Using the Snowflake SQLAlchemy toolkit with the Python Connector¶
Snowflake SQLAlchemy runs on the top of the Snowflake Connector for Python as a dialect to bridge a Snowflake database and SQLAlchemy applications.
For more information, see the dialect documentation.
Prerequisites¶
Snowflake Connector for Python¶
The only requirement for Snowflake SQLAlchemy is the Snowflake Connector for Python; however, the connector does not need to be installed because installing Snowflake SQLAlchemy automatically installs the connector.
Data analytics and web application frameworks (optional)¶
Snowflake SQLAlchemy can be used with pandas, Jupyter, and Pyramid, which provide higher levels of application frameworks for data analytics and web applications. However, building a working environment from scratch is not a trivial task, particularly for novice users. Installing the frameworks requires C compilers and tools, and choosing the right tools and versions is a hurdle that might deter users from using Python applications.
An easier way to build an environment is through Anaconda, which provides a complete, precompiled technology stack for all users, including non-Python experts
such as data analysts and students. For Anaconda installation instructions, see the Anaconda install documentation. The Snowflake SQLAlchemy package can
then be installed on top of Anaconda using pip
.
For more information, see the following documentation:
Installing Snowflake SQLAlchemy¶
The Snowflake SQLAlchemy package can be installed from the public PyPI repository using pip
:
pip install --upgrade snowflake-sqlalchemy
pip
automatically installs all required modules, including the Snowflake Connector for Python.
Note that the developer notes are hosted with the source code on GitHub.
Verifying your installation¶
Create a file (e.g.
validate.py
) that contains the following Python sample code, which connects to Snowflake and displays the Snowflake version:#!/usr/bin/env python from sqlalchemy import create_engine engine = create_engine( 'snowflake://{user}:{password}@{account_identifier}/'.format( user='<user_login_name>', password='<password>', account_identifier='<account_identifier>', ) ) try: connection = engine.connect() results = connection.execute('select current_version()').fetchone() print(results[0]) finally: connection.close() engine.dispose()
Replace
<user_login_name>
,<password>
, and<account_identifier>
with the appropriate values for your Snowflake account and user. For more details, see Connection Parameters (in this topic).Execute the sample code. For example, if you created a file named
validate.py
:python validate.py
The Snowflake version (e.g. 1.6.0
) should be displayed.
Snowflake-specific parameters and behavior¶
As much as possible, Snowflake SQLAlchemy provides compatible functionality for SQLAlchemy applications.
For information on using SQLAlchemy, see the SQLAlchemy documentation.
However, Snowflake SQLAlchemy also provides Snowflake-specific parameters and behavior, which are described in the following sections.
Connection parameters¶
Required parameters¶
Snowflake SQLAlchemy uses the following connection string syntax to connect to Snowflake and initiate a session:
'snowflake://<user_login_name>:<password>@<account_identifier>'
Where:
<user_login_name>
is the login name for your Snowflake user.<password>
is the password for your Snowflake user.<account_identifier>
is your account identifier. See Account identifiers.Note
Do not include the
snowflakecomputing.com
domain name as part of your account identifier. Snowflake automatically appends the domain name to your account identifier to create the required connection.
Additional connection parameters¶
You can optionally include the following additional information at the end of the connection string (after <account_name>
):
'snowflake://<user_login_name>:<password>@<account_identifier>/<database_name>/<schema_name>?warehouse=<warehouse_name>&role=<role_name>'
Where:
<database_name>
and<schema_name>
are the initial database and schema for the Snowflake session, separated by forward slashes (/
).warehouse=<warehouse_name>
androle=<role_name>'
are the initial warehouse and role for the session, specified as parameter strings, separated by question marks (?
).
Note
After login, the initial database, schema, warehouse, and role specified in the connection string can always be changed for the session.
Proxy server configuration¶
Proxy server parameters are not supported. Instead, use the supported environment variables to configure a proxy server. For information, see Using a proxy server.
Connection string examples¶
The following example calls the create_engine
method with the user name testuser1
, password 0123456
, account
identifier myorganization-myaccount
, database testdb
, schema public
, warehouse testwh
, and role myrole
:
from sqlalchemy import create_engine engine = create_engine( 'snowflake://testuser1:0123456@myorganization-myaccount/testdb/public?warehouse=testwh&role=myrole' )
For convenience, you can use the snowflake.sqlalchemy.URL
method to construct the connection string and connect to the database. The following example constructs the same connection string
from the previous example:
from snowflake.sqlalchemy import URL from sqlalchemy import create_engine engine = create_engine(URL( account = 'myorganization-myaccount', user = 'testuser1', password = '0123456', database = 'testdb', schema = 'public', warehouse = 'testwh', role='myrole', ))
Opening and closing a connection¶
Open a connection by executing engine.connect()
; avoid using engine.execute()
.
