Parse FCS (Flow Cytometry Standard) files v2.0-3.1. Extract events as NumPy arrays, read metadata/channels, convert to CSV/DataFrame, for flow cytometry data preprocessing.
license
BSD-3-Clause license
metadata
{"skill-author":"K-Dense Inc."}
verified
false
lastVerifiedAt
"2026-02-19T05:29:09.098Z"
source
builtin
trust_score
100
provenance_sha
ef7b4290346cd37a
FlowIO: Flow Cytometry Standard File Handler
Overview
FlowIO is a lightweight Python library for reading and writing Flow Cytometry Standard (FCS) files. Parse FCS metadata, extract event data, and create new FCS files with minimal dependencies. The library supports FCS versions 2.0, 3.0, and 3.1, making it ideal for backend services, data pipelines, and basic cytometry file operations.
When to Use This Skill
This skill should be used when:
FCS files requiring parsing or metadata extraction
Flow cytometry data needing conversion to NumPy arrays
Event data requiring export to FCS format
Multi-dataset FCS files needing separation
Channel information extraction (scatter, fluorescence, time)
Cytometry file validation or inspection
Pre-processing workflows before advanced analysis
Related Tools: For advanced flow cytometry analysis including compensation, gating, and FlowJo/GatingML support, recommend FlowKit library as a companion to FlowIO.
Installation
uv pip install flowio
Requires Python 3.9 or later.
Quick Start
Basic File Reading
from flowio import FlowData
# Read FCS file
flow_data = FlowData('experiment.fcs')
# Access basic informationprint(f"FCS Version: {flow_data.version}")
print(f"Events: {flow_data.event_count}")
print(f"Channels: {flow_data.pnn_labels}")
# Get event data as NumPy array
events = flow_data.as_array() # Shape: (events, channels)
Creating FCS Files
import numpy as np
from flowio import create_fcs
# Prepare data
data = np.array([[100, 200, 50], [, , ]])
channels = [, , ]
create_fcs(, data, channels)
150
180
60
# 2 events, 3 channels
'FSC-A'
'SSC-A'
'FL1-A'
# Create FCS file
'output.fcs'
Core Workflows
Reading and Parsing FCS Files
The FlowData class provides the primary interface for reading FCS files.
Standard Reading:
from flowio import FlowData
# Basic reading
flow = FlowData('sample.fcs')
# Access attributes
version = flow.version # '3.0', '3.1', etc.
event_count = flow.event_count # Number of events
channel_count = flow.channel_count # Number of channels
pnn_labels = flow.pnn_labels # Short channel names
pns_labels = flow.pns_labels # Descriptive stain names# Get event data
events = flow.as_array() # Preprocessed (gain, log scaling applied)
raw_events = flow.as_array(preprocess=False) # Raw data
Memory-Efficient Metadata Reading:
When only metadata is needed (no event data):
# Only parse TEXT segment, skip DATA and ANALYSIS
flow = FlowData('sample.fcs', only_text=True)
# Access metadata
metadata = flow.text # Dictionary of TEXT segment keywordsprint(metadata.get('$DATE')) # Acquisition dateprint(metadata.get('$CYT')) # Instrument name
Handling Problematic Files:
Some FCS files have offset discrepancies or errors:
# Ignore offset discrepancies between HEADER and TEXT sections
flow = FlowData('problematic.fcs', ignore_offset_discrepancy=True)
# Use HEADER offsets instead of TEXT offsets
flow = FlowData('problematic.fcs', use_header_offsets=True)
# Ignore offset errors entirely
flow = FlowData('problematic.fcs', ignore_offset_error=True)
Excluding Null Channels:
# Exclude specific channels during parsing
flow = FlowData('sample.fcs', null_channel_list=['Time', 'Null'])
Extracting Metadata and Channel Information
FCS files contain rich metadata in the TEXT segment.
# Get indices by channel type
scatter_idx = flow.scatter_indices # [0, 1] for FSC, SSC
fluoro_idx = flow.fluoro_indices # [2, 3, 4] for FL channels
time_idx = flow.time_index # Index of time channel (or None)# Access specific channel types
events = flow.as_array()
scatter_data = events[:, scatter_idx]
fluorescence_data = events[:, fluoro_idx]
ANALYSIS Segment:
If present, access processed results:
if flow.analysis:
analysis_keywords = flow.analysis # Dictionary of ANALYSIS keywordsprint(analysis_keywords)
Creating New FCS Files
Generate FCS files from NumPy arrays or other data sources.
Note: FlowIO exports as FCS 3.1 with single-precision floating-point data.
Exporting Modified Data
Modify existing FCS files and re-export them.
