##
26.4.2 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:

  Value | Disposition
  --- | ---

  `"error"` | turn matching warnings into exceptions

  `"ignore"` | never print matching warnings

  `"always"` | always print matching warnings

  `"default"` | print the first occurrence of matching warnings for each location where the warning is issued

  `"module"` | print the first occurrence of matching warnings for each module where the warning is issued

  `"once"` | print only the first occurrence of matching warnings, regardless of location

- 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`).

The warnings that are ignored by default may be enabled by passing -Wd to the interpreter. This enables default handling for all warnings, including those that are normally ignored by default. This is particular useful for enabling ImportWarning when debugging problems importing a developed package. ImportWarning can also be enabled explicitly in Python code using:

        warnings.simplefilter('default', ImportWarning)