ebmlite 1.1.0

A lightweight, "pure Python" library for parsing EBML (Extensible Binary Markup Language) data.

ebmlite README

ebmlite is a lightweight, "pure Python" library for parsing EBML (Extensible Binary Markup Language) data. It is designed to crawl through EBML files quickly and efficiently, and that's about it. ebmlite can also do basic EBML encoding, but more advanced EBML manipulation (e.g. with a proper DOM) are beyond its scope, and are better left to other libraries.

ebmlite is currently a work-in-progress. It is usable (we use it extensively), but does not (yet) implement the full EBML specification.

Parts of ebmlite were modeled after python-ebml, which we had previously been using, but is not a directly derivative work. ebmlite can import python-ebml schemata XML (to a limited degree), but that is the extent of its cross-compatibility.

EBML Overview (the short version)

EBML (Extensible Binary Markup Language) is a hierarchical tagged binary format, originally created for the Matroska project. The hierarchical structure of EBML bears some conceptual/functional similarity to XML, although the actual structure differs significantly.

In the raw, EBML elements consist of a numeric ID, the size of the element, and a payload. It is space-efficient; the lengths of the ID and size descriptors are variable, using prefix bits to indicate their lengths, a system similar to UTF-8. The mapping of IDs to names and payload data types is done via an external schema.

See the official specification for more information.

EBML Schemata

An EBML file is largely meaningless without a schema that defines its elements. The schema maps element IDs to names and data types; it also describes the structure (e.g. what elements can be children of other elements) and provides additional metadata. Note: ebmlite currently uses the structure for decoding only, and does not stringently enforce it.

ebmlite schemata are defined in XML. From these XML files, a Schema instance is created; within the Schema are Element subclasses for each element defined in the XML. Since the interpretation of an EBML file is almost entirely dependent on a schema, importing of EBML files is done through a Schema instance.

from ebmlite import loadSchema
schema = loadSchema('mide_ide.xml')
doc = schema.load('test_file.ebml')

ebmlite uses its own Schema definition syntax; it can also import python-ebml schemata. It does not (currently) use the official schema format.

Here is an example of an ebmlite schema, showing a simplified version of the definition of the standard EBML header elements:

<?xml version="1.0" encoding="utf-8"?>
<Schema>
    <MasterElement name="EBML" id="0x1A45DFA3" mandatory="1" multiple="0">
        <UIntegerElement name="EBMLVersion" id="0x4286" multiple="0" mandatory="1" />
        <UIntegerElement name="EBMLReadVersion" id="0x42F7" multiple="0" mandatory="1"/>
        <UIntegerElement name="EBMLMaxIDLength" id="0x42F2" multiple="0" mandatory="1"/>
        <UIntegerElement name="EBMLMaxSizeLength" id="0x42F3" multiple="0" mandatory="1"/>
        <StringElement name="DocType" id="0x4282" multiple="0" mandatory="1"/>
        <UIntegerElement name="DocTypeVersion" id="0x4287" multiple="0" mandatory="1"/>
        <BinaryElement name="Void" global="1" id="0xEC" multiple="1"/>
        <BinaryElement name="CRC-32" global="1" id="0xBF" multiple="0"/>
        <MasterElement name="SignatureSlot" global="1" id="0x1B538667" multiple="1">
            <UIntegerElement name="SignatureAlgo" id="0x7E8A" multiple="0"/>
            <UIntegerElement name="SignatureHash" id="0x7E9A" multiple="0"/>
            <BinaryElement name="SignaturePublicKey" id="0x7EA5" multiple="0"/>
            <BinaryElement name="Signature" id="0x7EB5" multiple="0"/>
            <MasterElement name="SignatureElements" id="0x7E5B" multiple="0">
                <MasterElement name="SignatureElementList" id="0x7E7B" multiple="1">
                    <BinaryElement name="SignedElement" id="0x6532" multiple="1"/>
                </MasterElement>
            </MasterElement>
        </MasterElement>
    </MasterElement>
    <!-- More definitions would follow... -->
</Schema>

Each element defined in the schema is a subclass of one of 8 Element base classes:

• MasterElement: An element containing other elements.
• IntegerElement: Contains a signed integer value of variable length.
• UIntegerElement: Contains an unsigned integer value of variable length.
• FloatElement: Contains a 32 or 64 bit floating point value.
• StringElement: Contains printable US-ASCII characters (0x20 to 0x7E).
• UnicodeElement: Contains UTF-8 string data.
• DateElement: Contains a timestamp, stored as nanoseconds since 2001-01-01T00:00:00 UTC as a 64 bit integer. ebmlite automatically translates this into a Python datetime.datetime object.
• BinaryElement: Contains binary data.

