warnings
— Warning control¶
Source code: 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()
; see
Exception Handling 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()
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.
See also
logging.captureWarnings()
allows you to handle all warnings with
the standard logging infrastructure.
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 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
class.
The following warnings category classes are currently defined:
Class |
Description |
---|---|
This is the base class of all warning
category classes. It is a subclass of
|
|
The default category for |
|
Base category for warnings about deprecated
features when those warnings are intended for
other Python developers (ignored by default,
unless triggered by code in |
|
Base category for warnings about dubious syntactic features. |
|
Base category for warnings about dubious runtime features. |
|
Base category for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python. |
|
Base category for warnings about features that will be deprecated in the future (ignored by default). |
|
Base category for warnings triggered during the process of importing a module (ignored by default). |
|
Base category for warnings related to Unicode. |
|
Base category for warnings related to resource usage. |
Changed in version 3.7: 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.
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:
Value
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.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
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).
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 The Warnings Filter.
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"
# and any subpackages of "mymodule"
Default Warning Filter¶
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 debug builds, the list of default warning filters is empty.
Changed in version 3.2: DeprecationWarning
is now ignored by default in addition to
PendingDeprecationWarning
.
Changed in version 3.7: DeprecationWarning
is once again shown by default when triggered
directly by code in __main__
.
Changed in version 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 (see above); it defaults to
UserWarning
. Alternatively message can be aWarning
instance, in which case category will be ignored andmessage.__class__
will be used. In this case the message text will bestr(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 ofdeprecation()
itself (since the latter would defeat the purpose of the warning message).source, if supplied, is the destroyed object which emitted a
ResourceWarning
.Changed in version 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 ofWarning
or message may be aWarning
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
.Changed in version 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 tosys.stderr
. You may replace this function with any callable by assigning towarnings.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 tosimplefilter()
.
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()
function. If the record argument isFalse
(the default) the context manager returnsNone
on entry. If record isTrue
, a list is returned 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()
.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 thewarnings
module itself.Note
The
catch_warnings
manager works by replacing and then later restoring the module’sshowwarning()
function and internal list of filter specifications. This means the context manager is modifying global state and therefore is not thread-safe.