spasmlang

Synopsis

spasmlang is a simple Python assembly language. It lets you generate CPython bytecode from a simple assembly-like syntax, and assembles it with a bundled C++ extension rather than a pure-Python bytecode library.

Supports CPython 3.10 through 3.14, and has no runtime dependencies.

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Table of Contents

• Installation
• Usage
• Examples
• Low-level API
• Architecture
• License

Installation

pip install spasmlang

Wheels are published for CPython 3.10–3.14 on Linux, macOS and Windows. Installing from source needs a C++17 compiler.

Usage

The spasmlang package provides a class, Assembly, that allows you to generate bytecode from a simple assembly-like syntax. See the examples below for a taste of its API. Where source-level assembly isn't practical, the low-level API lets you string instruction objects together directly.

You can also use the spasm command-line utility to compile assembly files directly to Python bytecode:

spasm example.pya  # generates example.pyc

Examples

This is how the classic "Hello, World!" program looks like, targeting the CPython 3.12 bytecode:

from spasm import Assembly

asm = Assembly()
asm.parse(
    r"""
    push_null
    load_const          print
    load_const          "Hello, World!"
    call                1
    return_value
    """
)
exec(asm.compile())

This is how you can compile the file example.pya to example.pyc to create a "Hello, World!" module, again targeting CPython 3.11:

# example.pya
    resume      0
    push_null
    load_name   $print
    load_const  "Hello, spasm!"
    precall     1
    call        1
    pop_top
    load_const  None
    return_value

Compile the assembly code with (assuming that you have installed spasmlang with CPython 3.11)

spasm example.pya

and then execute the generated module with e.g.

python3.11 -m example

This example shows how to create a module that exports a greet function that takes one argument, targeting CPython 3.11:

# greet.pya

code greet(who)
    resume                      0
    load_global                 (True, "print")
    load_const                  "Hello, "
    load_fast                   $who
    format_value                0
    build_string                2
    precall                     1
    call                        1
    return_value
end

    resume 0
    load_const                  .greet
    make_function               0
    store_name                  $greet
    load_const                  None
    return_value

Again, compile the assembly code with

spasm greet.pya

and test it with

$ python3.11 -c "from greet import greet; greet('spasmlang')"
Hello, spasmlang

Code blocks nest, and a block header can declare the variables the block shares with the one around it, which is what a closure needs:

code NAME(arguments)[cells]<frees>

The two trailing groups are optional and independent, so code f(x)<n> declares a free variable and no cells. Cells are this block's locals that a nested block captures; frees are the ones it captures from its own enclosing block. They have to be written down because nothing in the instruction stream tells the two apart — LOAD_DEREF $n looks the same either way.

This is lambda n: lambda: n in assembly, targeting CPython 3.12:

# adder.pya

code outer(n)[n]
    make_cell                   $n
    resume                      0

    code inner()<n>
        copy_free_vars          1
        resume                  0
        load_deref              $n
        return_value
    end

    load_closure                $n
    build_tuple                 1
    load_const                  .inner
    make_function               8
    return_value
end

    resume                      0
    load_const                  .outer
    make_function               0
    store_name                  $outer
    load_const                  None
    return_value

$ spasm adder.pya && python3.12 -c "from adder import outer; print(outer(42)())"
42

Note that n appears both as an argument of outer and in its cell list: from CPython 3.11 a captured parameter occupies a single frame slot that is at once a local and a cell, and MAKE_CELL is what turns it into one. inner is reachable only from outer, since a nested block is a constant of the block it is written in and not of the module.

Low-level API

Writing a snippet of assembly is the shortest way to get bytecode, but it is not always the right shape for the job. When the instruction stream is being computed rather than written — you are transforming an existing code object, generating a sequence whose length depends on runtime data, or injecting instrumentation into somebody else's function — you want to string instructions together as objects. That is what spasm.bytecode exposes, and it is the same layer the assembler itself is built on.

from spasm.bytecode import Bytecode, ExcEntry, Instr, Label

The data model

A Bytecode is a mutable, decoded code object. Bytecode.from_code(co) builds one from an existing code object and bc.to_code() encodes it back; the round trip is lossless, so decoding and re-encoding an untouched function gives back byte-identical co_code, line table and exception table.

Attribute	Meaning
instrs	The instruction list. Holds Instr objects and nothing else — no labels, no pseudo-entries.
end_labels	Labels for the position one past the last instruction.
exc_entries	The ExcEntry list making up the exception table (3.11+).
consts, names, varnames, freevars, cellvars	The code object's tables, as plain lists.
argcount, flags, firstlineno, filename, name, qualname	The remaining code object fields.

