author | bugtracker-ml@nttdocomo.com |
Sun, 04 Jul 2010 21:21:52 +0100 | |
changeset 84 | 05f4463787cf |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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:mod:`warnings` --- Warning control |
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=================================== |
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.. index:: single: warnings |
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.. module:: warnings |
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:synopsis: Issue warning messages and control their disposition. |
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.. versionadded:: 2.1 |
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Warning messages are typically issued in situations where it is useful to alert |
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the user of some condition in a program, where that condition (normally) doesn't |
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warrant raising an exception and terminating the program. For example, one |
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might want to issue a warning when a program uses an obsolete module. |
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Python programmers issue warnings by calling the :func:`warn` function defined |
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in this module. (C programmers use :cfunc:`PyErr_WarnEx`; see |
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:ref:`exceptionhandling` for details). |
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Warning messages are normally written to ``sys.stderr``, but their disposition |
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can be changed flexibly, from ignoring all warnings to turning them into |
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exceptions. The disposition of warnings can vary based on the warning category |
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(see below), the text of the warning message, and the source location where it |
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is issued. Repetitions of a particular warning for the same source location are |
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typically suppressed. |
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There are two stages in warning control: first, each time a warning is issued, a |
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determination is made whether a message should be issued or not; next, if a |
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message is to be issued, it is formatted and printed using a user-settable hook. |
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The determination whether to issue a warning message is controlled by the |
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warning filter, which is a sequence of matching rules and actions. Rules can be |
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added to the filter by calling :func:`filterwarnings` and reset to its default |
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state by calling :func:`resetwarnings`. |
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The printing of warning messages is done by calling :func:`showwarning`, which |
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may be overridden; the default implementation of this function formats the |
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message by calling :func:`formatwarning`, which is also available for use by |
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custom implementations. |
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.. _warning-categories: |
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Warning Categories |
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------------------ |
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There are a number of built-in exceptions that represent warning categories. |
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This categorization is useful to be able to filter out groups of warnings. The |
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following warnings category classes are currently defined: |
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+----------------------------------+-----------------------------------------------+ |
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| Class | Description | |
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+==================================+===============================================+ |
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| :exc:`Warning` | This is the base class of all warning | |
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| | category classes. It is a subclass of | |
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| | :exc:`Exception`. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`UserWarning` | The default category for :func:`warn`. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`DeprecationWarning` | Base category for warnings about deprecated | |
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| | features. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`SyntaxWarning` | Base category for warnings about dubious | |
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| | syntactic features. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`RuntimeWarning` | Base category for warnings about dubious | |
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| | runtime features. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`FutureWarning` | Base category for warnings about constructs | |
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| | that will change semantically in the future. | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`PendingDeprecationWarning` | Base category for warnings about features | |
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| | that will be deprecated in the future | |
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| | (ignored by default). | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`ImportWarning` | Base category for warnings triggered during | |
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| | the process of importing a module (ignored by | |
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| | default). | |
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+----------------------------------+-----------------------------------------------+ |
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| :exc:`UnicodeWarning` | Base category for warnings related to | |
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| | Unicode. | |
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+----------------------------------+-----------------------------------------------+ |
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While these are technically built-in exceptions, they are documented here, |
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because conceptually they belong to the warnings mechanism. |
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User code can define additional warning categories by subclassing one of the |
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standard warning categories. A warning category must always be a subclass of |
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the :exc:`Warning` class. |
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.. _warning-filter: |
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The Warnings Filter |
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------------------- |
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The warnings filter controls whether warnings are ignored, displayed, or turned |
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into errors (raising an exception). |
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101 |
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Conceptually, the warnings filter maintains an ordered list of filter |
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specifications; any specific warning is matched against each filter |
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specification in the list in turn until a match is found; the match determines |
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the disposition of the match. Each entry is a tuple of the form (*action*, |
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*message*, *category*, *module*, *lineno*), where: |
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107 |
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|
108 |
