AnnData

Overview

AnnData is a Python package for handling annotated data matrices, storing experimental measurements (X) alongside observation metadata (obs), variable metadata (var), and multi-dimensional annotations (obsm, varm, obsp, varp, uns). Originally designed for single-cell genomics through Scanpy, it now serves as a general-purpose framework for any annotated data requiring efficient storage, manipulation, and analysis.

When to Use This Skill

Use this skill when:

• Creating, reading, or writing AnnData objects
• Working with h5ad, zarr, or other genomics data formats
• Performing single-cell RNA-seq analysis
• Managing large datasets with sparse matrices or backed mode
• Concatenating multiple datasets or experimental batches
• Subsetting, filtering, or transforming annotated data
• Integrating with scanpy, scvi-tools, or other scverse ecosystem tools

Installation

uv pip install anndata          # 0.11+ (the API namespaces below assume >= 0.11)

# Optional extra for Dask-backed lazy reads (ad.experimental.read_lazy)
uv pip install 'anndata[dask]'

Quick Start

Creating an AnnData object

import anndata as ad
import numpy as np
import pandas as pd

# Minimal creation
X = np.random.rand(100, 2000)  # 100 cells × 2000 genes
adata = ad.AnnData(X)

# With metadata
obs = pd.DataFrame({
    'cell_type': ['T cell', 'B cell'] * 50,
    'sample': ['A', 'B'] * 50
}, index=[f'cell_{i}' for i in range(100)])

var = pd.DataFrame({
    'gene_name': [f'Gene_{i}' for i in range(2000)]
}, index=[f'ENSG{i:05d}' for i in range(2000)])

adata = ad.AnnData(X=X, obs=obs, var=var)

Reading data

import scanpy as sc  # 10x readers live in scanpy, not anndata

# Read h5ad file
adata = ad.read_h5ad('data.h5ad')

# Read with backed mode (for large files)
adata = ad.read_h5ad('large_data.h5ad', backed='r')

# Read other formats (these live under ad.io as of anndata 0.11)
adata = ad.io.read_csv('data.csv')
adata = ad.io.read_loom('data.loom')
adata = sc.read_10x_h5('filtered_feature_bc_matrix.h5')

API namespaces (anndata >= 0.11): all format readers/writers moved to the anndata.io module (ad.io.read_csv, ad.io.read_mtx, ad.io.read_loom, ad.io.read_elem, ...). The top-level ad.read_csv-style aliases still work but emit a DeprecationWarning. Exceptions: ad.read_h5ad, ad.read_zarr, adata.write_h5ad, and adata.write_zarr stay top-level with no warning.

10x readers (read_10x_h5, read_10x_mtx) live in scanpy (sc.read_10x_h5), not anndata — this skill defers analysis-specific I/O to scanpy.

Writing data

# Write h5ad file
adata.write_h5ad('output.h5ad')

# Write with compression
adata.write_h5ad('output.h5ad', compression='gzip')

# Write other formats
adata.write_zarr('output.zarr')
adata.write_csvs('output_dir/')

Basic operations

# Subset by conditions
t_cells = adata[adata.obs['cell_type'] == 'T cell']

# Subset by indices
subset = adata[0:50, 0:100]

# Add metadata
adata.obs['quality_score'] = np.random.rand(adata.n_obs)
adata.var['highly_variable'] = np.random.rand(adata.n_vars) > 0.8

# Access dimensions
print(f"{adata.n_obs} observations × {adata.n_vars} variables")

Core Capabilities

1. Data Structure

Understand the AnnData object structure including X, obs, var, layers, obsm, varm, obsp, varp, uns, and raw components.

See: references/data_structure.md for comprehensive information on:

• Core components (X, obs, var, layers, obsm, varm, obsp, varp, uns, raw)
• Creating AnnData objects from various sources
• Accessing and manipulating data components
• Memory-efficient practices

2. Input/Output Operations

Read and write data in various formats with support for compression, backed mode, and cloud storage.

