CSV's parsen

Data-pipelines schalen en optimaliseren met Polars

Liam Brannigan

Data Scientist & Polars Contributor

Finance-CSV-export

Chicago Finance Department
Generated: 2026-02-01
WARD;TYPE;STATUS;REQUEST_COST
11;Pothole in Street Complaint;Completed;125
42;Street Light Out;Open;88
27;Alley Light Out;Completed;61

🧹 Header staat niet op de eerste rij
🧹 Kolommen gescheiden door puntkomma's

Rijen overslaan

vendor_requests = pl.read_csv(
    "ward_service_requests.csv",
    skip_rows=2,

)

Extra headerrijen overslaan

vendor_requests = pl.read_csv(
    "ward_service_requests.csv",
    skip_rows=2,
    separator=";",
)

Geparste bestand controleren

vendor_requests.head(3)

shape: (3, 4)
| WARD | TYPE                        | STATUS    | REQUEST_COST |
| ---  | ---                         | ---       | ---          |
| i64  | str                         | str       | i64          |
|------|-----------------------------|-----------|--------------|
| 11   | Pothole in Street Complaint | Completed | 125          |
| 42   | Street Light Out            | Open      | 88           |
| 27   | Alley Light Out             | Completed | 61           |

Een probleem met schema-afleiding

WARD;TYPE;STATUS;REQUEST_COST
11;Pothole in Street Complaint;Completed;125
42;Street Light Out;Open;88
27;Alley Light Out;Completed;61

Eerste 100 rijen → schema afgeleid
Alleen ints → kolom als Int64

Schema-afleiding

WARD;TYPE;STATUS;REQUEST_COST
11;Pothole in Street Complaint;Completed;125
42;Street Light Out;Open;88
27;Alley Light Out;Completed;61
...
3;Rodent Baiting Service Request;Completed;61.5

Eerste 100 rijen → schema afgeleid
Alleen ints → kolom als Int64

Schema-afleiding

vendor_requests = pl.read_csv(
    "ward_service_requests.csv",
    separator=";",
    skip_rows=2,
    infer_schema_length=200,
)

Schema-afleiding

vendor_requests.schema

Schema({'WARD': Int64, 'TYPE': String, 'STATUS': String, 'REQUEST_COST': Float64})

Schema opgeven

vendor_requests = pl.read_csv(
    "ward_service_requests.csv",
    separator=";",
    skip_rows=2,
    schema={
        "WARD": pl.Int64,
        "TYPE": pl.String,
        "STATUS": pl.String,
        "REQUEST_COST": pl.Float64,
    },
)

Afgeleid schema overschrijven

vendor_requests = pl.read_csv(
    "ward_service_requests.csv",
    separator=";",
    skip_rows=2,
    schema_overrides={"REQUEST_COST": pl.Float64},
)

De override controleren

vendor_requests.schema

Schema({'WARD': Int64, 'TYPE': String, 'STATUS': String, 'REQUEST_COST': Float64})

Een datakwaliteitsprobleem

WARD;TYPE;STATUS;REQUEST_COST
11;Pothole in Street Complaint;Completed;125.0
unknown;Street Light Out;Open;88.5
27;Alley Light Out;Completed;61.0

Parseerfouten negeren

pl.read_csv(
    "ward_service_requests.csv",
    separator=";",
    skip_rows=2,
    infer_schema_length=200,
    ignore_errors=True,
)

Foute waarden → null
⚠️ Verbergt andere fouten

Null-waarden markeren

pl.read_csv(
    "ward_service_requests.csv",
    separator=";",
    skip_rows=2,
    infer_schema_length=200,
    null_values={"WARD": "unknown"},
)

Behoudt de bedoelde dtype
Veiliger dan ignore_errors

Laten we oefenen!

Data-pipelines schalen en optimaliseren met Polars