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Provides configuration-related objects.
Cedar Backup stores all of its configuration in an XML document 
    typically called cback.conf.  The standard location for 
    this document is in /etc, but users can specify a 
    different location if they want to.
The Config class is a Python object representation of a
    Cedar Backup XML configuration file.  The representation is two-way: 
    XML data can be used to create a Config object, and then 
    changes to the object can be propogated back to disk.  A 
    Config object can even be used to create a configuration 
    file from scratch programmatically.
The Config class is intended to be the only 
    Python-language interface to Cedar Backup configuration on disk.  Cedar
    Backup will use the class as its internal representation of 
    configuration, and applications external to Cedar Backup itself (such 
    as a hypothetical third-party configuration tool written in Python or a
    third party extension module) should also use the class when they need 
    to read and write configuration files.
The configuration file format has changed between Cedar Backup 1.x and Cedar Backup 2.x. Any Cedar Backup 1.x configuration file is also a valid Cedar Backup 2.x configuration file. However, it doesn't work to go the other direction, as the 2.x configuration files contains additional configuration is not accepted by older versions of the software.
A Config object can either be created 
    "empty", or can be created based on XML input (either in the 
    form of a string or read in from a file on disk). Generally speaking, 
    the XML input must result in a Config object which 
    passes the validations laid out below in the Validation 
    section.
An XML configuration file is composed of seven sections:
Each section is represented by an class in this module, and then the
    overall Config class is a composition of the various other
    classes.
Any configuration section that is missing in the XML document (or 
    has not been filled into an "empty" document) will just be 
    set to None in the object representation.  The same goes 
    for individual fields within each configuration section.  Keep in mind 
    that the document might not be completely valid if some sections or 
    fields aren't filled in - but that won't matter until validation takes 
    place (see the Validation section below).
By default, all string data that comes out of XML documents in 
    Python is unicode data (i.e. u"whatever").  This
    is fine for many things, but when it comes to filesystem paths, it can 
    cause us some problems.  We really want strings to be encoded in the 
    filesystem encoding rather than being unicode. So, most elements in 
    configuration which represent filesystem paths are coverted to plain 
    strings using util.encodePath.  The main exception is the various 
    absoluteExcludePath and relativeExcludePath 
    lists.  These are not converted, because they are generally only
    used for filtering, not for filesystem operations.
There are two main levels of validation in the Config 
    class and its children.  The first is field-level validation.  
    Field-level validation comes into play when a given field in an object 
    is assigned to or updated. We use Python's property 
    functionality to enforce specific validations on field values, and in 
    some places we even use customized list classes to enforce validations 
    on list members.  You should expect to catch a ValueError 
    exception when making assignments to configuration class fields.
The second level of validation is post-completion validation. Certain validations don't make sense until a document is fully "complete". We don't want these validations to apply all of the time, because it would make building up a document from scratch a real pain. For instance, we might have to do things in the right order to keep from throwing exceptions, etc.
All of these post-completion validations are encapsulated in the Config.validate method.  This method can be called at 
    any time by a client, and will always be called immediately after 
    creating a Config object from XML data and before 
    exporting a Config object to XML.  This way, we get decent
    ease-of-use but we also don't accept or emit invalid configuration 
    files.
The Config.validate implementation actually takes two passes to completely validate a configuration document. The first pass at validation is to ensure that the proper sections are filled into the document. There are default requirements, but the caller has the opportunity to override these defaults.
The second pass at validation ensures that any filled-in section 
    contains valid data.  Any section which is not set to None
    is validated according to the rules for that section (see below).
Reference Validations
No validations.
Extensions Validations
The list of actions may be either None or an empty list
    [] if desired. Each extended action must include a name, a
    module and a function.  Then, an extended action must include either an
    index or dependency information. Which one is required depends on which
    order mode is configured.
Options Validations
All fields must be filled in except the rsh command. The rcp and rsh commands are used as default values for all remote peers. Remote peers can also rely on the backup user as the default remote user name if they choose.
Peers Validations
Local peers must be completely filled in, including both name and collect directory. Remote peers must also fill in the name and collect directory, but can leave the remote user and rcp command unset. In this case, the remote user is assumed to match the backup user from the options section and rcp command is taken directly from the options section.
Collect Validations
The target directory must be filled in.  The collect mode, archive 
    mode and ignore file are all optional.  The list of absolute paths to 
    exclude and patterns to exclude may be either None or an 
    empty list [] if desired.