See: references/io_operations.md for details on:

• Native formats (h5ad, zarr)
• Alternative formats (CSV, MTX, Loom, 10X, Excel)
• Backed mode for large datasets
• Remote data access
• Format conversion
• Performance optimization

Common commands:

# Read/write h5ad
adata = ad.read_h5ad('data.h5ad', backed='r')
adata.write_h5ad('output.h5ad', compression='gzip')

# Read 10X data (10x readers live in scanpy, not anndata)
import scanpy as sc
adata = sc.read_10x_h5('filtered_feature_bc_matrix.h5')

# Read MTX format (.mtx is variables x observations; transpose so cells are rows)
adata = ad.io.read_mtx('matrix.mtx').T

3. Concatenation

Combine multiple AnnData objects along observations or variables with flexible join strategies.

See: references/concatenation.md for comprehensive coverage of:

• Basic concatenation (axis=0 for observations, axis=1 for variables)
• Join types (inner, outer)
• Merge strategies (same, unique, first, only)
• Tracking data sources with labels
• Lazy concatenation (AnnCollection)
• On-disk concatenation for large datasets

Common commands:

# Concatenate observations (combine samples)
adata = ad.concat(
    [adata1, adata2, adata3],
    axis=0,
    join='inner',
    label='batch',
    keys=['batch1', 'batch2', 'batch3']
)

# Concatenate variables (combine modalities)
adata = ad.concat([adata_rna, adata_protein], axis=1)

# Lazy concatenation
from anndata.experimental import AnnCollection
collection = AnnCollection(
    ['data1.h5ad', 'data2.h5ad'],
    join_obs='outer',
    label='dataset'
)

4. Data Manipulation

Transform, subset, filter, and reorganize data efficiently.

See: references/manipulation.md for detailed guidance on:

• Subsetting (by indices, names, boolean masks, metadata conditions)
• Transposition
• Copying (full copies vs views)
• Renaming (observations, variables, categories)
• Type conversions (strings to categoricals, sparse/dense)
• Adding/removing data components
• Reordering
• Quality control filtering

Common commands:

# Subset by metadata
filtered = adata[adata.obs['quality_score'] > 0.8]
hv_genes = adata[:, adata.var['highly_variable']]

# Transpose
adata_T = adata.T

# Copy vs view
view = adata[0:100, :]  # View (lightweight reference)
copy = adata[0:100, :].copy()  # Independent copy

# Convert strings to categoricals
adata.strings_to_categoricals()

5. Best Practices

Follow recommended patterns for memory efficiency, performance, and reproducibility.

See: references/best_practices.md for guidelines on:

• Memory management (sparse matrices, categoricals, backed mode)
• Views vs copies
• Data storage optimization
• Performance optimization
• Working with raw data
• Metadata management
• Reproducibility
• Error handling
• Integration with other tools
• Common pitfalls and solutions

Key recommendations:

# Use sparse matrices for sparse data
from scipy.sparse import csr_matrix
adata.X = csr_matrix(adata.X)

# Convert strings to categoricals
adata.strings_to_categoricals()

# Use backed mode for large files
adata = ad.read_h5ad('large.h5ad', backed='r')

# Store raw before filtering
adata.raw = adata.copy()
adata = adata[:, adata.var['highly_variable']]

Integration with Scverse Ecosystem

AnnData serves as the foundational data structure for the scverse ecosystem:

Scanpy (Single-cell analysis)

AnnData is scanpy's native object — once built/loaded, pass it straight in. Preprocessing, dimensionality reduction, clustering, and plotting (sc.pp.normalize_total, sc.pp.highly_variable_genes, sc.pp.pca, sc.pp.neighbors, sc.tl.umap, sc.tl.leiden, sc.pl.*) are scanpy's job, not anndata's — defer the analysis workflow there.

