Access to Python class attributes in struct definition

Author: andreax79

.readthedocs.yaml

@@ -1,7 +1,16 @@
 # .readthedocs.yaml
 version: 2
+
+# Set the OS and Python version
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
 mkdocs:
   configuration: mkdocs.yml
+
+# Declare the Python requirements required to build the documentation
 python:
    install:
    - requirements: requirements-dev.txt

README.md

@@ -197,6 +197,33 @@ pkg.length = 4
 pkg.data = [10, 20, 30, 40]
 ```
 
+### Python object attributes
+
+In struct definition, you can access Python object attributes using `self`.
+The value of expression accessing class attributes is evaluated at runtime.
+
+```python
+class RT11DirectoryEntry(cstruct.CStruct):
+
+    __byte_order__ = cstruct.LITTLE_ENDIAN
+    __def__ = """
+        struct RT11DirectoryEntry {
+            uint8_t type;
+            uint8_t clazz;
+            uint16_t raw_filename1;
+            uint16_t raw_filename2;
+            uint16_t raw_extension;
+            uint16_t length;
+            uint8_t job;
+            uint8_t channel;
+            uint16_t raw_creation_date;
+            uint16_t extra_bytes[self.extra_bytes_len];  /* The size of the array is determined at runtime */
+        };
+    """
+
+    extra_bytes_len: int = 0
+```
+
 ### Pack and Unpack
 
 A code example illustrating how to use

changelog.txt

@@ -182,3 +182,13 @@
 ### Improved
 
 - Python 3.12 support
+
+## [6.0] - 2025-01-16
+
+### Added
+
+- access to Python class attributes in struct definition
+
+### Improved
+
+- Python 3.13 support

cstruct/__init__.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #
@@ -24,7 +24,7 @@
 
 __author__ = "Andrea Bonomi <andrea.bonomi@gmail.com>"
 __license__ = "MIT"
-__version__ = "5.3"
+__version__ = "6.0"
 __date__ = "15 August 2013"
 
 from typing import Any, Dict, Optional, Type, Union

cstruct/abstract.py

@@ -1,7 +1,5 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
-#
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #
@@ -163,7 +161,7 @@ def set_flexible_array_length(self, flexible_array_length: Optional[int]) -> Non
             flexible_array: Optional[FieldType] = [x for x in self.__fields_types__.values() if x.flexible_array][0]
             if flexible_array is None:
                 raise CStructException("Flexible array not found in struct")
-            flexible_array.vlen = flexible_array_length
+            flexible_array.vlen_ex = flexible_array_length
 
     def unpack(self, buffer: Optional[Union[bytes, BinaryIO]], flexible_array_length: Optional[int] = None) -> bool:
         """
@@ -202,6 +200,17 @@ def pack(self) -> bytes:  # pragma: no cover
         """
         raise NotImplementedError
 
+    def pack_into(self, buffer: bytearray, offset: int = 0) -> None:
+        """
+        Pack the structure data into a buffer
+
+        Args:
+            buffer: target buffer (must be large enough to contain the packed structure)
+            offset: optional buffer offset
+        """
+        tmp = self.pack()
+        buffer[offset : offset + len(tmp)] = tmp
+
     def clear(self) -> None:
         self.unpack(None)
 
@@ -300,6 +309,9 @@ def __setstate__(self, state: bytes) -> bool:
 
 
 class CEnumMeta(EnumMeta):
+    __size__: int
+    __native_format__: str
+
     class WrapperDict(_EnumDict):
         def __setitem__(self, key: str, value: Any) -> None:
             env = None

cstruct/base.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #

cstruct/c_expr.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #
@@ -23,11 +23,12 @@
 #
 
 import ast
+import inspect
 import operator
-from typing import TYPE_CHECKING, Any, Callable, Dict, Type, Union
+from typing import TYPE_CHECKING, Any, Callable, Dict, Optional, Type, Union
 
