When is a field really created?¶
A field is created when someone ask to access it, or when another field is asked and the field is before it. So if you use a field in your field set constructor, one or more fields will be created. Example:
>>> from hachoir.field import Parser, Int8 >>> from hachoir.core.endian import BIG_ENDIAN >>> class Point(Parser): ... endian = BIG_ENDIAN ... def __init__(self, stream): ... Parser.__init__(self, stream) ... if self["color"].value == -1: ... self._description += " (no color)" ... ... def createFields(self): ... yield Int8(self, "color", "Point color (-1 for none)") ... yield Int8(self, "use_3d", "Does it use Z axis?") ... yield Int8(self, "x", "X axis value") ... yield Int8(self, "y", "Y axis value") ... if self["use_3d"] == 1: ... yield Int8(self, "z", "Z axis value") ...
In the constructor, the field “color” is asked. So the field list will contains one field (color):
>>> from hachoir.stream import StringInputStream >>> stream = StringInputStream(b"\x2A\x00\x04\x05") >>> p = Point(stream) >>> p.current_length 1
If you access another field, the field list will grow up until the requested field is reached:
>>> x = p["x"].value >>> p.current_length 3
Some field set methods which create new fields:
__getitem__(): feed field list until requested field is reached (or raise MissingField exception) ;
__len__(): create all fields ;
__iter__(): begin to iterate in existing fields, and the iterate in new fields until all fields are created ;
__contains__(): feed field list until requested field is reached, may create all fields if the field doesn’t exist ;
When is the size really computed?¶
The size attribute also interact with field list creation, but it’s mechanism is little bit more complex. By default, the whole field list have to be built before size value can be read. But you can specify field list size:
- if field list is not dynamic (e.g. doesn’t depend on flag), use class
- otherwise you can set _size instance attribute in the constructor.
>>> from hachoir.field import Parser >>> class FourBytes(Parser): ... endian = BIG_ENDIAN ... static_size = 32 ... def createFields(self): ... yield Integer(self, "four", "uint32") ... >>> class DynamicSize(Parser): ... endian = BIG_ENDIAN ... def __init__(self, stream, nb_items): ... Parser.__init__(self, stream) ... assert 0 < nb_items ... self.nb_items = nb_items ... self._size = nb_items * 32 # 32 is the size of one item ... ... def createFields(self): ... for index in range(self.nb_items): ... yield Integer(self, "item", "uint32") ... >>> a = FourBytes(stream) >>> b = DynamicSize(stream, 1) >>> a.size, b.size (32, 32) >>> # Check that nothing is really read from stream ... a.current_length, b.current_length (0, 0)
When the value of a field is read?¶
When a field is created, the value of the field doesn’t exist (equals to None). The value is really read when you read the field value using ‘value’ or ‘display’ field attributes. The value is then stored in the field.
Details about field name¶
The name of a field have to be unique in a field set because it is used as key in the field list. The argument ‘name’ of the Field constructor can be changed in the constructor, but should not (and cannot) be changed after that.
For arrays, you can use the ‘magic’ suffix «  » (e.g. “item”) which will be replaced by « [index] » where the number index is a counter starting a zero.
The “endian” is the way in which ‘’bytes’’ are stored. There are two important orders:
- « Big endian » in which most significant byte (big number) are written first (PowerPC / Motorola CPUs). It’s also the network byte order ;
- « Little endian » in which least significant byte (little number) are written first (Intel x86 CPUs).
The number 0x1020 is stored “0x10 0x20” in big endian and “0x20 0x10” in little endian.
The endian is global to a FieldSet and is a class attribute. Allowed values:
- BIG_ENDIAN ;
- NETWORK_ENDIAN (alias of BIG_ENDIAN) ;
Example to set endian:
>>> from hachoir.core.endian import LITTLE_ENDIAN >>> class UseLittleEndian(Parser): ... endian = LITTLE_ENDIAN ...
For sub-field sets, if endian is not specified, parent endian will be used.
Explore a field set using it’s path¶
Fields are stored in a tree. To explore the tree you have different tools:
- attribute root of a field which go to tree root ;
- attribute parent go to field parent (is None for tree root) ;
- and you can specify a path in __getitem__() argument.
There are different valid syntaxes for a path:
- path to a child of current node:
- path to a child of the parent:
- path from the root: