File I/O and Data Formats

Overview

PyOpenMS supports multiple mass spectrometry file formats for reading and writing. This guide covers file handling strategies and format-specific operations.

Supported Formats

Spectrum Data Formats

mzML: Standard XML-based format for mass spectrometry data
mzXML: Earlier XML-based format
mzData: XML format (deprecated but supported)

Identification Formats

idXML: OpenMS native identification format
mzIdentML: Standard XML format for identification data
pepXML: X! Tandem format
protXML: Protein identification format

Feature and Quantitation Formats

featureXML: OpenMS format for detected features
consensusXML: Format for consensus features across samples
mzTab: Tab-delimited format for reporting

Sequence and Library Formats

FASTA: Protein/peptide sequences
TraML: Transition lists for targeted experiments

Reading mzML Files

In-Memory Loading

Load entire file into memory (suitable for smaller files):

python

import pyopenms as ms

# Create experiment container
exp = ms.MSExperiment()

# Load file
ms.MzMLFile().load("sample.mzML", exp)

# Access data
print(f"Spectra: {exp.getNrSpectra()}")
print(f"Chromatograms: {exp.getNrChromatograms()}")

Indexed Access

Efficient random access for large files:

python

# Create indexed access
indexed_mzml = ms.IndexedMzMLFileLoader()
indexed_mzml.load("large_file.mzML")

# Get specific spectrum by index
spec = indexed_mzml.getSpectrumById(100)

# Access by native ID
spec = indexed_mzml.getSpectrumByNativeId("scan=5000")

Streaming / On-Disc Access

Memory-efficient processing for very large files uses OnDiscMSExperiment, which parses the index of an indexed mzML and loads spectra on demand instead of holding the whole run in memory. (The old MSExperimentConsumer subclassing pattern is not available in pyOpenMS 3.5.)

python

# Requires an indexed mzML. Write one with the write-index option set:
exp = ms.MSExperiment()
ms.MzMLFile().load("large.mzML", exp)
f = ms.MzMLFile()
opt = f.getOptions(); opt.setWriteIndex(True); f.setOptions(opt)
f.store("large_indexed.mzML", exp)

# Now access spectra lazily, one at a time
od = ms.OnDiscMSExperiment()
if od.openFile("large_indexed.mzML"):
    count = 0
    for i in range(od.getNrSpectra()):
        spec = od.getSpectrum(i)   # loaded from disk on demand
        if spec.getMSLevel() == 2:
            count += 1
    print(f"Processed {count} MS2 spectra")

Cached Access

A cached binary representation trades a little disk space for faster repeated reads. Use the static CachedmzML.store/load methods:

python

# Write a cached representation
exp = ms.MSExperiment()
ms.MzMLFile().load("sample.mzML", exp)
ms.CachedmzML().store("sample.cachedmzML", exp)

# Load it back for on-demand spectrum access
cached = ms.CachedmzML()
ms.CachedmzML().load("sample.cachedmzML", cached)
print(f"{cached.getNrSpectra()} spectra")
spec = cached.getSpectrum(0)

Writing mzML Files

Basic Writing

python

# Create or modify experiment
exp = ms.MSExperiment()
# ... add spectra ...

# Write to file
ms.MzMLFile().store("output.mzML", exp)

Compression Options

python

# Configure compression
file_handler = ms.MzMLFile()

options = ms.PeakFileOptions()
options.setCompression(True)  # Enable compression
file_handler.setOptions(options)

file_handler.store("compressed.mzML", exp)

Reading Identification Data

idXML Format

python

# Load identification results
protein_ids = []                              # protein IDs: plain list
# pyOpenMS 3.5+: peptide IDs must be a PeptideIdentificationList, not a plain list
peptide_ids = ms.PeptideIdentificationList()

ms.IdXMLFile().load("identifications.idXML", protein_ids, peptide_ids)

# Access peptide identifications
for peptide_id in peptide_ids:
    print(f"RT: {peptide_id.getRT()}")
    print(f"MZ: {peptide_id.getMZ()}")

    # Get peptide hits
    for hit in peptide_id.getHits():
        print(f"  Sequence: {hit.getSequence().toString()}")
        print(f"  Score: {hit.getScore()}")
        print(f"  Charge: {hit.getCharge()}")

mzIdentML Format

python

# Read mzIdentML
protein_ids = []                              # protein IDs: plain list
peptide_ids = ms.PeptideIdentificationList()  # pyOpenMS 3.5+: not a plain list

ms.MzIdentMLFile().load("results.mzid", protein_ids, peptide_ids)

pepXML Format

python

# Load pepXML
protein_ids = []                              # protein IDs: plain list
peptide_ids = ms.PeptideIdentificationList()  # pyOpenMS 3.5+: not a plain list

ms.PepXMLFile().load("results.pep.xml", protein_ids, peptide_ids)

