exception
.
|
|
UserWarning
|
The default category for
warn()
.
|
|
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
|
Base category for warnings about dubious syntactic features.
|
|
RuntimeWarning
|
Base category for warnings about dubious runtime features.
|
|
FutureWarning
|
Base category for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
|
|
PendingDeprecationWarning
|
Base category for warnings about features that will be deprecated in the future (ignored by default).
|
|
ImportWarning
|
Base category for warnings triggered during the process of importing a module (ignored by default).
|
|
UnicodeWarning
|
Base category for warnings related to Unicode.
|
|
BytesWarning
|
Base category for warnings related to
bytes
and
bytearray
.
|
|
ResourceWarning
|
Base category for warnings related to resource usage (ignored by default).
|
3.7 版改变：
Previously
DeprecationWarning
and
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.
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, 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
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 start of the fully qualified module name must match, case-sensitively. In
-W
and
PYTHONWARNINGS
,
模块
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
模块
.
-
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).
Repeated Warning Suppression Criteria
¶
The filters that suppress repeated warnings apply the following criteria to determine if a warning is considered a repeat:
-
"default"
: A warning is considered a repeat only if the (
message
,
category
,
模块
,
lineno
) are all the same.
-
"module"
: A warning is considered a repeat if the (
message
,
category
,
模块
) are the same, ignoring the line number.
-
"once"
: A warning is considered a repeat if the (
message
,
category
) are the same, ignoring the module and line number.
描述警告过滤
¶
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
；
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
警告过滤
. 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"
默认警告过滤
¶
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
在
调试构建
, the list of default warning filters is empty.
3.2 版改变：
DeprecationWarning
is now ignored by default in addition to
PendingDeprecationWarning
.
3.7 版改变：
DeprecationWarning
is once again shown by default when triggered directly by code in
__main__
.
3.7 版改变：
BytesWarning
no longer appears in the default filter list and is instead configured via
sys.warnoptions
当
-b
is specified twice.
覆写默认过滤
¶
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__"))
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
,
*
,
skip_file_prefixes
=
()
)
¶
-
Issue a warning, or maybe ignore it or raise an exception. The
category
argument, if given, must be a
warning category class
；默认为
UserWarning
。另外，
message
可以是
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 deprecated_api(message):
warnings.warn(message, DeprecationWarning, stacklevel=2)
This makes the warning refer to
deprecated_api
’s caller, rather than to the source of
deprecated_api
itself (since the latter would defeat the purpose of the warning message).
The
skip_file_prefixes
keyword argument can be used to indicate which stack frames are ignored when counting stack levels. This can be useful when you want the warning to always appear at call sites outside of a package when a constant
stacklevel
does not fit all call paths or is otherwise challenging to maintain. If supplied, it must be a tuple of strings. When prefixes are supplied, stacklevel is implicitly overridden to be
max(2,
stacklevel)
. To cause a warning to be attributed to the caller from outside of the current package you might write:
# example/lower.py
_warn_skips = (os.path.dirname(__file__),)
def one_way(r_luxury_yacht=None, t_wobbler_mangrove=None):
if r_luxury_yacht:
warnings.warn("Please migrate to t_wobbler_mangrove=.",
skip_file_prefixes=_warn_skips)
# example/higher.py
from . import lower
def another_way(**kw):
lower.one_way(**kw)
This makes the warning refer to both the
example.lower.one_way()
and
package.higher.another_way()
call sites only from calling code living outside of
example
包。
source
, if supplied, is the destroyed object which emitted a
ResourceWarning
.
3.6 版改变：
添加
source
参数。
3.12 版改变：
添加
skip_file_prefixes
.
-
warnings.
warn_explicit
(
message
,
category
,
filename
,
lineno
,
模块
=
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：
添加
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
=
警告
,
模块
=
''
,
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
=
警告
,
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()
.
-
@
warnings.
弃用
(
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 checkers
will also generate a diagnostic on usage of the deprecated object.
用法：
from warnings import deprecated
from typing import overload
@deprecated("Use B instead")
class A:
pass
@deprecated("Use g instead")
def f():
pass
@overload
@deprecated("int support is deprecated")
def g(x: int) -> int: ...
@overload
def g(x: str) -> int: ...
The warning specified by
category
will be emitted at runtime on use of deprecated objects. For functions, that happens on calls; for classes, on instantiation and on creation of subclasses. If the
category
is
None
, no warning is emitted at runtime. The
stacklevel
determines where the warning is emitted. If it is
1
(the default), the warning is emitted at the direct caller of the deprecated object; if it is higher, it is emitted further up the stack. Static type checker behavior is not affected by the
category
and
stacklevel
自变量。
The deprecation message passed to the decorator is saved in the
__deprecated__
attribute on the decorated object. If applied to an overload, the decorator must be after the
@overload
decorator for the attribute to exist on the overload as returned by
typing.get_overloads()
.
Added in version 3.13:
见
PEP 702
.
可用上下文管理器
¶
-
class
warnings.
catch_warnings
(
*
,
record
=
False
,
模块
=
None
,
action
=
None
,
category
=
警告
,
lineno
=
0
,
append
=
False
)
¶
-
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()
.
The
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.
若
action
argument is not
None
, the remaining arguments are passed to
simplefilter()
as if it were called immediately on entering the context.
见
警告过滤
for the meaning of the
category
and
lineno
参数。
注意
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.
3.11 版改变：
添加
action
,
category
,
lineno
，和
append
参数。
