"""sstruct.py -- SuperStruct
Higher level layer on top of the struct module, enabling to
bind names to struct elements. The interface is similar to
struct, except the objects passed and returned are not tuples
(or argument lists), but dictionaries or instances.
Just like struct, we use format strings to describe a data
structure, except we use one line per element. Lines are
separated by newlines or semi-colons. Each line contains
either one of the special struct characters ('@', '=', '<',
'>' or '!') or a 'name:formatchar' combo (eg. 'myFloat:f').
Repetitions, like the struct module offers them are not useful
in this context, except for fixed length strings (eg. 'myInt:5h'
is not allowed but 'myString:5s' is). The 'x' format character
(pad byte) is treated as 'special', since it is by definition
anonymous. Extra whitespace is allowed everywhere.
The sstruct module offers one feature that the "normal" struct
module doesn't: support for fixed point numbers. These are spelled
as "n.mF", where n is the number of bits before the point, and m
the number of bits after the point. Fixed point numbers get
converted to floats.
pack(format, object):
'object' is either a dictionary or an instance (or actually
anything that has a __dict__ attribute). If it is a dictionary,
its keys are used for names. If it is an instance, it's
attributes are used to grab struct elements from. Returns
a string containing the data.
unpack(format, data, object=None)
If 'object' is omitted (or None), a new dictionary will be
returned. If 'object' is a dictionary, it will be used to add
struct elements to. If it is an instance (or in fact anything
that has a __dict__ attribute), an attribute will be added for
each struct element. In the latter two cases, 'object' itself
is returned.
unpack2(format, data, object=None)
Convenience function. Same as unpack, except data may be longer
than needed. The returned value is a tuple: (object, leftoverdata).
calcsize(format)
like struct.calcsize(), but uses our own format strings:
it returns the size of the data in bytes.
"""
# XXX I would like to support pascal strings, too, but I'm not
# sure if that's wise. Would be nice if struct supported them
# "properly", but that would certainly break calcsize()...
__version__ = "1.2"
__copyright__ = "Copyright 1998, Just van Rossum <just@letterror.com>"
import struct
import re
import types
error = "sstruct.error"
def pack(format, object):
formatstring, names, fixes = getformat(format)
elements = []
if type(object) is not types.DictType:
object = object.__dict__
for name in names:
value = object[name]
if fixes.has_key(name):
# fixed point conversion
value = int(round(value*fixes[name]))
elements.append(value)
data = apply(struct.pack, (formatstring,) + tuple(elements))
return data
def unpack(format, data, object=None):
if object is None:
object = {}
formatstring, names, fixes = getformat(format)
if type(object) is types.DictType:
dict = object
else:
dict = object.__dict__
elements = struct.unpack(formatstring, data)
for i in range(len(names)):
name = names[i]
value = elements[i]
if fixes.has_key(name):
# fixed point conversion
value = value / fixes[name]
dict[name] = value
return object
def unpack2(format, data, object=None):
length = calcsize(format)
return unpack(format, data[:length], object), data[length:]
def calcsize(format):
formatstring, names, fixes = getformat(format)
return struct.calcsize(formatstring)
# matches "name:formatchar" (whitespace is allowed)
_elementRE = re.compile(
"\s*" # whitespace
"([A-Za-z_][A-Za-z_0-9]*)" # name (python identifier)
"\s*:\s*" # whitespace : whitespace
"([cbBhHiIlLfd]|[0-9]+[ps]|" # formatchar...
"([0-9]+)\.([0-9]+)(F))" # ...formatchar
"\s*" # whitespace
"(#.*)?$" # [comment] + end of string
)
# matches the special struct format chars and 'x' (pad byte)
_extraRE = re.compile("\s*([x@=<>!])\s*(#.*)?$")
# matches an "empty" string, possibly containing whitespace and/or a comment
_emptyRE = re.compile("\s*(#.*)?$")
_fixedpointmappings = {
8: "b",
16: "h",
32: "l"}
_formatcache = {}
def getformat(format):
try:
formatstring, names, fixes = _formatcache[format]
except KeyError:
lines = re.split("[\n;]", format)
formatstring = ""
names = []
fixes = {}
for line in lines:
if _emptyRE.match(line):
continue
m = _extraRE.match(line)
if m:
formatchar = m.group(1)
if formatchar <> 'x' and formatstring:
raise error, "a special format char must be first"
else:
m = _elementRE.match(line)
if not m:
raise error, "syntax error in format: '%s'" % line
name = m.group(1)
names.append(name)
formatchar = m.group(2)
if m.group(3):
# fixed point
before = int(m.group(3))
after = int(m.group(4))
bits = before + after
if bits not in [8, 16, 32]:
raise error, "fixed point must be 8, 16 or 32 bits long"
formatchar = _fixedpointmappings[bits]
assert m.group(5) == "F"
fixes[name] = float(1 << after)
formatstring = formatstring + formatchar
_formatcache[format] = formatstring, names, fixes
return formatstring, names, fixes
def _test():
format = """
# comments are allowed
> # big endian (see documentation for struct)
# empty lines are allowed:
ashort: h
along: l
abyte: b # a byte
achar: c
astr: 5s
afloat: f; adouble: d # multiple "statements" are allowed
afixed: 16.16F
"""
print 'size:', calcsize(format)
class foo:
pass
i = foo()
i.ashort = 0x7fff
i.along = 0x7fffffff
i.abyte = 0x7f
i.achar = "a"
i.astr = "12345"
i.afloat = 0.5
i.adouble = 0.5
i.afixed = 1.5
data = pack(format, i)
print 'data:', `data`
print unpack(format, data)
i2 = foo()
unpack(format, data, i2)
print vars(i2)
if __name__ == "__main__":
_test()
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