* *action* is one of the following strings: |
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|
109 |
|
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|
110 |
+---------------+----------------------------------------------+ |
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|
111 |
| Value | Disposition | |
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|
112 |
+===============+==============================================+ |
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|
113 |
| ``"error"`` | turn matching warnings into exceptions | |
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|
114 |
+---------------+----------------------------------------------+ |
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|
115 |
| ``"ignore"`` | never print matching warnings | |
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|
116 |
+---------------+----------------------------------------------+ |
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|
117 |
| ``"always"`` | always print matching warnings | |
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|
118 |
+---------------+----------------------------------------------+ |
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|
119 |
| ``"default"`` | print the first occurrence of matching | |
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|
120 |
| | warnings for each location where the warning | |
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|
121 |
| | is issued | |
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122 |
+---------------+----------------------------------------------+ |
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123 |
| ``"module"`` | print the first occurrence of matching | |
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|
124 |
| | warnings for each module where the warning | |
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|
125 |
| | is issued | |
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|
126 |
+---------------+----------------------------------------------+ |
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|
127 |
| ``"once"`` | print only the first occurrence of matching | |
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|
128 |
| | warnings, regardless of location | |
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|
129 |
+---------------+----------------------------------------------+ |
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|
130 |
|
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131 |
* *message* is a string containing a regular expression that the warning message |
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132 |
must match (the match is compiled to always be case-insensitive) |
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|
133 |
|
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|
134 |
* *category* is a class (a subclass of :exc:`Warning`) of which the warning |
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|
135 |
category must be a subclass in order to match |
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|
136 |
|
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|
137 |
* *module* is a string containing a regular expression that the module name must |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
138 |
match (the match is compiled to be case-sensitive) |
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|
139 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
140 |
* *lineno* is an integer that the line number where the warning occurred must |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
141 |
match, or ``0`` to match all line numbers |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
142 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
143 |
Since the :exc:`Warning` class is derived from the built-in :exc:`Exception` |
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|
144 |
class, to turn a warning into an error we simply raise ``category(message)``. |
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|
145 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
146 |
The warnings filter is initialized by :option:`-W` options passed to the Python |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
147 |
interpreter command line. The interpreter saves the arguments for all |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
148 |
:option:`-W` options without interpretation in ``sys.warnoptions``; the |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
149 |
:mod:`warnings` module parses these when it is first imported (invalid options |
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|
150 |
are ignored, after printing a message to ``sys.stderr``). |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
151 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
152 |
The warnings that are ignored by default may be enabled by passing :option:`-Wd` |
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|
153 |
to the interpreter. This enables default handling for all warnings, including |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
154 |
those that are normally ignored by default. This is particular useful for |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
155 |
enabling ImportWarning when debugging problems importing a developed package. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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|
156 |
ImportWarning can also be enabled explicitly in Python code using:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
157 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
158 |
warnings.simplefilter('default', ImportWarning) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
159 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
160 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
161 |
.. _warning-suppress: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
162 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
163 |
Temporarily Suppressing Warnings |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
164 |
-------------------------------- |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
165 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
166 |
If you are using code that you know will raise a warning, such as a deprecated |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
167 |
function, but do not want to see the warning, then it is possible to suppress |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
168 |
the warning using the :class:`catch_warnings` context manager:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
169 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
170 |
import warnings |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
171 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
172 |
def fxn(): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
173 |
warnings.warn("deprecated", DeprecationWarning) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
174 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
175 |
with warnings.catch_warnings(): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
176 |
warnings.simplefilter("ignore") |
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|
177 |
fxn() |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
178 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
179 |
While within the context manager all warnings will simply be ignored. This |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
180 |
allows you to use known-deprecated code without having to see the warning while |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
181 |
not suppressing the warning for other code that might not be aware of its use |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
182 |
of deprecated code. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
183 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
184 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
185 |
.. _warning-testing: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
186 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
187 |
Testing Warnings |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
188 |
---------------- |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
189 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
190 |
To test warnings raised by code, use the :class:`catch_warnings` context |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
191 |
manager. With it you can temporarily mutate the warnings filter to facilitate |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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diff
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|
192 |
