Python API

FOSU is in active development; its API may change without compatibility shims. Parsing returns eager objects and lists, detached from native memory. All records are read-only native objects with eager Python fields, declared and typed in _model.py. Attribute reads do not allocate numeric values. Lists remain ordinary mutable Python lists.

Install and use

Requires CPython 3.10+. Stable-ABI wheels target glibc and musl Linux on x86-64 and AArch64, plus macOS 11+ on Intel and Apple Silicon. One wheel per platform supports every compatible CPython version. Windows x86-64 is supported both natively and through the Linux x86-64 wheel under WSL.

python -m pip install fosu

Installing from a source checkout requires a C++20 compiler. An ordinary isolated pip install . provisions CMake and Ninja when needed; Unix Makefiles are also supported when Ninja is unavailable. --no-build-isolation makes the caller responsible for all build tools.

import fosu

beatmap = fosu.parse_file("map.osu")
for note in beatmap.hit_objects:
    if isinstance(note, fosu.Slider):
        print(note.time, note.length, note.control_points)

parse_file(path) accepts strings, bytes, and os.PathLike. parse(data) accepts buffer-protocol objects, including bytes, bytearray, memoryview, and arrays. Non-bytes buffers are copied to an immutable snapshot; encode text explicitly.

Inputs are bounded by available address space. Allocation failures raise MemoryError, and file failures raise OSError. Native parsing releases the GIL; Python value construction holds it. Concurrent calls return independent results. Malformed records are skipped and counted in beatmap.stats.malformed_lines; success does not certify playability.

Performance

FOSU 0.5.0 (798b810), measured on 2026-09-20 using CPython 3.12.14 on an Intel Core i7-8700 under Linux/WSL2. All rows use the same 1,004 mutually accepted all-mode maps. Times include eager result construction and release; lower is better. Warm-file measurements include opening and reading page-cached files.

Figures are medians of complete passes, not fastest individual parses. Six passes per API were collected; passes more than 5% above their API's unfiltered median are excluded as presumed interference, leaving five or six per result. The parsers expose different models: OsuPyParser also performs derived-statistic work. See the comparison and measured variation for result contracts, or the feature-cost tables for slider geometry, gameplay, mods, and ARM measurements.

Section selection

listing = fosu.parse_file(
    "map.osu",
    sections=fosu.Sections.METADATA | fosu.Sections.DIFFICULTY,
)

Both entry points accept keyword-only sections, defaulting to Sections.ALL. Members are GENERAL, EDITOR, METADATA, DIFFICULTY, EVENTS, TIMING_POINTS, COLOURS, and HIT_OBJECTS. Combine them with |. Skipped sections retain defaults and empty lists; Sections(0) selects none. Unsupported mask bits are rejected.

Include GENERAL for mode-dependent difficulty rules (such as mania CircleSize) and EVENTS for break-dependent combo rules. Only selected sections contribute malformed-line counts.

Results and important distinctions

The model classes in _model.py define the fields and convenience methods. Shared fields use C++ names. Important Python-specific behavior:

• hit_objects contains Circle, Slider, Spinner, or HoldNote, in stable timestamp order. Narrow the union with isinstance.
• Times are milliseconds. Slider endpoints are 0 by default. Pass calculate_slider_end_times=True to parse or parse_file to calculate them using curve distance, timing and repeats. Do not use slider endpoints unless calculation was requested. Accessing the field never computes or caches anything. Other object types still have numeric endpoints. Omitted sections use their default settings. Hit samples and slider edge fields remain text.
• Slider control_points includes the head position, unlike the native point range. curve_segments preserves modern segment boundaries and explicit B-spline degrees. slides=2 means forward and back.
• tag_list and bookmark_list are parsed conveniences alongside the tags and bookmarks text fields.
• IDs and preview time map the -1 sentinel to None.
• Strings decode with UTF-8 surrogateescape, preserving undecodable bytes.
• Inherited timing-point NaN beat lengths are preserved; consumers must not treat them as ordinary slider velocities.

Records support equality, repr, deepcopy, and pickle. They are not dataclasses: dataclasses.asdict and dataclasses.replace do not apply. Public record classes cannot be subclassed, and attributes cannot be assigned or deleted. Constructors accept positional or keyword arguments and retain the supplied values without runtime type validation. Parsed numeric fields are always eagerly converted to their documented Python types.

List contents can be changed without affecting the input or another parse result. Such changes do not recompute derived data. All records participate in cyclic garbage collection, including cycles consumers create through lists or values passed to constructors.

Pass calculate_slider_paths=True to retain each slider's path (otherwise None). fosu.slider_position_at(slider.path, progress) is a pure query over that polyline, with progress clamped to [0, 1]. Returned PathPoint coordinates are relative to the head; add the slider's x/y for playfield coordinates. Paths alone do not calculate end times. Requesting both reuses their distance. Native code exposes the same query and Beatmap.slider_paths, indexed by slider.

calculate_slider_events=True additionally populates slider.events with HEAD, TICK, REPEAT, LEGACY_LAST_TICK, and TAIL records in official generator order, grouped by slider-span traversal. This is not necessarily timestamp order: a legacy last tick may be timed before a late tick or the repeat beginning its final span. The legacy event is the effective historical tail judgement, not an additional score or combo event; the real TAIL and slider end_time remain unchanged. This option includes path and end-time calculation. Each record has time, span index/start time, path progress, and a position relative to the head. Native code exposes Beatmap.slider_events, indexed by slider. Without the option, events are empty. These are path events using the decoded slider's timing, not a converted ruleset's nested hitobjects: no samples or catch conversion. Expansion beyond 1,048,576 events per map raises MemoryError rather than silently dropping events.

apply_stacking=True applies unmodded osu!standard stacking after parsing, including the pre-v6 algorithm. Hit-object x/y and absolute slider control points are adjusted before returning. obj.raw_position() subtracts the stacking offset from the current x/y, or returns x/y when stacking is absent. It can have small floating-point rounding differences. Coordinates are floats in both interfaces, including when stacking is disabled. Each object also retains stack_height and stack_offset in its Stacking value for inspection; do not add the offset again. Relative path/event positions remain unchanged: add them to the adjusted head. It includes path and end-time calculation, but not events. Native results use Beatmap.stacking, indexed by hit object. Other modes are unchanged; Python stacking is None when not calculated. Include GENERAL, DIFFICULTY, TIMING_POINTS and HIT_OBJECTS when selecting sections for meaningful results.

mods= accepts combined Mods values. EZ and HR adjust difficulty settings for osu!standard and osu!taiko; HR also reflects standard hit objects and slider control points vertically. EZ/HR require GENERAL and DIFFICULTY in sections. They are not yet supported for osu!catch or osu!mania because those modes require converted fruit offsets or resolved hit windows; support is planned. DT, NIGHTCORE, and HT support every mode and divide gameplay timeline values by 1.5, 1.5, and 0.75 respectively. This includes hit objects, calculated slider events, timing-point offsets and uninherited beat lengths, and breaks. General and Editor timestamp metadata remains in source-map time.

See compatibility for supported behavior and limitations; there is no ruleset conversion.

CPU selection

fosu.backend reports "avx2", "neon", or "scalar". Set FOSU_BACKEND=auto|scalar|avx2|neon before importing to select an engine. Unsupported explicit requests raise ImportError; selection stays fixed for the loaded extension. See development checks.