Get schema of parquet file in Python

Question

Is there any python library that can be used to just get the schema of a parquet file?

Currently we are loading the parquet file into dataframe in Spark and getting schema from the dataframe to display in some UI of the application. But initializing spark-context and loading data frame and getting the schema from dataframe is time consuming activity. So looking for an alternative way to just get the schema.

Answer 1

This function returns the schema of a local URI representing a parquet file. The schema is returned as a usable Pandas dataframe. The function does not read the whole file, just the schema.

import pandas as pd
import pyarrow.parquet


def read_parquet_schema_df(uri: str) -> pd.DataFrame:
    """Return a Pandas dataframe corresponding to the schema of a local URI of a parquet file.

    The returned dataframe has the columns: column, pa_dtype
    """
    # Ref: https://stackoverflow.com/a/64288036/
    schema = pyarrow.parquet.read_schema(uri, memory_map=True)
    schema = pd.DataFrame(({"column": name, "pa_dtype": str(pa_dtype)} for name, pa_dtype in zip(schema.names, schema.types)))
    schema = schema.reindex(columns=["column", "pa_dtype"], fill_value=pd.NA)  # Ensures columns in case the parquet file has an empty dataframe.
    return schema

It was tested with the following versions of the used third-party packages:

$ pip list | egrep 'pandas|pyarrow'
pandas             1.1.3
pyarrow            1.0.1

Answer 2

This is supported by using pyarrow (https://github.com/apache/arrow/).

from pyarrow.parquet import ParquetFile
# Source is either the filename or an Arrow file handle (which could be on HDFS)
ParquetFile(source).metadata

Note: We merged the code for this only yesterday, so you need to build it from source, see https://github.com/apache/arrow/commit/f44b6a3b91a15461804dd7877840a557caa52e4e

Answer 3

In addition to the answer by @mehdio, in case your parquet is a directory (e.g. a parquet generated by spark), to read the schema / column names:

import pyarrow.parquet as pq
pfile = pq.read_table("file.parquet")
print("Column names: {}".format(pfile.column_names))
print("Schema: {}".format(pfile.schema))

Answer 4

There's now an easiest way with the read_schema method. Note that it returns actually a dict where your schema is a bytes literal, so you need an extra step to convert your schema into a proper python dict.

from pyarrow.parquet import read_schema
import json

schema = read_schema(source)
schema_dict = json.loads(schema.metadata[b'org.apache.spark.sql.parquet.row.metadata'])['fields']

Answer 5

The simplest and lightest way I could find to retrieve a schema is using the fastparquet library:

from fastparquet import ParquetFile
    
pf = ParquetFile('file.parquet')
print(pf.schema)

Answer 6

As other commentors have mentioned, PyArrow is the easiest way to grab the schema of a Parquet file with Python. My answer goes into more detail about the schema that's returned by PyArrow and the metadata that's stored in Parquet files.

import pyarrow.parquet as pq

table = pq.read_table(path)
table.schema # returns the schema

Here's how to create a PyArrow schema (this is the object that's returned by table.schema):

import pyarrow as pa

pa.schema([
    pa.field("id", pa.int64(), True),
    pa.field("last_name", pa.string(), True),
    pa.field("position", pa.string(), True)])

Each PyArrow Field has name, type, nullable, and metadata properties. See here for more details on how to write custom file / column metadata to Parquet files with PyArrow.

The type property is for PyArrow DataType objects. pa.int64() and pa.string() are examples of PyArrow DataTypes.

Make sure you understand about column level metadata like min / max. That'll help you understand some of the cool features like predicate pushdown filtering that Parquet files allow for in big data systems.

Answer 7

Polars provides a dedicated method for parsing the schema of a parquet file without loading the actual data:

import polars as pl
schema = pl.read_parquet_schema("file.parquet")