your testing. For instance, do the following to capture all raised warnings to |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
193 |
check:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
194 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
195 |
import warnings |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
196 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
197 |
def fxn(): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
198 |
warnings.warn("deprecated", DeprecationWarning) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
199 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
200 |
with warnings.catch_warnings(record=True) as w: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
201 |
# Cause all warnings to always be triggered. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
202 |
warnings.simplefilter("always") |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
203 |
# Trigger a warning. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
204 |
fxn() |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
205 |
# Verify some things |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
206 |
assert len(w) == 1 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
207 |
assert isinstance(w[-1].category, DeprecationWarning) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
208 |
assert "deprecated" in str(w[-1].message) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
209 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
210 |
One can also cause all warnings to be exceptions by using ``error`` instead of |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
211 |
``always``. One thing to be aware of is that if a warning has already been |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
212 |
raised because of a ``once``/``default`` rule, then no matter what filters are |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
213 |
set the warning will not be seen again unless the warnings registry related to |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
214 |
the warning has been cleared. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
215 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
216 |
Once the context manager exits, the warnings filter is restored to its state |
2fb8b9db1c86
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when the context was entered. This prevents tests from changing the warnings |
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filter in unexpected ways between tests and leading to indeterminate test |
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results. The :func:`showwarning` function in the module is also restored to |
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its original value. |
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|
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When testing multiple operations that raise the same kind of warning, it |
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is important to test them in a manner that confirms each operation is raising |
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a new warning (e.g. set warnings to be raised as exceptions and check the |
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operations raise exceptions, check that the length of the warning list |
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continues to increase after each operation, or else delete the previous |
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entries from the warnings list before each new operation). |
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|
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|
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.. _warning-functions: |
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231 |
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Available Functions |
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------------------- |
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|
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.. function:: warn(message[, category[, stacklevel]]) |
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Issue a warning, or maybe ignore it or raise an exception. The *category* |
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argument, if given, must be a warning category class (see above); it defaults to |
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:exc:`UserWarning`. Alternatively *message* can be a :exc:`Warning` instance, |
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in which case *category* will be ignored and ``message.__class__`` will be used. |
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In this case the message text will be ``str(message)``. This function raises an |
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exception if the particular warning issued is changed into an error by the |
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warnings filter see above. The *stacklevel* argument can be used by wrapper |
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functions written in Python, like this:: |
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def deprecation(message): |
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warnings.warn(message, DeprecationWarning, stacklevel=2) |
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|
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This makes the warning refer to :func:`deprecation`'s caller, rather than to the |
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source of :func:`deprecation` itself (since the latter would defeat the purpose |
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of the warning message). |
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253 |
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254 |
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.. function:: warn_explicit(message, category, filename, lineno[, module[, registry[, module_globals]]]) |
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256 |
|
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This is a low-level interface to the functionality of :func:`warn`, passing in |
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explicitly the message, category, filename and line number, and optionally the |
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module name and the registry (which should be the ``__warningregistry__`` |
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dictionary of the module). The module name defaults to the filename with |
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``.py`` stripped; if no registry is passed, the warning is never suppressed. |
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*message* must be a string and *category* a subclass of :exc:`Warning` or |
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*message* may be a :exc:`Warning` instance, in which case *category* will be |
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|
264 |
ignored. |
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|
265 |
|
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*module_globals*, if supplied, should be the global namespace in use by the code |
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for which the warning is issued. (This argument is used to support displaying |
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source for modules found in zipfiles or other non-filesystem import |
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|
269 |
sources). |
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|
270 |
|
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.. versionchanged:: 2.5 |
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|
272 |
Added the *module_globals* parameter. |
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|
273 |
|
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274 |
|
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.. function:: warnpy3k(message[, category[, stacklevel]]) |
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276 |
|
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277 |
Issue a warning related to Python 3.x deprecation. Warnings are only shown |
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|
278 |
when Python is started with the -3 option. Like :func:`warn` *message* must |
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|
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be a string and *category* a subclass of :exc:`Warning`. :func:`warnpy3k` |
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|
280 |
is using :exc:`DeprecationWarning` as default warning class. |
