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"""StarStruct element class.""" |
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import struct |
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import re |
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from starstruct.element import register, Element |
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from starstruct.modes import Mode |
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@register |
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class ElementString(Element): |
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""" |
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A StarStruct element for strings, because standard string treatment of |
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pack/unpack can be inconvenient. |
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This element will encode and decode string type elements from and to forms |
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that are easier to use and manage. |
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""" |
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def __init__(self, field, mode=Mode.Native, alignment=1): |
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"""Initialize a StarStruct element object.""" |
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# All of the type checks have already been performed by the class |
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# factory |
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self.name = field[0] |
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# The ref attribute is required for all elements, but base element |
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# types don't have one |
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self.ref = None |
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self._mode = mode |
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self._alignment = alignment |
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# Validate that the format specifiers are valid struct formats, this |
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# doesn't have to be done now because the format will be checked when |
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# any struct functions are called, but it's better to inform the user of |
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# any errors earlier. |
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# The easiest way to perform this check is to create a "Struct" class |
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# instance, this will also increase the efficiency of all struct related |
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# functions called. |
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self.format = mode.value + field[1] |
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self._struct = struct.Struct(self.format) |
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@staticmethod |
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def valid(field): |
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""" |
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Validation function to determine if a field tuple represents a valid |
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string element type. |
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""" |
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return len(field) == 2 \ |
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and isinstance(field[1], str) \ |
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and re.match(r'\d*[csp]', field[1]) |
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def validate(self, msg): |
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""" |
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Ensure that the supplied message contains the required information for |
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this element object to operate. |
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The "string" element requires no further validation. |
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""" |
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pass |
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def update(self, mode=None, alignment=None): |
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"""change the mode of the struct format""" |
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if alignment: |
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self._alignment = alignment |
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if mode: |
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self._mode = mode |
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self.format = mode.value + self.format[1:] |
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# recreate the struct with the new format |
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self._struct = struct.Struct(self.format) |
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def pack(self, msg): |
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"""Pack the provided values into the supplied buffer.""" |
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# Ensure that the input is of the proper form to be packed |
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val = msg[self.name] |
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size = struct.calcsize(self.format) |
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assert len(val) <= size |
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if self.format[-1] in ('s', 'p'): |
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if not isinstance(val, bytes): |
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assert isinstance(val, str) |
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val = val.encode() |
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if self.format[-1] == 'p' and len(val) < size: |
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# 'p' (pascal strings) must be the exact size of the format |
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val += b'\x00' * (size - len(val)) |
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data = self._struct.pack(val) |
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else: # 'c' |
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if not all(isinstance(c, bytes) for c in val): |
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if isinstance(val, bytes): |
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val = [bytes([c]) for c in val] |
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else: |
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# last option, it could be a string, or a list of strings |
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assert (isinstance(val, list) and |
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all(isinstance(c, str) for c in val)) or \ |
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isinstance(val, str) |
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val = [c.encode() for c in val] |
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if len(val) < size: |
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val.extend([b'\x00'] * (size - len(val))) |
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data = self._struct.pack(*val) |
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# If the data does not meet the alignment, add some padding |
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missing_bytes = len(data) % self._alignment |
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if missing_bytes: |
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data += b'\x00' * missing_bytes |
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return data |
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def unpack(self, msg, buf): |
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"""Unpack data from the supplied buffer using the initialized format.""" |
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ret = self._struct.unpack_from(buf, 0) |
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# Remember to remove any alignment-based padding |
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extra_bytes = self._alignment - 1 - (struct.calcsize(self.format) % |
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self._alignment) |
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unused = buf[struct.calcsize(self.format) + extra_bytes:] |
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val = ret[0] |
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if self.format[-1] in 's': |
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# for 's' formats, convert to a string and strip padding |
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val = val.decode().strip('\x00') |
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elif self.format[-1] in 'p': |
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# for 'p' formats, convert to a string, but leave the padding |
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val = val.decode() |
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else: # 'c' |
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# Just in case we have some ints in the message |
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val = [c.decode() if not isinstance(c, int) |
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else chr(c) |
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for c in val] |
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return (val, unused) |
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def make(self, msg): |
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"""Return a string of the expected format""" |
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val = msg[self.name] |
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size = struct.calcsize(self.format) |
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assert len(val) <= size |
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# If the supplied value is a list of chars, or a list of bytes, turn |
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# it into a string for ease of processing. |
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if isinstance(val, list): |
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if all(isinstance(c, bytes) for c in val): |
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val = ''.join([c.decode() for c in val]) |
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elif all(isinstance(c, str) for c in val): |
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val = ''.join([c for c in val]) |
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else: |
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error = 'Invalid value for string element: {}' |
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raise TypeError(error.format(val)) |
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elif isinstance(val, bytes): |
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# If the supplied value is a byes, decode it into a normal string |
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val = val.decode() |
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# 'p' (pascal strings) and 'c' (char list) must be the exact size of |
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# the format |
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if self.format[-1] in ('p', 'c') and len(val) < size: |
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val += '\x00' * (size - len(val)) |
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1 |
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# Lastly, 'c' (char list) formats are expected to be a list of |
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# characters rather than a string. |
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if self.format[-1] == 'c': |
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val = [c for c in val] |
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return val |
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sprout42 /
StarStruct
Generally, there is nothing wrong with usage of
*or**arguments. For readability of the code base, we suggest to not over-use these language constructs though.For more information, we can recommend this blog post from Ned Batchelder including its comments which also touches this aspect.