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chemigram.core.exif

chemigram.core.exif

Read camera/lens metadata from raw files for L1 vocabulary binding.

Uses exifread (pure-Python, no native deps) to extract the EXIF fields chemigram cares about: Make, Model, LensModel, FocalLength. Per RFC-015 / ADR-053, downstream binding (:mod:chemigram.core.binding) is exact-match on (make, model, lens_model) — this module only extracts.

Per ADR-007 (BYOA), the dependency stays in chemigram.core's graph because EXIF is structural metadata, not AI capability — every raw a photographer drops in needs identity resolution before any vocabulary applies. PyExifTool (faster, more complete) is rejected for v1 because it requires the exiftool binary as an external dep.

Public API
  • :func:read_exif — extract EXIF from a raw file
  • :class:ExifData — frozen dataclass with the four fields we model
  • :class:ExifReadError — exception raised on unreadable input

ExifReadError

Bases: Exception

Raised when EXIF cannot be read from a file.

ExifData dataclass 

ExifData(make, model, lens_model, focal_length_mm)

The EXIF fields chemigram cares about for L1 binding.

String fields are whitespace- and null-stripped. Missing string fields become empty strings (not None) so callers don't need to special-case absence vs. presence-of-empty.

read_exif 

read_exif(path)

Read relevant EXIF tags from a raw file.

Parameters:

Name Type Description Default
path Path

path to a raw (NEF, ARW, RAF, CR2, ...).

 required

Returns:

Type Description
ExifData

class:ExifData with whitespace-stripped strings; missing
ExifData

string fields become ""; missing focal_length_mm
ExifData

becomes None.

Raises:

Type Description
ExifReadError

corrupt or unreadable file.
FileNotFoundError

path does not exist.
Source code in src/chemigram/core/exif.py 
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def read_exif(path: Path) -> ExifData:
    """Read relevant EXIF tags from a raw file.

    Args:
        path: path to a raw (NEF, ARW, RAF, CR2, ...).

    Returns:
        :class:`ExifData` with whitespace-stripped strings; missing
        string fields become ``""``; missing ``focal_length_mm``
        becomes ``None``.

    Raises:
        ExifReadError: corrupt or unreadable file.
        FileNotFoundError: ``path`` does not exist.
    """
    if not path.exists():
        raise FileNotFoundError(path)

    try:
        with path.open("rb") as fh:
            tags = exifread.process_file(fh, details=False)
    except (OSError, ValueError, KeyError, IndexError, TypeError, AttributeError) as exc:
        # exifread doesn't expose a single error type; these cover the
        # families we've observed (corrupt files, malformed IFD pointers,
        # truncated streams). Letting other exceptions propagate is
        # intentional — they signal genuine bugs, not bad input.
        raise ExifReadError(f"failed to read EXIF from {path}: {exc}") from exc

    make = _stringify_tag(tags.get("Image Make"))
    model = _stringify_tag(tags.get("Image Model"))
    lens_model = _stringify_tag(tags.get("EXIF LensModel")) or _stringify_tag(
        tags.get("MakerNote LensModel")
    )
    focal_length = _focal_length_mm(tags.get("EXIF FocalLength"))

    return ExifData(
        make=make,
        model=model,
        lens_model=lens_model,
        focal_length_mm=focal_length,
    )