Element definitions have several attributes:

• name (string): The Element subclass' name.
• id (integer): The Element subclass' EBML ID.
• global (bool, optional): If "true" (e.g. 1 or True), the element may appear in any location in an EBML file, not just where it appears in the schema. This is equivalent to a depth of -1 in a python-ebml schema
• length (integer, optional): A fixed size to use when encoding the element, overriding the EBML variable length encoding. Use to create byte-aligned structures.
• multiple (bool, optional, default=1): Indicates that the element can appear more than once within the same parent. Currently partially enforced for encoding.
• mandatory (bool, optional, default=0): Indicates that the element must be present. Not currently enforced.
• precache (bool, optional, default varies by type): Indicates that the element's value should be read and cached when the element is parsed, rather than 'lazy-loaded' when explicitly accessed. Can be used to reduce the number of seeks when working with an EBML file after it has been imported. Simple numeric element types have this enabled by default; master, binary, and string/Unicode elements do not.

There are two additional, special-case Element subclasses which are not subclassed:

• UnknownElement: Instantiated for elements with IDs that do not appear in the schema. Its payload is treated as binary data. The UnknownElement itself does not appear in the Schema. Unlike other Element subclasses, its ID can vary from instance to instance.
• VoidElement: "Void" (ID 0xEC) is a standard EBML element, typically used for padding. If the Schema defines the Void element, it is replaced by this special-case element. The contents of its payload are ignored.

The structure of the schema's XML defines the structure of the EBML document; children of a MasterElement in the schema are valid child element types in the EBML. An Element type can appear multiple times in a schema; i.e. if its type can appear as a child of different parent types. Only the first definition requires both name and id attributes. Successive definitions can be abbreviated to just the name and/or id; they will inherit all the other attributes of the first definition. Successive definitions must not have contradictory attributes, however.

<Schema>
    <MasterElement name="Parent1" id="0x5210">
        <!-- first definition of child: has all attributes -->
        <IntegerElement name="SharedChild" id="0x5211" precache="1" length="8"/>
    </MasterElement>

    <!-- Proper reuse of a child element -->
    <MasterElement name="Parent2" id="0x5220">
        <!-- second definition of child: only name (preferred) or ID required -->
        <IntegerElement name="SharedChild"/>
    </MasterElement>
    <MasterElement name="Parent3" id="0x5230">
        <!-- third definition of child: only name (preferred) or ID required -->
        <IntegerElement id="0x5211"/>
    </MasterElement>

    <!-- A bad reuse! This will raise an exception when the schema is parsed. -->
    <MasterElement name="Parent3" id="0x5230">
        <!-- BAD REDEFINITION: attribute(s) contradict initial definition! -->
        <IntegerElement name="SharedChild" id="0xBAD1D"/>
    </MasterElement>
</Schema>

Note: As seen in the example above, ebmlite allows an EBML document to have multiple elements at its root level. Several other EBML libraries do this as well, but this is apparently counter to the official spec. Officially, an EBML document should have only a single root element, similar to an XML file.

ebmlite

Schema

The Schema class is a factory used to encode and decode EBML files. When it's initialized, it scans through the schema file and creates a new class for each element present in the file; then, when encoding or decoding files, it references these classes in order to encapsulate everything safely.

Documents

Documents are subclasses of MasterElements, which act as an interface to EBML files and act as the root node of the EBML tree. Each Schema also creates a Document subclass to use, and the base Document class will not function without class variables defined by the Schema.

