3.20.2 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 match 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:
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"error" |
turn matching warnings into exceptions |
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"ignore" |
never print matching warnings |
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"always" |
always print matching warnings |
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"default" |
print the first occurrence of matching warnings for
each location where the warning is issued |
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"module" |
print the first occurrence of matching warnings for
each module where the warning is issued |
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"once" |
print only the first occurrence of matching
warnings, regardless of location |
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- message is a string containing a regular expression that
the warning message must match (the match 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 match 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).
The warnings filter is initialized by -W
options passed to the Python interpreter command line. The interpreter
saves the arguments for all -W options 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).
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