--- jupytext: formats: md:myst text_representation: extension: .md format_name: myst --- # How to prepare datasets This guide shows how to assemble input datasets for coffea processors. The goal is to build a `fileset` mapping that the {class}`coffea.processor.Runner` can chunk and distribute efficiently. ## Fileset formats Coffea supports three fileset formats. The most flexible format (recommended) maps each file to its tree name: ### Format 1: File-to-tree mapping (recommended) ```python from coffea import processor from coffea.nanoevents import NanoAODSchema fileset = { "DYJets": { "files": { "/store/mc/.../nano_dy.root": "Events", "/store/mc/.../nano_dy_2.root": "Events", }, "metadata": {"year": 2018, "is_mc": True}, }, "DataMu": { "files": { "/store/data/.../nano_data.root": "Events", }, "metadata": {"year": 2018, "is_mc": False}, }, } runner = processor.Runner( executor=processor.IterativeExecutor(), schema=NanoAODSchema, ) result = runner(fileset, processor_instance=my_processor) ``` - The top-level keys label datasets; they propagate into `events.metadata['dataset']` - `files` is a dictionary mapping file paths to tree names (e.g., `"Events"`) - `metadata` is merged into each chunk's metadata dictionary and is available inside your processor via `events.metadata` - This format allows different tree names per file ### Format 2: Single tree name per dataset If all files in a dataset use the same tree name, you can specify it once: ```python fileset = { "DYJets": { "treename": "Events", "files": [ "/store/mc/.../nano_dy.root", "/store/mc/.../nano_dy_2.root", ], "metadata": {"year": 2018, "is_mc": True}, }, } result = runner(fileset, processor_instance=my_processor) ``` ### Format 3: Global tree name If all files across all datasets use the same tree name, pass it to the runner: ```python fileset = { "DYJets": { "files": ["/store/mc/.../nano_dy.root"], "metadata": {"year": 2018}, }, } result = runner(fileset, processor_instance=my_processor, treename="Events") ``` **Note:** Format 1 (file-to-tree mapping) is most flexible and is recommended for new analyses. ## Mix local and remote files Uproot accepts local paths, XRootD URLs, and glob patterns. Coffea simply forwards them to uproot. ```python fileset = { "DYJets": { "files": { "root://cmsxrootd.fnal.gov//store/mc/Run3Summer23/.../nano_1.root": "Events", "root://cmsxrootd.fnal.gov//store/mc/Run3Summer23/.../nano_2.root": "Events", }, "metadata": {"is_mc": True}, }, "LocalTest": { "files": { "nano_test.root": "Events", }, "metadata": {"is_mc": False}, }, } ``` Chunks can span files in different storage systems; coffea relies on uproot to stream the data. ## Discover files programmatically `coffea.dataset_tools.rucio_utils` helps when the list of files lives in Rucio or in JSON manifests. ```python from coffea.dataset_tools.rucio_utils import get_dataset_files_replicas outfiles, outsites, sites_counts = get_dataset_files_replicas( dataset="/DYJetsToLL_M-50_TuneCP5_13TeV-madgraphMLM-pythia8/RunIISummer20UL18NanoAODv9-106X_upgrade2018_realistic_v16_L1v1-v1/NANOAODSIM", ) # chose site from which to extract files site = max(sites_counts, key=sites_counts.get) files_by_site = [] for i, (files, sites) in enumerate(zip(outfiles, outsites)): iS = sites.index(site) files_by_site.append(files[iS]) # Convert list to dict mapping files to tree name (Format 1) fileset = { "DYJets": { "files": {f: "Events" for f in files_by_site}, "metadata": {"is_mc": True}, } } # Or use Format 2 if all files have same tree fileset_alt = { "DYJets": { "treename": "Events", "files": files_by_site, "metadata": {"is_mc": True}, } } ``` Output example: ```python print("site:\n", site) print("\nfileset:\n", fileset) print("\nfileset_alt:\n", fileset_alt) ``` ```python site: T2_DE_DESY fileset: {'DYJets': {'files': {'root://dcache-cms-xrootd.desy.de:1094//store/mc/RunIISummer20UL18NanoAODv9/DYJetsToLL_M-50_TuneCP5_13TeV-madgraphMLM-pythia8/NANOAODSIM/106X_upgrade2018_realistic_v16_L1v1-v1/120000/0CF0CDED-7582-7A49-84CD-\ 0E5F73DE27B0.root': 'Events', 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chunk and is available as `events.metadata`. ```python fileset = { "TTJets": { "files": { "ttbar.root": "Events", }, "metadata": { "cross_section": 831.76, "year": 2022, }, }, } ``` Inside your processor: ```python def process(self, events): dataset = events.metadata["dataset"] # "TTJets" xsec = events.metadata.get("cross_section", 1.0) year = events.metadata.get("year") # Use metadata in your analysis ... ``` Use metadata for cross sections, era tags, or analysis-specific flags. ## Keep the fileset small during development Create reduced filesets for unit tests by limiting the number of files. ```python # Take only first file from each dataset mini_fileset = {} for dataset, info in fileset.items(): first_file = list(info["files"].items())[0] mini_fileset[dataset] = { "files": {first_file[0]: first_file[1]}, "metadata": info.get("metadata", {}), } ``` Run your processor with `mini_fileset` locally, then switch back to the full fileset for production. You can also use `NanoEventsFactory.from_root(..., entry_stop=N)` to limit the number of events processed. ## Tips & tricks - Use `NanoEventsFactory.from_root(..., entry_stop=N)` alongside a reduced fileset to validate schemas before launching large jobs. - Preserve bookkeeping like `n_events` and the sum of generator weights in metadata so weight calculations stay consistent across processors. - Keep JSON filesets under version control; they document exactly which inputs were analyzed. - When combining many small files, merge them upstream if possible—large numbers of tiny files increase scheduler overhead during preprocessing.