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Data Ingestion

This page containes examples for data ingestion to Python using DuckDB. First, import the DuckDB page:

import duckdb

Then, proceed with any of the following sections.

CSV Files

CSV files can be read using the read_csv function, called either from within Python or directly from within SQL. By default, the read_csv function attempts to auto-detect the CSV settings by sampling from the provided file.

Read from a file using fully auto-detected settings:

duckdb.read_csv("example.csv")

Read multiple CSV files from a folder:

duckdb.read_csv("folder/*.csv")

Specify options on how the CSV is formatted internally:

duckdb.read_csv("example.csv", header = False, sep = ",")

Override types of the first two columns:

duckdb.read_csv("example.csv", dtype = ["int", "varchar"])

Directly read a CSV file from within SQL:

duckdb.sql("SELECT * FROM 'example.csv'")

Call read_csv from within SQL:

duckdb.sql("SELECT * FROM read_csv('example.csv')")

See the CSV Import page for more information.

Parquet Files

Parquet files can be read using the read_parquet function, called either from within Python or directly from within SQL.

Read from a single Parquet file:

duckdb.read_parquet("example.parquet")

Read multiple Parquet files from a folder:

duckdb.read_parquet("folder/*.parquet")

Read a Parquet file over https:

duckdb.read_parquet("https://some.url/some_file.parquet")

Read a list of Parquet files:

duckdb.read_parquet(["file1.parquet", "file2.parquet", "file3.parquet"])

Directly read a Parquet file from within SQL:

duckdb.sql("SELECT * FROM 'example.parquet'")

Call read_parquet from within SQL:

duckdb.sql("SELECT * FROM read_parquet('example.parquet')")

See the Parquet Loading page for more information.

JSON Files

JSON files can be read using the read_json function, called either from within Python or directly from within SQL. By default, the read_json function will automatically detect if a file contains newline-delimited JSON or regular JSON, and will detect the schema of the objects stored within the JSON file.

Read from a single JSON file:

duckdb.read_json("example.json")

Read multiple JSON files from a folder:

duckdb.read_json("folder/*.json")

Directly read a JSON file from within SQL:

duckdb.sql("SELECT * FROM 'example.json'")

Call read_json from within SQL:

duckdb.sql("SELECT * FROM read_json_auto('example.json')")

Directly Accessing DataFrames and Arrow Objects

DuckDB is automatically able to query certain Python variables by referring to their variable name (as if it was a table). These types include the following: Pandas DataFrame, Polars DataFrame, Polars LazyFrame, NumPy arrays, relations, and Arrow objects. Accessing these is made possible by replacement scans.

DuckDB supports querying multiple types of Apache Arrow objects including tables, datasets, RecordBatchReaders, and scanners. See the Python guides for more examples.

import duckdb
import pandas as pd
test_df = pd.DataFrame.from_dict({"i": [1, 2, 3, 4], "j": ["one", "two", "three", "four"]})
print(duckdb.sql("SELECT * FROM test_df").fetchall())
[(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]

DuckDB also supports “registering” a DataFrame or Arrow object as a virtual table, comparable to a SQL VIEW. This is useful when querying a DataFrame/Arrow object that is stored in another way (as a class variable, or a value in a dictionary). Below is a Pandas example:

If your Pandas DataFrame is stored in another location, here is an example of manually registering it:

import duckdb
import pandas as pd
my_dictionary = {}
my_dictionary["test_df"] = pd.DataFrame.from_dict({"i": [1, 2, 3, 4], "j": ["one", "two", "three", "four"]})
duckdb.register("test_df_view", my_dictionary["test_df"])
print(duckdb.sql("SELECT * FROM test_df_view").fetchall())
[(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]

You can also create a persistent table in DuckDB from the contents of the DataFrame (or the view):

# create a new table from the contents of a DataFrame
con.execute("CREATE TABLE test_df_table AS SELECT * FROM test_df")
# insert into an existing table from the contents of a DataFrame
con.execute("INSERT INTO test_df_table SELECT * FROM test_df")

Pandas DataFrames – object Columns

pandas.DataFrame columns of an object dtype require some special care, since this stores values of arbitrary type. To convert these columns to DuckDB, we first go through an analyze phase before converting the values. In this analyze phase a sample of all the rows of the column are analyzed to determine the target type. This sample size is by default set to 1000. If the type picked during the analyze step is incorrect, this will result in a “Failed to cast value:” error, in which case you will need to increase the sample size. The sample size can be changed by setting the pandas_analyze_sample config option.

# example setting the sample size to 100k
duckdb.execute("SET GLOBAL pandas_analyze_sample = 100_000")

Registering Objects

You can register Python objects as DuckDB tables using the DuckDBPyConnection.register() function.

The precedence of objects with the same name is as follows:

  • Objects explicitly registered via DuckDBPyConnection.register()
  • Native DuckDB tables and views
  • Replacement scans
About this page

Last modified: 2024-06-15