import scanpy as sc

sc.pp.filter_cells(adata, min_genes=200)  # scanpy mutates the AnnData in place
# ... continue the analysis pipeline in scanpy

Muon (Multimodal data)

import muon as mu

# Combine RNA and protein data
mdata = mu.MuData({'rna': adata_rna, 'protein': adata_protein})

PyTorch integration

from anndata.experimental import AnnLoader

# Create DataLoader for deep learning (also accepts an AnnCollection)
dataloader = AnnLoader(adata, batch_size=128, shuffle=True)

for batch in dataloader:
    X = batch["X"]      # dict-style access; tensors, not attributes
    labels = batch["obs"]["cell_type"]
    # Train model

Common Workflows

Single-cell data lifecycle (the anndata-owned parts)

Load, compute simple QC metrics on obs/var, snapshot raw, subset, and write. The normalize/log1p/HVG/cluster steps belong to scanpy — hand off there.

import anndata as ad
import scanpy as sc

# 1. Load (10x readers live in scanpy)
adata = sc.read_10x_h5('filtered_feature_bc_matrix.h5')

# 2. Quick QC metrics on obs/var, then mask-subset (pure anndata wrangling)
adata.obs['n_genes'] = (adata.X > 0).sum(axis=1)
adata.obs['n_counts'] = adata.X.sum(axis=1)
adata = adata[(adata.obs['n_genes'] > 200) & (adata.obs['n_counts'] < 50000)].copy()

# 3. Snapshot raw before any gene filtering
adata.raw = adata.copy()

# 4. Hand off normalization / HVG / clustering to scanpy, then come back:
#    sc.pp.normalize_total / sc.pp.log1p / sc.pp.highly_variable_genes / ...
adata = adata[:, adata.var['highly_variable']].copy()  # subset is anndata's job

# 5. Save processed data
adata.write_h5ad('processed.h5ad', compression='gzip')

Batch integration (concatenate, then defer correction)

# Load and concatenate batches with source labels — this is anndata's job
adatas = [ad.read_h5ad(p) for p in ['batch1.h5ad', 'batch2.h5ad', 'batch3.h5ad']]
adata = ad.concat(
    adatas,
    label='batch',
    keys=['batch1', 'batch2', 'batch3'],
    join='inner',
)

# Batch correction and downstream analysis (combat / pca / neighbors / umap)
# are scanpy territory — pass `adata` to scanpy from here.

Working with large datasets

# Open in backed mode
adata = ad.read_h5ad('100GB_dataset.h5ad', backed='r')

# Filter based on metadata (no data loading)
high_quality = adata[adata.obs['quality_score'] > 0.8]

# Load filtered subset
adata_subset = high_quality.to_memory()

# Process subset
process(adata_subset)

# Or process in chunks
chunk_size = 1000
for i in range(0, adata.n_obs, chunk_size):
    chunk = adata[i:i+chunk_size, :].to_memory()
    process(chunk)

Troubleshooting

Out of memory errors

Use backed mode or convert to sparse matrices:

# Backed mode
adata = ad.read_h5ad('file.h5ad', backed='r')

# Sparse matrices
from scipy.sparse import csr_matrix
adata.X = csr_matrix(adata.X)

Slow file reading

Use compression and appropriate formats:

# Optimize for storage
adata.strings_to_categoricals()
adata.write_h5ad('file.h5ad', compression='gzip')

# Use Zarr for cloud storage
adata.write_zarr('file.zarr', chunks=(1000, 1000))

Index alignment issues

Always align external data on index:

# Wrong
adata.obs['new_col'] = external_data['values']

# Correct
adata.obs['new_col'] = external_data.set_index('cell_id').loc[adata.obs_names, 'values']

Additional Resources

• Official documentation: https://anndata.readthedocs.io/
• Scanpy tutorials: https://scanpy.readthedocs.io/
• Scverse ecosystem: https://scverse.org/
• GitHub repository: https://github.com/scverse/anndata