Approach 1: Using write_fcs() Method:
from flowio import FlowData
# Read original file
flow = FlowData('original.fcs')
# Write with updated metadata
flow.write_fcs('modified.fcs', metadata={'$SRC': 'Modified data'})
Approach 2: Extract, Modify, and Recreate:
For modifying event data:
from flowio import FlowData, create_fcs
# Read and extract data
flow = FlowData('original.fcs')
events = flow.as_array(preprocess=False)
# Modify event data
events[:, 0] = events[:, 0] * 1.5# Scale first channel# Create new FCS file with modified data
create_fcs('modified.fcs',
events,
flow.pnn_labels,
opt_channel_names=flow.pns_labels,
metadata=flow.text)
Handling Multi-Dataset FCS Files
Some FCS files contain multiple datasets in a single file.
Detecting Multi-Dataset Files:
from flowio import FlowData, MultipleDataSetsError
try:
flow = FlowData('sample.fcs')
except MultipleDataSetsError:
print("File contains multiple datasets")
# Use read_multiple_data_sets() instead
Reading All Datasets:
from flowio import read_multiple_data_sets
# Read all datasets from file
datasets = read_multiple_data_sets('multi_dataset.fcs')
print(f"Found {len(datasets)} datasets")
# Process each datasetfor i, dataset inenumerate(datasets):
print(f"\nDataset {i}:")
print(f" Events: {dataset.event_count}")
print(f" Channels: {dataset.pnn_labels}")
# Get event data for this dataset
events = dataset.as_array()
print(f" Shape: {events.shape}")
print(f" Mean values: {events.mean(axis=0)}")
Reading Specific Dataset:
from flowio import FlowData
# Read first dataset (nextdata_offset=0)
first_dataset = FlowData('multi.fcs', nextdata_offset=0)
# Read second dataset using NEXTDATA offset from first
next_offset = int(first_dataset.text['$NEXTDATA'])
if next_offset > 0:
second_dataset = FlowData('multi.fcs', nextdata_offset=next_offset)
Data Preprocessing
FlowIO applies standard FCS preprocessing transformations when preprocess=True.
Preprocessing Steps:
Gain Scaling: Multiply values by PnG (gain) keyword
Logarithmic Transformation: Apply PnE exponential transformation if present
Formula: value = a * 10^(b * raw_value) where PnE = "a,b"
Time Scaling: Convert time values to appropriate units
Controlling Preprocessing:
# Preprocessed data (default)
preprocessed = flow.as_array(preprocess=True)
# Raw data (no transformations)
raw = flow.as_array(preprocess=False)
Error Handling
Handle common FlowIO exceptions appropriately.
from flowio import (
FlowData,
FCSParsingError,
DataOffsetDiscrepancyError,
MultipleDataSetsError
)
try:
flow = FlowData('sample.fcs')
events = flow.as_array()
except FCSParsingError as e:
print(f"Failed to parse FCS file: {e}")
# Try with relaxed parsing
flow = FlowData('sample.fcs', ignore_offset_error=True)
except DataOffsetDiscrepancyError as e:
print(f"Offset discrepancy detected: {e}")
# Use ignore_offset_discrepancy parameter
flow = FlowData('sample.fcs', ignore_offset_discrepancy=True)
except MultipleDataSetsError as e:
print(f"Multiple datasets detected: {e}")
# Use read_multiple_data_sets insteadfrom flowio import read_multiple_data_sets
datasets = read_multiple_data_sets('sample.fcs')
except Exception as e:
print(f"Unexpected error: {e}")
Common Use Cases
Inspecting FCS File Contents
Quick exploration of FCS file structure:
from flowio import FlowData
flow = FlowData('unknown.fcs')
print("=" * 50)
print(f"File: {flow.name}")
print(f"Version: {flow.version}")
print(f"Size: {flow.file_size:,} bytes")
print("=" * 50)
print(f"\nEvents: {flow.event_count:,}")
print(f"Channels: {flow.channel_count}")
print("\nChannel Information:")
for i, (pnn, pns) inenumerate(zip(flow.pnn_labels, flow.pns_labels)):
ch_type = "scatter"if i in flow.scatter_indices else \
"fluoro"if i in flow.fluoro_indices else \
"time"if i == flow.time_index else"other"print(f" [{i}] {pnn:10s} | {pns:30s} | {ch_type}")
print("\nKey Metadata:")
for key in ['$DATE', '$BTIM', '$ETIM', '$CYT', '$INST', '$SRC']:
value = flow.text.get(key, 'N/A')
print(f" {key:15s}: {value}")
Batch Processing Multiple Files
Process a directory of FCS files:
from pathlib import Path
from flowio import FlowData
import pandas as pd
# Find all FCS files
fcs_files = list(Path('data/').glob('*.fcs'))
# Extract summary information
summaries = []
for fcs_path in fcs_files:
try:
flow = FlowData(str(fcs_path), only_text=True)
summaries.append({
'filename': fcs_path.name,
'version': flow.version,
'events': flow.event_count,
'channels': flow.channel_count,
'date': flow.text.get('$DATE', 'N/A')
})
except Exception as e:
print(f"Error processing {fcs_path.name}: {e}")
# Create summary DataFrame
df = pd.DataFrame(summaries)