Each collect directory entry must contain an absolute path to 
    collect, and then must either be able to take collect mode, archive 
    mode and ignore file configuration from the parent 
    CollectConfig object, or must set each value on its own.  
    The list of absolute paths to exclude, relative paths to exclude and 
    patterns to exclude may be either None or an empty list 
    [] if desired.  Any list of absolute paths to exclude or 
    patterns to exclude will be combined with the same list in the 
    CollectConfig object to make the complete list for a given
    directory.
Stage Validations
The target directory must be filled in.  There must be at least one 
    peer (remote or local) between the two lists of peers.  A list with no 
    entries can be either None or an empty list 
    [] if desired.
If a set of peers is provided, this configuration completely overrides configuration in the peers configuration section, and the same validations apply.
Store Validations
The device type and drive speed are optional, and all other values are required (missing booleans will be set to defaults, which is OK).
The image writer functionality in the writer module is 
    supposed to be able to handle a device speed of None.  Any
    caller which needs a "real" (non-None) value for
    the device type can use DEFAULT_DEVICE_TYPE, which is 
    guaranteed to be sensible.
Purge Validations
The list of purge directories may be either None or an 
    empty list [] if desired.  All purge directories must 
    contain a path and a retain days value.
Author: Kenneth J. Pronovici <pronovic@ieee.org>
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| ActionDependencies Class representing dependencies associated with an extended action. | |||
| ActionHook Class representing a hook associated with an action. | |||
| PreActionHook Class representing a pre-action hook associated with an action. | |||
| PostActionHook Class representing a pre-action hook associated with an action. | |||
| ExtendedAction Class representing an extended action. | |||
| CommandOverride Class representing a piece of Cedar Backup command override configuration. | |||
| CollectFile Class representing a Cedar Backup collect file. | |||
| CollectDir Class representing a Cedar Backup collect directory. | |||
| PurgeDir Class representing a Cedar Backup purge directory. | |||
| LocalPeer Class representing a Cedar Backup peer. | |||
| RemotePeer Class representing a Cedar Backup peer. | |||
| ReferenceConfig Class representing a Cedar Backup reference configuration. | |||
| ExtensionsConfig Class representing Cedar Backup extensions configuration. | |||
| OptionsConfig Class representing a Cedar Backup global options configuration. | |||
| PeersConfig Class representing Cedar Backup global peer configuration. | |||
| CollectConfig Class representing a Cedar Backup collect configuration. | |||
| StageConfig Class representing a Cedar Backup stage configuration. | |||
| StoreConfig Class representing a Cedar Backup store configuration. | |||
| PurgeConfig Class representing a Cedar Backup purge configuration. | |||
| Config Class representing a Cedar Backup XML configuration document. | |||
| ByteQuantity Class representing a byte quantity. | |||
| BlankBehavior Class representing optimized store-action media blanking behavior. | |||
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| DEFAULT_DEVICE_TYPE = The default device type. | |||
| DEFAULT_MEDIA_TYPE = The default media type. | |||
| VALID_DEVICE_TYPES = List of valid device types. | |||
| VALID_MEDIA_TYPES = List of valid media types. | |||
| VALID_COLLECT_MODES = List of valid collect modes. | |||
| VALID_ARCHIVE_MODES = List of valid archive modes. | |||
| VALID_ORDER_MODES = List of valid extension order modes. | |||
| logger = <logging.Logger object> | |||
| VALID_CD_MEDIA_TYPES =  | |||
| VALID_DVD_MEDIA_TYPES =  | |||
| VALID_COMPRESS_MODES = List of valid compress modes. | |||
| VALID_BLANK_MODES =  | |||
| VALID_BYTE_UNITS =  | |||
| VALID_FAILURE_MODES =  | |||
| REWRITABLE_MEDIA_TYPES =  | |||
| ACTION_NAME_REGEX =  | |||
| __package__ =  | |||
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 Read a byte size value from an XML document. A byte size value is an interpreted string value. If the string value ends with "MB" or "GB", then the string before that is interpreted as megabytes or gigabytes. Otherwise, it is intepreted as bytes. 
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 Adds a text node as the next child of a parent, to contain a byte size. If the  The size in bytes will be normalized. If it is larger than 1.0 GB, it will be shown in GB ("1.0 GB"). If it is larger than 1.0 MB ("1.0 MB"), it will be shown in MB. Otherwise, it will be shown in bytes ("423413"). 
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| VALID_MEDIA_TYPESList of valid media types.
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