# Avoid this. engine = create_engine(...) engine.execute(<SQL>) engine.dispose() # Do this. engine = create_engine(...) connection = engine.connect() try: connection.execute(<SQL>) finally: connection.close() engine.dispose()
Note
Make certain to close the connection by executing connection.close()
before engine.dispose()
; otherwise, the Python Garbage collector removes the resources required to communicate
with Snowflake, preventing the Python connector from closing the session properly.
If you plan to use explicit transactions, you must disable the AUTOCOMMIT execution option in SQLAlchemy.
For more information, see AUTOCOMMIT execution option in SQLAlchemy..
By default, SQLAlchemy enables this option. When this option is enabled, INSERT, UPDATE, and DELETE statements are committed automatically upon execution, even when these statements are run within an explicit transaction.
To disable AUTOCOMMIT, pass autocommit=False
to the Connection.execution_options()
method. For example:
# Disable AUTOCOMMIT if you need to use an explicit transaction.
with engine.connect().execution_options(autocommit=False) as connection:
try:
connection.execute("BEGIN")
connection.execute("INSERT INTO test_table VALUES (88888, 'X', 434354)")
connection.execute("INSERT INTO test_table VALUES (99999, 'Y', 453654654)")
connection.execute("COMMIT")
except Exception as e:
connection.execute("ROLLBACK")
finally:
connection.close()
engine.dispose()
Auto-increment behavior¶
Auto-incrementing a value requires the Sequence
object. Include the Sequence
object in the primary key column to automatically increment the value as each new record is inserted.
For example:
t = Table('mytable', metadata, Column('id', Integer, Sequence('id_seq'), primary_key=True), Column(...), ... )
Object name case handling¶
Snowflake stores all case-insensitive object names in uppercase text. In contrast, SQLAlchemy considers all lowercase object names to be case-insensitive. Snowflake SQLAlchemy converts the object
name case during schema-level communication (i.e. during table and index reflection). If you use uppercase object names, SQLAlchemy assumes they are case-sensitive and encloses the names with quotes.
This behavior will cause mismatches against data dictionary data received from Snowflake, so unless identifier names have been truly created as case sensitive using quotes (e.g. "TestDb"
),
all lowercase names should be used on the SQLAlchemy side.
Index support¶
Snowflake does not utilize indexes, so neither does Snowflake SQLAlchemy.
Numpy data type support¶
Snowflake SQLAlchemy supports binding and fetching NumPy
data types. Binding is always supported. To enable fetching NumPy
data types, add numpy=True
to the connection
parameters.
The following NumPy
data types are supported:
numpy.int64
numpy.float64
numpy.datetime64
The following example shows the round trip of numpy.datetime64
data:
import numpy as np import pandas as pd engine = create_engine(URL( account = 'myorganization-myaccount', user = 'testuser1', password = 'pass', database = 'db', schema = 'public', warehouse = 'testwh', role='myrole', numpy=True, )) specific_date = np.datetime64('2016-03-04T12:03:05.123456789Z') with engine.connect() as connection: conenction.exec_sql_query( "CREATE OR REPLACE TABLE ts_tbl(c1 TIMESTAMP_NTZ)") connection.exec_sql_query( "INSERT INTO ts_tbl(c1) values(%s)", (specific_date,) ) df = pd.read_sql_query("SELECT * FROM ts_tbl", connection) assert df.c1.values[0] == specific_date
Cache column metadata¶
SQLAlchemy provides the runtime inspection API to get the runtime information about the various objects. One of the common use case is get all tables and their column metadata in a schema in order to construct a schema catalog.
For more information, see runtime inspection API. For an example managing database schema migrations with SQLAlchemy, alembic .
A pseudo code flow is as follows:
inspector = inspect(engine) schema = inspector.default_schema_name for table_name in inspector.get_table_names(schema): column_metadata = inspector.get_columns(table_name, schema) primary_keys = inspector.get_primary_keys(table_name, schema) foreign_keys = inspector.get_foreign_keys(table_name, schema) ...
In this flow, a potential problem is it may take quite a while as queries run on each table. The results are cached but getting column metadata is expensive.
To mitigate the problem, Snowflake SQLAlchemy takes a flag cache_column_metadata=True
such that all of column metadata for all tables are cached when get_table_names
is called and
the rest of get_columns
, get_primary_keys
and get_foreign_keys
can take advantage of the cache.
engine = create_engine(URL( account = 'myorganization-myaccount', user = 'testuser1', password = 'pass', database = 'db', schema = 'public', warehouse = 'testwh', role='myrole', cache_column_metadata=True, ))
Note
Memory usage will go up higher as all of column metadata are cached associated with Inspector
object. Use the flag only if you need to get all of column metadata.
