Python API

Python Installation

DuckDB can be installed for Python using the following command:

pip install duckdb

Simple Usage

The standard DuckDB Python API provides a SQL interface compliant with the DB-API 2.0 specification described by PEP 249. It is based on the SQLite Python API.

To use the module, you must first create a Connection object that represents the database. The connection object takes as parameter the database file to read and write from. The special value :memory: can be used to create an in-memory database. Note that for an in-memory database no data is persisted to disk (i.e. all data is lost when you exit the Python process).

import duckdb
con = duckdb.connect(':memory:')

Once you have a Connection, you can create a Cursor object and call its execute() method to perform SQL commands:

c = con.cursor()

# create a table
c.execute("CREATE TABLE items(item VARCHAR, value DECIMAL(10,2), count INTEGER)")
# insert an item into the table
c.execute("INSERT INTO items VALUES ('jeans', 20.0, 1)")

# retrieve the item again
c.execute("SELECT * FROM items")
# [['jeans', 20.0, 1]]

Efficient Retrieval

When retrieving the data from DuckDB back into Python, the standard method of calling fetchall() is inefficient as individual Python objects need to be created for every value in the result set. When retrieving a lot of data, this can become very costly.

In DuckDB, there are two additional methods that can be used to efficiently retrieve dat: fetchnumpy() and fetchdf(). fetchnumpy() fetches the data as a dictionary of NumPy arrays. fetchdf() fetches the data as a Pandas DataFrame.

Below is an example of using this functionality:

# fetch as pandas data frame
df = c.execute("SELECT * FROM items").fetchdf()
#    item  value  count
# 0  jeans   20.0      1

# fetch as dictionary of numpy arrays
arr = c.execute("SELECT * FROM items").fetchnumpy()
# {'item': masked_array(data=['jeans'], mask=False, fill_value='?', dtype=object),
#  'value': masked_array(data=[20.], mask=False, fill_value=1e+20),
#  'count': masked_array(data=[1], mask=False, fill_value=999999, dtype=int32)}