An Instr carries op (settable as an opname string or as an int), arg, and the position attributes lineno, col_offset, end_lineno and end_col.

There is no stacksize: co_stacksize is always computed from the instruction stream by to_code(), and is neither stored nor settable.

Bytecode() with no arguments is an empty template — no instructions, empty tables, argcount 0, flags 0, firstlineno 1, filename <string>, name and qualname <bytecode> — so building from scratch is a matter of filling in the fields you care about. Bytecode(instrs) seeds the instruction list in one go; either way bc.instrs is a live list you can mutate in place.

import sys
import types

from spasm.bytecode import Bytecode, Instr, infer_flags

bc = Bytecode()
if sys.version_info >= (3, 11):
    bc.instrs.append(Instr("RESUME", 0))
bc.instrs += [
    Instr("LOAD_FAST", "x"),
    Instr("LOAD_FAST", "y"),
    Instr("BINARY_OP" if sys.version_info >= (3, 11) else "BINARY_ADD", 0),
    Instr("RETURN_VALUE"),
]
bc.name = bc.qualname = "add"
bc.argcount = 2
bc.varnames = ["x", "y"]
bc.flags = infer_flags(bc, is_function=True)

add = types.FunctionType(bc.to_code(), {})
assert add(1, 2) == 3

infer_flags derives the co_flags a code object being built from scratch needs, as far as that can be done from the code alone: it cannot see *args/**kwargs or lexical nesting, so CO_VARARGS, CO_VARKEYWORDS and CO_NESTED are left to the caller.

Instruction arguments

Arguments that name a table entry are given as the value, not as the index, and the entry is created if it isn't there already:

Argument kind	What to pass
co_consts (LOAD_CONST, …)	The constant itself.
co_varnames (LOAD_FAST, STORE_FAST, …)	The variable name, as a string.
free/cell variables (LOAD_DEREF, …)	The variable name, as a string.
Jump targets	A Label.
co_names (LOAD_GLOBAL, LOAD_ATTR, STORE_NAME, …)	An int, from encode_name_arg().
Everything else	The raw int oparg.

Name arguments are the one exception, because they are not just an index: from 3.11 LOAD_GLOBAL and from 3.12 LOAD_ATTR pack a flag bit alongside it. encode_name_arg(bc, opname, name, flag=False) interns the name and returns the oparg, shifting the index where the version calls for it.

Two more helpers cover opargs that are conceptually symbolic: compare_oparg(Compare.LT) for COMPARE_OP, whose encoding moved twice between 3.10 and 3.13, and the BinaryOp enum for BINARY_OP on 3.11+, whose members are used as the oparg directly.

If you would rather work with indices, add_const(), add_name() and add_varname() intern a value and hand back its index.

The superinstructions that pack two variable indices into a single oparg — LOAD_FAST_LOAD_FAST and friends — take a plain int, since a single name cannot express what they encode.

Jumps and labels

Jump targets are Label objects rather than offsets, which is what makes an instruction list editable: inserting or removing instructions shifts every offset in the code object, but a label stays attached to the instruction it points at.

Make one with bc.new_label(), append it to the labels list of the instruction it should land on, and pass it as the argument of the jump. For a target one past the end of the code, append it to bc.end_labels instead. bc.label_positions() maps every label to the index in instrs it currently resolves to.

bc = Bytecode()
falsy = bc.new_label()

instrs = [Instr("LOAD_FAST", "x")]
if sys.version_info >= (3, 13):
    instrs.append(Instr("TO_BOOL"))  # 3.13+ POP_JUMP_IF_* only accepts a bool
instrs += [
    Instr("POP_JUMP_IF_FALSE", falsy),
    Instr("LOAD_CONST", "truthy"),
    Instr("RETURN_VALUE"),
    Instr("LOAD_CONST", "falsy"),
    Instr("RETURN_VALUE"),
]
instrs[-2].labels.append(falsy)
bc.instrs += instrs

Nothing here needs to know how far the jump reaches: the encoder picks the relative or absolute form the opcode wants, and grows EXTENDED_ARG prefixes to a fixed point when an oparg does not fit in a byte — which it has to iterate, since growing one prefix moves every target after it.

