magical_py

Native Python bindings for magical_rs, a zero-dependency Rust library that identifies files by their magic bytes.

This is not a drop-in replacement for python-magic, and it is not trying to be one. python-magic returns a string like "image/png" and leaves you to parse it. This returns a typed value that carries its media type and conventional extension with it.

Install

pip install magical-py

There is a binary wheel for every platform we publish for. One abi3 wheel covers CPython 3.8 and later, so you will not have to build from source.

FileKind covers all 114 formats the Rust crate detects, and the enum is declared in Python rather than generated, so it is a real enum.Enum and your editor completes FileKind. with every member and its docstring.

Use

from magical_py import FileKind, detect

kind = detect("photo.jpg")

kind is FileKind.Jpg      # True, a real enum member your editor can complete
kind.value                 # 'jpg', a stable id for serialising
kind.mime                  # 'image/jpeg'
kind.extension             # 'jpg'
kind.description           # 'JPEG'

For data you already have in memory:

from magical_py import detect_bytes

detect_bytes(header)       # FileKind.Png | None

detect takes anything open for binary reading as readily as a path, so a file you already have, a socket, a pipe and a zipfile.ZipExtFile all work the same way:

with open("disc.iso", "rb") as handle:
    kind = detect(handle)

read_header(source, max_bytes=...) is the piece that only reads, for when you want the bytes rather than a verdict about them. What detect and detect_bytes read is a window, and max_bytes_read is how you set it.

Both return None when nothing matches, and raise the usual OSError subclasses when a file cannot be read:

try:
    kind = detect(path)
except FileNotFoundError:
    kind = None

What it looks like

Each member carries its display name as its __doc__, which is the attribute the REPL and help() read. Python 3.9 and later render it for all 114 members; Python 3.8 omits it from help(), so on 3.8 read __doc__ directly as below.

>>> FileKind.Png.__doc__
'PNG'
>>> FileKind.ISO.__doc__
'ISO 9660'
>>> FileKind.PkgZip.description
'Zip / JAR / APK'

Things worth knowing

Detection reads magic bytes only. The file name is never consulted, so a .jpg that actually contains PNG data is reported as FileKind.Png. That is the behaviour you want from a detection library, and it is also why the extension is advisory rather than something to switch on.

Some media types are None. FileKind.mime and FileKind.extension return None when no media type is registered or verified for that format. None means "there isn't one", never "we guessed". A wrong media type would be a documentation bug of the same kind as a wrong magic byte, and this project does not ship those.

detect reads 36,870 bytes. ISO 9660 stores its magic at offset 36,865, so detecting it means reading that far into the file. detect does that for you. magical_py.bytes_read() reports the figure, read_header(path, max_bytes=...) spends it, and max_bytes_read bounds the read on the detection calls themselves. Do not hardcode 2048 and assume you are done.

A short answer and a wrong answer look alike without a window. Give detect_bytes 2,048 bytes cut from an ISO and it returns None, because the magic sits at offset 36,865 and never entered the buffer. A bare None reads as "this is not an ISO", which is not what happened. Name the window and the same call still returns None, and now it means "nothing whose magic fits in 2,048 bytes matched" — a claim about the read, which a caller can reason from:

detect_bytes(header[:2048], max_bytes_read=2048)   # None, about the window
detect("disc.iso", max_bytes_read=2048)            # and reads only 2048 bytes

max_bytes_read is keyword-only and defaults to bytes_read(). Every format in the table declares a read size of at least 2,048, so a window below that excludes PNG along with everything else. That is the trade being made: a cheaper read, and fewer formats reachable inside it.

Some signatures are only two bytes. 9 formats match on nothing but a two-byte prefix: Arj, Bitmap, Gzip, MP3, MSDOS, Pcx, Pickle, SerializedJavaData and Zlib. Those are the values the formats themselves specify, so a file beginning with those bytes is reported as that format even if it is not really one. Each one is listed with its bytes in the magical_rs README.