VARIANT, ARRAY, and OBJECT support¶
Snowflake SQLAlchemy supports fetching VARIANT
, ARRAY
and OBJECT
data types. All types are converted into str
in Python so that you can convert them to native data
types using json.loads
.
This example shows how to create a table including VARIANT
, ARRAY
, and OBJECT
data type columns:
from snowflake.sqlalchemy import (VARIANT, ARRAY, OBJECT) ... t = Table('my_semi_structured_datatype_table', metadata, Column('va', VARIANT), Column('ob', OBJECT), Column('ar', ARRAY)) metdata.create_all(engine)
In order to retrieve VARIANT
, ARRAY
, and OBJECT
data type columns and convert them to the native Python data types, fetch data and call the json.loads
method as follows:
import json connection = engine.connect() results = connection.execute(select([t])) row = results.fetchone() data_variant = json.loads(row[0]) data_object = json.loads(row[1]) data_array = json.loads(row[2])
CLUSTER BY support¶
Snowflake SQLAlchemy supports the CLUSTER BY
parameter for tables. For information about the parameter, see CREATE TABLE.
This example shows how to create a table with two columns, id
and name
, as the clustering key:
t = Table('myuser', metadata, Column('id', Integer, primary_key=True), Column('name', String), snowflake_clusterby=['id', 'name'], ... ) metadata.create_all(engine)
Alembic support¶
Alembic is a database migration tool on top of SQLAlchemy
. Snowflake SQLAlchemy works by adding the following code to alembic/env.py
so that Alembic can recognize Snowflake SQLAlchemy.
from alembic.ddl.impl import DefaultImpl class SnowflakeImpl(DefaultImpl): __dialect__ = 'snowflake'
See Alembic Documentation for general usage.
Key pair authentication support¶
Snowflake SQLAlchemy supports key pair authentication by leveraging the functionality of Snowflake Connector for Python. See Using key-pair authentication and key-pair rotation for steps to create the private and public keys.
The private key parameter is passed through connect_args
as follows:
... from snowflake.sqlalchemy import URL from sqlalchemy import create_engine from cryptography.hazmat.backends import default_backend from cryptography.hazmat.primitives import serialization with open("rsa_key.p8", "rb") as key: p_key= serialization.load_pem_private_key( key.read(), password=os.environ['PRIVATE_KEY_PASSPHRASE'].encode(), backend=default_backend() ) pkb = p_key.private_bytes( encoding=serialization.Encoding.DER, format=serialization.PrivateFormat.PKCS8, encryption_algorithm=serialization.NoEncryption()) engine = create_engine(URL( account='abc123', user='testuser1', ), connect_args={ 'private_key': pkb, }, )
Where PRIVATE_KEY_PASSPHRASE
is a passphrase to decrypt the private key file, rsa_key.p8
.
The snowflake.sqlalchemy.URL
method does not support private key parameters.
Merge command support¶
Snowflake SQLAlchemy supports performing an upsert with its MergeInto
custom expression. See MERGE for full documentation.
Use it as follows:
from sqlalchemy.orm import sessionmaker from sqlalchemy import MetaData, create_engine from snowflake.sqlalchemy import MergeInto engine = create_engine(db.url, echo=False) session = sessionmaker(bind=engine)() connection = engine.connect() meta = MetaData() meta.reflect(bind=session.bind) t1 = meta.tables['t1'] t2 = meta.tables['t2'] merge = MergeInto(target=t1, source=t2, on=t1.c.t1key == t2.c.t2key) merge.when_matched_then_delete().where(t2.c.marked == 1) merge.when_matched_then_update().where(t2.c.isnewstatus == 1).values(val = t2.c.newval, status=t2.c.newstatus) merge.when_matched_then_update().values(val=t2.c.newval) merge.when_not_matched_then_insert().values(val=t2.c.newval, status=t2.c.newstatus) connection.execute(merge)
CopyIntoStorage support¶
Snowflake SQLAlchemy supports saving tables and query results into different Snowflake stages, Azure Containers, and AWS buckets with
its custom CopyIntoStorage
expression. See COPY INTO <location> for full documentation.
Use it as follows:
from sqlalchemy.orm import sessionmaker from sqlalchemy import MetaData, create_engine from snowflake.sqlalchemy import CopyIntoStorage, AWSBucket, CSVFormatter engine = create_engine(db.url, echo=False) session = sessionmaker(bind=engine)() connection = engine.connect() meta = MetaData() meta.reflect(bind=session.bind) users = meta.tables['users'] copy_into = CopyIntoStorage(from_=users, into=AWSBucket.from_uri('s3://my_private_backup').encryption_aws_sse_kms('1234abcd-12ab-34cd-56ef-1234567890ab'), formatter=CSVFormatter().null_if(['null', 'Null'])) connection.execute(copy_into)