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Python/ast.c

cpython 3.14 @ ab2d84fe1023/Python/ast.c

Python/ast.c bridges the pegen parser output and the compiler. It validates the AST (_PyAST_Validate), converts a Python-level ast object back to a C mod_ty (PyAST_obj2mod), and provides the asdl_seq memory allocation helpers used throughout the AST construction code.

Map​

LinesSymbolRole
1-200_PyAST_ValidateStructural AST validation: check for invalid node combinations
201-500PyAST_obj2modConvert Python ast.Module object to C mod_ty
501-700asdl_seq_new, asdl_int_seq_newSequence allocation from PyArena
701-960ast_for_* helpersConvert Python AST node children to C AST nodes

Reading​

_PyAST_Validate​

_PyAST_Validate is called after parsing to catch structural errors that the grammar alone cannot detect. It walks the AST and raises ValueError for forbidden patterns such as a starred expression in an assignment target where it is not allowed, or a return statement outside a function.

// CPython: Python/ast.c:142 validate_expr
static int
validate_expr(struct validator *state, expr_ty exp, expr_context_ty ctx)
{
    switch (exp->kind) {
    case BoolOp_kind:
        if (asdl_seq_LEN(exp->v.BoolOp.values) < 2) {
            PyErr_SetString(PyExc_ValueError, "BoolOp with less than 2 values");
            return 0;
        }
        ...
    }
}

PyAST_obj2mod​

PyAST_obj2mod(ast_obj, arena, mode) takes a Python ast.Module, ast.Expression, or ast.Interactive node and converts its children to C AST nodes using PyArena allocation. This is the entry point for compile(ast.parse(src), ...).

PyArena allocation​

All AST nodes are allocated from a PyArena slab. When the compilation unit is done, the arena is freed as a single block. Individual node deallocation is never needed.

gopy notes​

parser/pegen/ produces C-equivalent AST nodes directly without a round-trip through Python objects. _PyAST_Validate checks are replicated in the pegen action helpers in parser/pegen/action_helpers_gen.go. PyAST_obj2mod is not needed in gopy since the compiler consumes the pegen AST directly.