Utils

The functions provided by util.py will expose the majority of functionality needed to users, without the need to interface too deeply with this library. The following functions are provided:

• util.toXml(el, [parent=None,] [offsets=True,] [sizes=True,] [types=True,] [ids=True]):
  Recursively converts EBML elements into xml elements.
  Argument el: an EBML element or document.
  Optional argument parent: The resulting XML element's parent element, if any.
  Optional argument offsets: If True, create an offset attributes for each generated XML element, containing the corresponding EBML element's offset.
  Optional argument sizes: If True, create size attributes containing the corresponding EBML element's size.
  Optional argument types: If True, create type attributes containing the name of the corresponding EBML element type.
  Optional argument ids: If True, create id attributes containing the corresponding EBML element's EBML ID.
  Returns the root of an XML tree created using the xml.etree.ElementTree built-in class.

• util.xmlElement2ebml(xmlEl, ebmlFile, schema, [sizeLength=4,] [unknown=True]):
  Recursively converts XML elements tonight into EBML elements.
  Argument xmlEl: The XML element. Its tag must match an element defined in the schema.
  Argument ebmlFile: An open file-like stream, to which the EBML data will be written.
  Argument schema: An ebmlite.core.Schema instance to use when writing the EBML document.
  Optional argument sizeLength:
  Optional argument unknown: If True, unknown element names will be allowed, provided their XML elements include an id attribute with the EBML ID (in hexadecimal).
  Returns the length of the encoded element, including header and children.
  Raises NameError: raised if an xml element is not present in the schema and unknown is False, OR if the xml element does not have an ID.

• util.xml2ebml(xmlFile, ebmlFile, schema, [sizeLength=4,] [headers=True,] [unknown=True]): Argument xmlFile: The XML source. Can be a filename, an open file-like stream, or a parsed XML document.
  Argument ebmlFile: The EBML file to write. Can be a filename or an open file-like stream.
  Argument schema: The EBML schema to use. Can be a filename or an instance of a Schema.
  Optional argument sizeLength: The default length of each element's size descriptor. Must be large enough to store the largest 'master' element. If an XML element has a sizeLength attribute, it will override this.
  Optional argument headers: If True, generate the standard EBML EBML element if the XML document does not contain one.
  Optional argument unknown: If True, unknown element names will be allowed, provided their XML elements include an id attribute with the EBML ID (in hexadecimal).
  Returns the size of the ebml file in bytes.
  Raises NameError: raises if an xml element is not present in the schema.

• util.loadXml(xmlFile, schema, [ebmlFile=None]):
  Helpful utility to load an EBML document from an XML file.
  Argument xmlFile: The XML source. Can be a filename, an open file-like stream, or a parsed XML document.
  Argument schema: The EBML schema to use. Can be a filename or an instance of a Schema.
  Optional Argument ebmlFile: The name of the temporary EBML file to write, or :memory: to use RAM (like sqlite3). Defaults to an automatically-generated temporary file.
  Returns the root node of the specified EBML file

• util.pprint:
  Test function to recursively crawl an EBML document or element and print its structure, with child elements shown indented.
  Argument el: An instance of a Document or Element subclass.
  Argument values: If True, show elements' values.
  Optional Argument out: A file-like stream to which to write.
  Optional argument indent: The string containing the character(s) used for each indentation.

Utils can also be called from the command line with the following syntax:

python util.py {xml2ebml|ebml2xml|view} {FILE1.ebml|FILE1.xml} SCHEMA.xml [-o {FILE2.xml|FILE2.ebml}] [-c|--clobber] [-p|--pretty]

The program requires you to specify a mode: xml2ebml, ebml2xml, or view. The first two modes convert xml files to ebml files and ebml files to xml files, respectively; the last mode formats an IDE file to be human-readable.
FILE1: The location of the ebml or xml file to convert/view.
SCHEMA: The location of the schema to use when interpreting these files.
FILE2: The location to output to; otherwise, the output is directed into the console.
-c|--clobber: If FILE2 exists, then overwrite it, otherwise the program will fail.
-p|--pretty: Prints the output in a human-readable format.

To Do

• Complete documentation and example code.
• See @todo items in the Python files (i.e. core.py).