Reading Feature Data

featureXML

python

# Load features
feature_map = ms.FeatureMap()
ms.FeatureXMLFile().load("features.featureXML", feature_map)

# Access features
for feature in feature_map:
    print(f"RT: {feature.getRT()}")
    print(f"MZ: {feature.getMZ()}")
    print(f"Intensity: {feature.getIntensity()}")
    print(f"Quality: {feature.getOverallQuality()}")

consensusXML

python

# Load consensus features
consensus_map = ms.ConsensusMap()
ms.ConsensusXMLFile().load("consensus.consensusXML", consensus_map)

# Access consensus features
for consensus_feature in consensus_map:
    print(f"RT: {consensus_feature.getRT()}")
    print(f"MZ: {consensus_feature.getMZ()}")

    # Get feature handles (sub-features from different maps)
    for handle in consensus_feature.getFeatureList():
        map_index = handle.getMapIndex()
        intensity = handle.getIntensity()
        print(f"  Map {map_index}: {intensity}")

Reading FASTA Files

python

# Load protein sequences
fasta_entries = []
ms.FASTAFile().load("database.fasta", fasta_entries)

for entry in fasta_entries:
    print(f"Identifier: {entry.identifier}")
    print(f"Description: {entry.description}")
    print(f"Sequence: {entry.sequence}")

Reading TraML Files

python

# Load transition lists for targeted experiments
targeted_exp = ms.TargetedExperiment()
ms.TraMLFile().load("transitions.TraML", targeted_exp)

# Access transitions
for transition in targeted_exp.getTransitions():
    print(f"Precursor MZ: {transition.getPrecursorMZ()}")
    print(f"Product MZ: {transition.getProductMZ()}")

Writing mzTab Files

python

# Create mzTab for reporting
mztab = ms.MzTab()

# Add metadata
metadata = mztab.getMetaData()
metadata.mz_tab_version.set("1.0.0")
metadata.title.set("Proteomics Analysis Results")

# Add protein data
protein_section = mztab.getProteinSectionRows()
# ... populate protein data ...

# Write to file
ms.MzTabFile().store("report.mzTab", mztab)

Format Conversion

mzXML to mzML

python

# Read mzXML
exp = ms.MSExperiment()
ms.MzXMLFile().load("data.mzXML", exp)

# Write as mzML
ms.MzMLFile().store("data.mzML", exp)

Extract Chromatograms from mzML

python

# Load experiment
exp = ms.MSExperiment()
ms.MzMLFile().load("data.mzML", exp)

# Extract specific chromatogram
for chrom in exp.getChromatograms():
    if chrom.getNativeID() == "TIC":
        rt, intensity = chrom.get_peaks()
        print(f"TIC has {len(rt)} data points")

File Metadata

Access mzML Metadata

python

# Load file
exp = ms.MSExperiment()
ms.MzMLFile().load("sample.mzML", exp)

# Get experimental settings
exp_settings = exp.getExperimentalSettings()

# Instrument info
instrument = exp_settings.getInstrument()
print(f"Instrument: {instrument.getName()}")
print(f"Model: {instrument.getModel()}")

# Sample info
sample = exp_settings.getSample()
print(f"Sample name: {sample.getName()}")

# Source files
for source_file in exp_settings.getSourceFiles():
    print(f"Source: {source_file.getNameOfFile()}")

Best Practices

Memory Management

For large files:

Use indexed or streaming access instead of full in-memory loading
Process data in chunks
Clear data structures when no longer needed

python

# Good for large files
indexed_mzml = ms.IndexedMzMLFileLoader()
indexed_mzml.load("huge_file.mzML")

# Process spectra one at a time
for i in range(indexed_mzml.getNrSpectra()):
    spec = indexed_mzml.getSpectrumById(i)
    # Process spectrum
    # Spectrum automatically cleaned up after processing

Error Handling

python

try:
    exp = ms.MSExperiment()
    ms.MzMLFile().load("data.mzML", exp)
except Exception as e:
    print(f"Failed to load file: {e}")

File Validation

python

# Check if file exists and is readable
import os

if os.path.exists("data.mzML") and os.path.isfile("data.mzML"):
    exp = ms.MSExperiment()
    ms.MzMLFile().load("data.mzML", exp)
else:
    print("File not found")