Two rules look at more than a fixed pattern. FileKind.ScriptExecute requires a / later on the first line, so a bare #! does not count; FileKind.WEBP requires the full RIFF....WEBP layout. The shebang rule exists because #!AMR is the literal magic of AMR audio: without the / check, the script rule claimed those files and they became undetectable.

Asking the table a question

detect answers one question — what is this file — and answers it with the first table entry that matched. Four more ask about the table itself.

What is this format matched on? FileKind.rule says so, without a call:

>>> FileKind.ISO.rule.offsets
(32769, 34817, 36865)
>>> FileKind.Png.rule.signatures
(b'\x89PNG\r\n\x1a\n',)
>>> FileKind.Png.rule.max_bytes_read
2048

describe(kind) is the same thing spelled as a function, and signature_table() returns all 114 entries in the order detection tries them. Walk that list and stop at the first match and you get exactly what detect gives, which is the point: it is the answer to "why did my file come back as this".

Would this one format match? FileKind.matches asks about a single format, ignoring the order:

>>> FileKind.Ktx2.matches(ktx2_bytes)   # True, and detect says Ktx2
True
>>> FileKind.Ktx.matches(ktx2_bytes)    # also True; detect will never say this
True
>>> detect_bytes(ktx2_bytes) is FileKind.Ktx2
True

Ktx's magic is a prefix of Ktx2's, so a KTX2 file is reported as KTX2 and is unreachable as KTX. Qcow is shadowed by Qcow2 the same way, and those two pairs are the only ones in the table — tests/test_signatures.py walks all 114 entries and fails if a third appears.

matches also ignores max_bytes_read, which is the other half of what it does not answer. detect_bytes(PNG, max_bytes_read=1) returns None because no rule's declared read size fits in one byte; FileKind.Png.matches(PNG) is True, because a per-format question has no window to declare.

What does a smaller window cost me? read_limits() reports the crate's own figures instead of making you remember them:

>>> limits = read_limits()
>>> limits.default_max_bytes_read, limits.iso_max_bytes_read
(2048, 36870)
>>> limits.tar_max_bytes_read        # what a tar-only reader needs
262
>>> limits == read_limits()          # a fresh, frozen object each call
True

DEFAULT_MAX_BYTES_READ is the same 2,048 bound as a module constant, for the common case of naming the window you actually intend to read.

The two rules that decide with a function report uses_predicate and no signatures, because there are no bytes to report. ScriptExecute and WEBP are decided by Rust functions over the buffer, and listing the bytes they happen to start with would misdescribe how they work:

>>> FileKind.ScriptExecute.rule.uses_predicate, FileKind.ScriptExecute.rule.signatures
(True, ())

Detection levels

The Rust crate has five detection levels. Level 1 is detect and detect_bytes, the table of 114 formats above. The other four are custom rules, and three of them have a Python counterpart. They differ in what a match costs, which is the whole reason they are three things.

Level 5 is raw pointers. It is unsafe by definition and is not exposed.

Level 2: say what the format looks like

match_types_custom returns the first rule that matched, or the fallback you pass as the last argument. match_types_custom_all returns every kind that matched, in rule order.

from magical_py import MagicCustom, match_types_custom

PNG = bytes([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A])

rules = [
    MagicCustom("gif", [b"GIF87a", b"GIF89a"], [0]),
    MagicCustom("png", [PNG], [0]),
]

match_types_custom(PNG, rules, "unknown")     # 'png'
match_types_custom(b"zzz", rules, "unknown")  # 'unknown', the fallback

A signature matches at any of the offsets, so one rule can cover every variant of a format. Order is priority order: the first match wins and no later rule is tested, which is what makes putting a cheap signature rule first worthwhile.