print(df)
Converting FCS to CSV
Export event data to CSV format:
from flowio import FlowData
import pandas as pd
# Read FCS file
flow = FlowData('sample.fcs')
# Convert to DataFrame
df = pd.DataFrame(
flow.as_array(),
columns=flow.pnn_labels
)
# Add metadata as attributes
df.attrs['fcs_version'] = flow.version
df.attrs['instrument'] = flow.text.get('$CYT', 'Unknown')
# Export to CSV
df.to_csv('output.csv', index=False)
print(f"Exported {len(df)} events to CSV")
Filtering Events and Re-exporting
Apply filters and save filtered data:
from flowio import FlowData, create_fcs
import numpy as np
# Read original file
flow = FlowData('sample.fcs')
events = flow.as_array(preprocess=False)
# Apply filtering (example: threshold on first channel)
fsc_idx = 0
threshold = 500
mask = events[:, fsc_idx] > threshold
filtered_events = events[mask]
print(f"Original events: {len(events)}")
print(f"Filtered events: {len(filtered_events)}")
# Create new FCS file with filtered data
create_fcs('filtered.fcs',
filtered_events,
flow.pnn_labels,
opt_channel_names=flow.pns_labels,
metadata={**flow.text, '$SRC': 'Filtered data'})
Extracting Specific Channels
Extract and process specific channels:
from flowio import FlowData
import numpy as np
flow = FlowData('sample.fcs')
events = flow.as_array()
# Extract fluorescence channels only
fluoro_indices = flow.fluoro_indices
fluoro_data = events[:, fluoro_indices]
fluoro_names = [flow.pnn_labels[i] for i in fluoro_indices]
print(f"Fluorescence channels: {fluoro_names}")
print(f"Shape: {fluoro_data.shape}")
# Calculate statistics per channelfor i, name inenumerate(fluoro_names):
channel_data = fluoro_data[:, i]
print(f"\n{name}:")
print(f" Mean: {channel_data.mean():.2f}")
print(f" Median: {np.median(channel_data):.2f}")
print(f" Std Dev: {channel_data.std():.2f}")
Best Practices
Memory Efficiency: Use only_text=True when event data is not needed
Error Handling: Wrap file operations in try-except blocks for robust code
Multi-Dataset Detection: Check for MultipleDataSetsError and use appropriate function
Preprocessing Control: Explicitly set preprocess parameter based on analysis needs
Offset Issues: If parsing fails, try ignore_offset_discrepancy=True parameter
Channel Validation: Verify channel counts and names match expectations before processing
Metadata Preservation: When modifying files, preserve original TEXT segment keywords
Advanced Topics
Understanding FCS File Structure
FCS files consist of four segments:
HEADER: FCS version and byte offsets for other segments
For comprehensive API documentation including all parameters, methods, exceptions, and FCS keyword reference, consult the detailed reference file:
Read:references/api_reference.md
The reference includes:
Complete FlowData class documentation
All utility functions (read_multiple_data_sets, create_fcs)
Exception classes and handling
FCS file structure details
Common TEXT segment keywords
Extended example workflows
When working with complex FCS operations or encountering unusual file formats, load this reference for detailed guidance.
Integration Notes
NumPy Arrays: All event data is returned as NumPy ndarrays with shape (events, channels)
Pandas DataFrames: Easily convert to DataFrames for analysis:
import pandas as pd
df = pd.DataFrame(flow.as_array(), columns=flow.pnn_labels)
FlowKit Integration: For advanced analysis (compensation, gating, FlowJo support), use FlowKit library which builds on FlowIO's parsing capabilities
Web Applications: FlowIO's minimal dependencies make it ideal for web backend services processing FCS uploads
Troubleshooting
Problem: "Offset discrepancy error"
Solution: Use ignore_offset_discrepancy=True parameter
Problem: "Multiple datasets error"
Solution: Use read_multiple_data_sets() function instead of FlowData constructor
Problem: Out of memory with large files
Solution: Use only_text=True for metadata-only operations, or process events in chunks
Problem: Unexpected channel counts
Solution: Check for null channels; use null_channel_list parameter to exclude them
Problem: Cannot modify event data in place
Solution: FlowIO doesn't support direct modification; extract data, modify, then use create_fcs() to save
Summary
FlowIO provides essential FCS file handling capabilities for flow cytometry workflows. Use it for parsing, metadata extraction, and file creation. For simple file operations and data extraction, FlowIO is sufficient. For complex analysis including compensation and gating, integrate with FlowKit or other specialized tools.
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