79 lines
2.6 KiB
Python
79 lines
2.6 KiB
Python
"""Download Census 2021 children by five-year age band per LSOA.
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Source: NOMIS (ONS Census 2021 — TS007A dataset, age by five-year bands)
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License: Open Government Licence v3.0
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Used to estimate how many primary-age (4-10) and secondary-age (11-15)
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children live in each LSOA, which drives the school catchment model. Census
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bands don't align with school phases, so phase totals take fractional shares
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of the 0-4, 10-14 and 15-19 bands (one fifth per single year of age).
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"""
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import argparse
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from pathlib import Path
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import polars as pl
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from pipeline.utils import ENGLAND_LSOA_COUNT_2021, download_nomis_csv
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# NOMIS API: Census 2021 TS007A (age, five-year bands) by LSOA 2021 (TYPE151).
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# c2021_age_19 codes: 1 = 0-4, 2 = 5-9, 3 = 10-14, 4 = 15-19.
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BASE_URL = (
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"https://www.nomisweb.co.uk/api/v01/dataset/NM_2020_1.data.csv"
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"?date=latest&geography=TYPE151&measures=20100&c2021_age_19=1,2,3,4"
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"&select=GEOGRAPHY_CODE,C2021_AGE_19,OBS_VALUE"
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)
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AGE_BAND_COLUMNS = {
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1: "aged_0_4",
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2: "aged_5_9",
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3: "aged_10_14",
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4: "aged_15_19",
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}
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def download_and_convert(output_path: Path) -> None:
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print("Downloading Census 2021 LSOA age bands from NOMIS...")
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df = download_nomis_csv(BASE_URL)
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print(f"Total rows: {df.height}")
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result = (
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df.rename({"GEOGRAPHY_CODE": "lsoa21"})
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.pivot(on="C2021_AGE_19", index="lsoa21", values="OBS_VALUE")
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.rename({str(code): name for code, name in AGE_BAND_COLUMNS.items()})
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.with_columns(pl.col(name).cast(pl.UInt32) for name in AGE_BAND_COLUMNS.values())
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.filter(pl.col("lsoa21").str.starts_with("E"))
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.sort("lsoa21")
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)
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missing = [c for c in AGE_BAND_COLUMNS.values() if c not in result.columns]
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if missing:
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raise ValueError(f"NOMIS response missing age bands: {missing}")
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print(f"England LSOAs: {result.height}")
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if result.height != ENGLAND_LSOA_COUNT_2021:
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raise ValueError(
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f"Expected {ENGLAND_LSOA_COUNT_2021} England LSOAs, "
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f"got {result.height}: truncated NOMIS download?"
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)
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for name in AGE_BAND_COLUMNS.values():
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print(f" {name}: total {result[name].sum():,}")
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output_path.parent.mkdir(parents=True, exist_ok=True)
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result.write_parquet(output_path, compression="zstd")
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print(f"Saved to {output_path}")
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def main() -> None:
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parser = argparse.ArgumentParser(
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description="Download Census 2021 age bands (children) by LSOA"
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)
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parser.add_argument(
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"--output", type=Path, required=True, help="Output parquet file path"
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)
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args = parser.parse_args()
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download_and_convert(args.output)
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if __name__ == "__main__":
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main()
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