 from .base import DEFINES, STRUCTS
-from .exceptions import EvalError
+from .exceptions import ContextNotFound, EvalError
 
 if TYPE_CHECKING:
     from .abstract import AbstractCStruct
@@ -65,7 +66,36 @@ def c_eval(expr: str) -> Union[int, float]:
         raise EvalError
 
 
+def eval_attribute_node(node: ast.Attribute) -> Union[int, float]:
+    """
+    Evaluate node attribute, e.g. 'self.x'
+    Only 'self' is allowed. The attribute must be a number.
+
+    Args:
+        node: attribute node
+
+    Returns:
+        result: the attribute value
+
+    Raises:
+        EvalError: expression result is not a number, or not self attribute
+        ContextNotFound: context is not defined
+    """
+    if not node.value or node.value.id != "self":  # type: ignore
+        raise EvalError("only self is allowed")
+    context = get_cstruct_context()
+    if context is None:
+        raise ContextNotFound("context is not defined")
+    result = getattr(context, node.attr)
+    if not isinstance(result, (int, float)):
+        raise EvalError("expression result is not a number")
+    return result
+
+
 def eval_node(node: ast.stmt) -> Union[int, float]:
+    if isinstance(node, ast.Attribute):
+        return eval_attribute_node(node)
+
     handler = OPS[type(node)]
     result = handler(node)
     if isinstance(result, bool):  # convert bool to int
@@ -116,6 +146,20 @@ def eval_call(node) -> Union[int, float]:
     raise KeyError(node.func.id)
 
 
+def get_cstruct_context() -> Optional["AbstractCStruct"]:
+    """
+    Get the calling CStruct instance from the stack (if any)
+    """
+    from .abstract import AbstractCStruct
+
+    stack = inspect.stack()
+    for frame in stack:
+        caller_self = frame.frame.f_locals.get("self")
+        if isinstance(caller_self, AbstractCStruct):
+            return caller_self
+    return None
+
+
 try:
     Constant = ast.Constant
 except AttributeError:  # python < 3.8

cstruct/c_parser.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #
@@ -24,11 +24,21 @@
 
 import re
 from collections import OrderedDict
-from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Type, Union
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Tuple,
+    Type,
+    Union,
+)
 
 from .base import DEFINES, ENUMS, STRUCTS, TYPEDEFS
 from .c_expr import c_eval
-from .exceptions import CStructException, ParserError
+from .exceptions import CStructException, EvalError, ParserError
 from .field import FieldType, Kind, calculate_padding
 from .native_types import get_native_type
 
@@ -41,7 +51,7 @@
 SPACES = [" ", "\t", "\n"]
 
 
-class Tokens(object):
+class Tokens:
     def __init__(self, text: str) -> None:
         # remove the comments
         text = re.sub(r"//.*?$|/\*.*?\*/", "", text, flags=re.S | re.MULTILINE)
@@ -59,7 +69,7 @@ def __init__(self, text: str) -> None:
         text = "\n".join(lines)
         self.tokens = self.tokenize(text)
 
-    def tokenize(self, text) -> List[str]:
+    def tokenize(self, text: str) -> List[str]:
         tokens: List[str] = []
         t: List[str] = []
         for c in text:
@@ -72,7 +82,7 @@ def tokenize(self, text) -> List[str]:
             else:
                 t.append(c)
         if t:
-            tokens.append(t.getvalue())
+            tokens.append("".join(t))
         return tokens
 
     def pop(self) -> str:
@@ -101,7 +111,8 @@ def __str__(self) -> str:
         return str(self.tokens)
 
 
-def parse_length(tokens: Tokens, next_token: str, vlen: int, flexible_array: bool) -> Tuple[str, int, bool]:
+def parse_length(tokens: Tokens, next_token: str, flexible_array: bool) -> Tuple[str, Union[int, Callable[[], int]], bool]:
+    # Extract t_vlen
     t = next_token.split("[")