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|
281 |
|
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282 |
|
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283 |
.. function:: showwarning(message, category, filename, lineno[, file[, line]]) |
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|
284 |
|
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285 |
Write a warning to a file. The default implementation calls |
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|
286 |
``formatwarning(message, category, filename, lineno, line)`` and writes the |
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|
287 |
resulting string to *file*, which defaults to ``sys.stderr``. You may replace |
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|
288 |
this function with an alternative implementation by assigning to |
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|
289 |
``warnings.showwarning``. |
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|
290 |
*line* is a line of source code to be included in the warning |
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|
291 |
message; if *line* is not supplied, :func:`showwarning` will |
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|
292 |
try to read the line specified by *filename* and *lineno*. |
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|
293 |
|
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294 |
.. versionchanged:: 2.6 |
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|
295 |
Added the *line* argument. Implementations that lack the new argument |
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|
296 |
will trigger a :exc:`DeprecationWarning`. |
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|
297 |
|
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298 |
|
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.. function:: formatwarning(message, category, filename, lineno[, line]) |
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|
300 |
|
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301 |
Format a warning the standard way. This returns a string which may contain |
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|
302 |
embedded newlines and ends in a newline. *line* is |
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|
303 |
a line of source code to be included in the warning message; if *line* is not supplied, |
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|
304 |
:func:`formatwarning` will try to read the line specified by *filename* and *lineno*. |
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|
305 |
|
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|
306 |
.. versionchanged:: 2.6 |
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|
307 |
Added the *line* argument. |
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|
308 |
|
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|
309 |
|
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|
310 |
.. function:: filterwarnings(action[, message[, category[, module[, lineno[, append]]]]]) |
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|
311 |
|
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|
312 |
Insert an entry into the list of warnings filters. The entry is inserted at the |
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|
313 |
front by default; if *append* is true, it is inserted at the end. This checks |
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|
314 |
the types of the arguments, compiles the message and module regular expressions, |
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|
315 |
and inserts them as a tuple in the list of warnings filters. Entries closer to |
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|
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the front of the list override entries later in the list, if both match a |
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|
317 |
particular warning. Omitted arguments default to a value that matches |
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|
318 |
everything. |
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|
319 |
|
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|
320 |
|
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|
321 |
.. function:: simplefilter(action[, category[, lineno[, append]]]) |
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|
322 |
|
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Insert a simple entry into the list of warnings filters. The meaning of the |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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function parameters is as for :func:`filterwarnings`, but regular expressions |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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are not needed as the filter inserted always matches any message in any module |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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as long as the category and line number match. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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Initial QEMU (symbian-qemu-0.9.1-12) import
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328 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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.. function:: resetwarnings() |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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Reset the warnings filter. This discards the effect of all previous calls to |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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:func:`filterwarnings`, including that of the :option:`-W` command line options |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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and calls to :func:`simplefilter`. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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Available Context Managers |
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-------------------------- |
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.. class:: catch_warnings([\*, record=False, module=None]) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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A context manager that copies and, upon exit, restores the warnings filter |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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and the :func:`showwarning` function. |
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If the *record* argument is :const:`False` (the default) the context manager |
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returns :class:`None` on entry. If *record* is :const:`True`, a list is |
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returned that is progressively populated with objects as seen by a custom |
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:func:`showwarning` function (which also suppresses output to ``sys.stdout``). |
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Each object in the list has attributes with the same names as the arguments to |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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:func:`showwarning`. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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The *module* argument takes a module that will be used instead of the |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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module returned when you import :mod:`warnings` whose filter will be |
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protected. This argument exists primarily for testing the :mod:`warnings` |
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module itself. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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Initial QEMU (symbian-qemu-0.9.1-12) import
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.. note:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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In Python 3.0, the arguments to the constructor for |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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:class:`catch_warnings` are keyword-only arguments. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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Initial QEMU (symbian-qemu-0.9.1-12) import
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.. versionadded:: 2.6 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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