Transforming an existing code object

Instrumentation is the case the API is really shaped for: decode, splice, and encode back.

import sys

from spasm.bytecode import Bytecode, Instr, encode_name_arg

PY311 = sys.version_info >= (3, 11)


def f(a, b):
    return a * b


bc = Bytecode.from_code(f.__code__)

# The flag bit makes LOAD_GLOBAL push the NULL that the call sequence wants,
# in whichever order this version expects it; before 3.11 there is no NULL.
flag = {"flag": True} if PY311 else {}
trace = [
    Instr("LOAD_GLOBAL", encode_name_arg(bc, "LOAD_GLOBAL", "print", **flag)),
    Instr("LOAD_CONST", "called!"),
    Instr("CALL" if PY311 else "CALL_FUNCTION", 1),
    Instr("POP_TOP"),
]

# RESUME has to stay the first instruction of a 3.11+ code object.
at = 1 if PY311 else 0
for instr in trace:
    instr.lineno = bc.instrs[at].lineno
bc.instrs[at:at] = trace

f.__code__ = bc.to_code()
f(3, 4)  # prints "called!" and returns 12

Give inserted instructions a lineno explicitly. Nothing forces you to, but the line table is what tracebacks and debuggers read, and an instrumented function whose lines have drifted is unpleasant to debug.

Exception table entries

From 3.11 on, exception handling is table-driven rather than done with block instructions, and bc.exc_entries is that table. An ExcEntry is three labels — the protected region's start and (exclusive) stop, and the handler — plus the stack depth the handler is entered at and whether the interpreter should push lasti before the exception.

from spasm.bytecode import Bytecode, ExcEntry, Instr


def risky(x):
    return 1 / x


bc = Bytecode.from_code(risky.__code__)

start = bc.new_label()
handler = bc.new_label()
bc.instrs[1].labels.append(start)  # everything after RESUME is protected

recover = [
    Instr("POP_TOP"),  # the exception the interpreter pushed for us
    Instr("LOAD_CONST", None),
    Instr("RETURN_VALUE"),
]
recover[0].labels.append(handler)
for instr in recover:
    instr.lineno = bc.instrs[-1].lineno
bc.instrs += recover

# stop is exclusive, so the handler's own label doubles as the end of the
# region it handles.
bc.exc_entries.append(ExcEntry(start, handler, handler))

risky.__code__ = bc.to_code()
assert risky(0) is None

depth defaults to being inferred, which works whenever the depth at the start of the protected region follows from normal control flow. Where it does not — a handler reachable only through another handler's cleanup path, say — to_code() raises instead of guessing, and you pass the depth yourself.

What is checked, and what is not

to_code() computes co_stacksize, resolves labels, encodes the line and exception tables and rejects opcodes the interpreter reserves for itself: the INSTRUMENTED_* family, ENTER_EXECUTOR, CACHE and the pseudo-opcodes. That last check exists because handing some of them to PyCode_New does not raise, it crashes.

Beyond that, this is an assembly language: it will faithfully encode a stack effect that does not balance, a LOAD_FAST reading a variable that was never stored, or a jump into the middle of an instruction's inline caches, and the interpreter will fault on it. Bytecode that is valid runs; bytecode that merely assembles need not.

Finally, opcodes and calling conventions move between releases — RESUME and BINARY_OP arrived in 3.11, POP_JUMP_IF_FALSE wants a real bool on the stack from 3.13, the NULL a call needs is pushed in a different order in 3.13 than in 3.12 — so code written against this API targets an interpreter version in a way that source-level spasm snippets partly hide. The dis module for the version you are targeting is the reference.

Architecture

spasmlang is two layers.

spasm._core is a C++ extension implementing the bytecode data model — Bytecode, Instr, Label, ExcEntry — along with code object decoding and encoding, line table and exception table handling, and stack depth computation. It carries opcode tables generated at build time for the exact interpreter it is compiled against (see setup.py), which is why there is one wheel per Python minor version rather than a single abi3 wheel.

spasm.bytecode is a thin Python layer over it holding the parts that are version-dependent bookkeeping rather than data structure: the Compare and BinaryOp symbolic opargs and their per-version encodings, name-argument packing for LOAD_GLOBAL/LOAD_ATTR, and co_flags inference. spasm.asm builds on both and stays concerned with parsing and assembly.

Stack depth (co_stacksize) is computed for you. Exception table entry depths are inferred too, but only where that can be done exactly — see the note in spasm/bytecode.py and src/stackdepth.cpp; to_code() raises rather than emit a depth it cannot derive correctly.

This code was previously developed as a separate bytecode-native package and has since been absorbed here.

License

spasmlang is distributed under the terms of the MIT license.