When the bytes alone are not enough, add predicates:

from magical_py import MagicCustom, MatchRules

def is_utf8(data: bytes) -> bool:
    try:
        data.decode("utf-8")
    except UnicodeDecodeError:
        return False
    return True

rule = MagicCustom("text", rules=MatchRules.all(is_utf8))
rule.matches(header)  # True or False

MatchRules.all needs every predicate, MatchRules.any needs one, and MatchRules.with_fn takes a single one. All three stop as soon as they can. Signatures and predicates are alternatives, not a conjunction: a rule with rules set will not also compare signatures, and asking for both raises ValueError rather than quietly ignoring one of them.

Level 3: decide in Python

Reach for level 3 when the format is not known when you write the code: a rule from a config file, chosen by a plugin, assembled in a loop. match_dyn_types returns the first kind that matched, or None; match_dyn_types_all returns every one, in rule order.

from magical_py import DynMagicCustom, match_dyn_types, match_dyn_types_all

rules = [
    DynMagicCustom(lambda data: data.startswith(b"RIFF"), "riff"),
    DynMagicCustom(lambda data: data[4:8] == b"WEBP", "webp"),
]

match_dyn_types(header, rules)      # a kind, or None
match_dyn_types_all(header, rules)  # every kind that matched, in rule order

Kinds do not have to agree, so one list can mix strings, enums, dataclasses and None. Nothing is converted on the way through; each rule hands back the exact object it was given.

Level 4: decide with something that takes time

match_async_dyn_types and match_async_dyn_types_all are the level 3 pair, awaited.

import asyncio

from magical_py import AsyncDynMagic, match_async_dyn_types

async def registered_checksum(data: bytes) -> bool:
    async with session.get(registry_url) as response:
        return await response.text() == data[:32].hex()

rule = AsyncDynMagic(registered_checksum, "verified")

kind = asyncio.run(match_async_dyn_types(header, [rule]))

The matcher is awaited on the loop you are already on. That is the difference from the crate, which drives its own future from Rust, and it is why level 4 needs no feature flag and no extra dependency, unlike cargo add magical_rs --features magical_async_dyn. A matcher that is handed a coroutine object instead of a function is a TypeError, because a rule gets matched against many buffers and a coroutine can only be awaited once.

Choosing one

Level 2 for a format you know. It is one call with no Python in it, and the difference between that and a Python callable is exactly the difference between the two rows above.

Level 3 for a format you do not know yet, or a rule that is data. Level 4 only when the decision genuinely has to wait for something, since awaiting is worth it exactly when there is something to await.

Examples

Six runnable scripts live in bindings/python/examples/. Each one builds the files it reads from the magic bytes in the format tables above and writes them to a temporary directory, so nothing has to be checked in and every example runs from a bare install:

python 01_detect_a_file.py

tests/test_examples.py runs every one of them and checks the values they print, so an example that stops working fails the suite instead of quietly misleading someone.

Type checking

The package ships a py.typed marker and stubs for the compiled module, and it is checked with pyright in strict mode. pythonVersion is pinned to 3.8 so the checker enforces the same floor as the wheel.

from magical_py import FileKind

# Narrowed to FileKind after the None check.
kind: FileKind | None = detect("photo.jpg")
if kind is not None:
    reveal_type(kind)     # FileKind
    reveal_type(kind.mime)  # str | None

Development

Managed with uv.

uv sync                # create the venv and install maturin, pytest, pyright
uv run maturin develop # build the extension into the venv
uv run pytest          # run the tests
uv run pyright         # type check in strict mode
uv build               # build the wheel and the sdist

python/magical_py/_kinds.py is generated. Edit the metadata table in ../../scripts/gen_kinds.ps1 and re-run it; tests/test_drift.py fails if the enum and the Rust detection table disagree.

The repository's helper scripts live in ../../scripts/, not next to the code they maintain. gates.sh is the one this section describes: it runs the build, the tests, pyright, rustfmt and clippy in one go, and CI runs the same file, so a local run and a CI run cannot disagree about what "green" means.

License

MIT, the same as magical_rs.