On-disk format


These docs are written for anndata 0.7. Files written before this version may differ in some conventions, but will still be read by newer versions of the library.

AnnData objects are saved on disk to hierarchichal array stores like HDF5 (via H5py) and Zarr. This allows us to have very similar structures in disk and on memory.

In general, AnnData objects can hold three kinds of values: (1) dense arrays, (2) sparse arrays, and (3) DataFrames. As an example we’ll look into a typical .h5ad object that’s been through an analysis. This structure should be largely equivalent to Zarr structure, though there are a few minor differences.

>>> import h5py
>>> f = h5py.File("02_processed.h5ad", "r")
>>> list(f.keys())
['X', 'layers', 'obs', 'obsm', 'uns', 'var', 'varm']

Dense arrays

Dense arrays have the most simple representation on disk, as they have native equivalents in H5py Datasets and Zarr Arrays. We can see an example of this with dimensionality reductions stored in the obsm group:

>>> f["obsm"].visititems(print)
X_pca <HDF5 dataset "X_pca": shape (38410, 50), type "<f4">
X_umap <HDF5 dataset "X_umap": shape (38410, 2), type "<f4">

Sparse arrays

Sparse arrays don’t have a native representations in HDF5 or Zarr, so we’ve defined our own based on their in-memory structure. Currently two sparse data formats are supported by AnnData objects, CSC and CSR (corresponding to scipy.sparse.csc_matrix and scipy.sparse.csr_matrix respectivley). These formats represent a two-dimensional sparse array with three one-dimensional arrays, indptr, indices, and data.


A full description of these formats is out of scope for this document, but are easy to find.

We represent a sparse array as a Group on-disk, where the kind and shape of the sparse array is defined in the Group’s attributes:

>>> dict(f["X"].attrs)
{'encoding-type': 'csr_matrix',
 'encoding-version': '0.1.0',
 'shape': array([38410, 27899])}

Inside the group are the three constituent arrays:

>>> f["X"].visititems(print)
data <HDF5 dataset "data": shape (41459314,), type "<f4">
indices <HDF5 dataset "indices": shape (41459314,), type "<i4">
indptr <HDF5 dataset "indptr": shape (38411,), type "<i4">


DataFrames are saved as a columnar format in a group, so each column of a DataFrame gets its own dataset. To maintain efficiency with categorical values, only the numeric codes are stored for each row, while categories values are saved in a reserved subgroup __categories.

Dataframes can be identified from other groups by their attributes:

>>> dict(f["obs"].attrs)
{'_index': 'Cell',
 'column-order': array(['sample', 'cell_type', 'n_genes_by_counts',
       'log1p_n_genes_by_counts', 'total_counts', 'log1p_total_counts',
       'pct_counts_in_top_50_genes', 'pct_counts_in_top_100_genes',
       'pct_counts_in_top_200_genes', 'pct_counts_in_top_500_genes',
       'total_counts_mito', 'log1p_total_counts_mito', 'pct_counts_mito',
       'label_by_score'], dtype=object),
 'encoding-type': 'dataframe',
 'encoding-version': '0.1.0'}

These attributes identify the column used as an index, the order of the original columns, and some type information.

>>> f["obs"].visititems(print)
Cell <HDF5 dataset "Cell": shape (38410,), type "|O">
__categories <HDF5 group "/obs/__categories" (3 members)>
__categories/cell_type <HDF5 dataset "cell_type": shape (22,), type "|O">
__categories/label_by_score <HDF5 dataset "label_by_score": shape (16,), type "|O">
__categories/sample <HDF5 dataset "sample": shape (41,), type "|O">
cell_type <HDF5 dataset "cell_type": shape (38410,), type "|i1">
label_by_score <HDF5 dataset "label_by_score": shape (38410,), type "|i1">
log1p_n_genes_by_counts <HDF5 dataset "log1p_n_genes_by_counts": shape (38410,), type "<f8">

Categorical Series can be identified by the presence of the attribute "categories", which contains a pointer to the categories’ values:

>>> dict(f["obs/cell_type"].attrs)
{'categories': <HDF5 object reference>}


In zarr, as there are no reference objects, the categories attribute is an absolute path to the category values.

Other values


Mappings are simply stored as Group s on disk. These are distinct from DataFrames and sparse arrays since they don’t have any special attributes. A Group is created for any Mapping in the AnnData object, including the standard obsm, varm, layers, and uns. Notably, this definition is used recursively within uns:

>>> f["uns"].visititems(print)
pca <HDF5 group "/uns/pca" (2 members)>
pca/variance <HDF5 dataset "variance": shape (50,), type "<f4">
pca/variance_ratio <HDF5 dataset "variance_ratio": shape (50,), type "<f4">


Zero dimensional arrays are used for scalar values (i.e. single values like strings, numbers or booleans). These should only occur inside of uns, and are common inside of saved parameters:

>>> f["uns/neighbors/params"].visititems(print)
method <HDF5 dataset "method": shape (), type "|O">
metric <HDF5 dataset "metric": shape (), type "|O">
n_neighbors <HDF5 dataset "n_neighbors": shape (), type "<i8">
>>> f["uns/neighbors/params/metric"][()]