     if len(t) != 2:
         raise ParserError(f"Error parsing: `{next_token}`")
@@ -114,14 +125,19 @@ def parse_length(tokens: Tokens, next_token: str, vlen: int, flexible_array: boo
     t_vlen = vlen_part.split("]")[0].strip()
     vlen_expr.append(vlen_part.split("]")[0].strip())
     t_vlen = " ".join(vlen_expr)
+    # Evaluate t_vlen
+    vlen: Union[int, Callable[[], int]]
     if not t_vlen:
+        # If the length expression is empty, this is a flex array
         flexible_array = True
         vlen = 0
     else:
+        # Evaluate the length expression
+        # If the length expression is not a constant, it is evaluated at runtime
         try:
-            vlen = c_eval(t_vlen)
-        except (ValueError, TypeError):
-            vlen = int(t_vlen)
+            vlen = int(c_eval(t_vlen))
+        except EvalError:
+            vlen = lambda: int(c_eval(t_vlen))
     return next_token, vlen, flexible_array
 
 
@@ -133,7 +149,7 @@ def parse_type(tokens: Tokens, __cls__: Type["AbstractCStruct"], byte_order: Opt
     if c_type in ["signed", "unsigned", "struct", "union", "enum"] and len(tokens) > 1:
         c_type = c_type + " " + tokens.pop()
 
-    vlen = 1
+    vlen: Union[int, Callable[[], int]] = 1
     flexible_array = False
 
     if not c_type.endswith("{"):
@@ -148,20 +164,21 @@ def parse_type(tokens: Tokens, __cls__: Type["AbstractCStruct"], byte_order: Opt
             c_type = "void *"
         # parse length
         if "[" in next_token:
-            next_token, vlen, flexible_array = parse_length(tokens, next_token, vlen, flexible_array)
+            next_token, vlen, flexible_array = parse_length(tokens, next_token, flexible_array)
         tokens.push(next_token)
         # resolve typedefs
         while c_type in TYPEDEFS:
             c_type = TYPEDEFS[c_type]
 
     # calculate fmt
+    ref: Union[None, Type[AbstractCEnum], Type[AbstractCStruct]]
     if c_type.startswith("struct ") or c_type.startswith("union "):  # struct/union
         c_type, tail = c_type.split(" ", 1)
         kind = Kind.STRUCT if c_type == "struct" else Kind.UNION
         if tokens.get() == "{":  # Named nested struct
             tokens.push(tail)
             tokens.push(c_type)
-            ref: Union[Type[AbstractCEnum], Type[AbstractCStruct]] = __cls__.parse(tokens, __name__=tail, __byte_order__=byte_order)
+            ref = __cls__.parse(tokens, __name__=tail, __byte_order__=byte_order)
         elif tail == "{":  # Unnamed nested struct
             tokens.push(tail)
             tokens.push(c_type)
@@ -428,7 +445,7 @@ def parse_struct(
             raise ParserError(f"Invalid reserved member name `{vname}`")
         # parse length
         if "[" in vname:
-            vname, field_type.vlen, field_type.flexible_array = parse_length(tokens, vname, 1, flexible_array)
+            vname, field_type.vlen_ex, field_type.flexible_array = parse_length(tokens, vname, flexible_array)
             flexible_array = flexible_array or field_type.flexible_array
         # anonymous nested union
         if vname == ";" and field_type.ref is not None and (__is_union__ or field_type.ref.__is_union__):

cstruct/cstruct.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #

cstruct/exceptions.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-# Copyright (c) 2013-2019 Andrea Bonomi <andrea.bonomi@gmail.com>
+# Copyright (c) 2013-2025 Andrea Bonomi <andrea.bonomi@gmail.com>
 #
 # Published under the terms of the MIT license.
 #
@@ -27,6 +27,7 @@
     "CStructException",
     "ParserError",
     "EvalError",
+    "ContextNotFound",
 ]
 
 
@@ -44,3 +45,7 @@ class ParserError(CStructException):
 
 class EvalError(CStructException):
     pass
+
+
+class ContextNotFound(EvalError):
+    pass