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Showing content from https://cloud.google.com/spanner/docs/getting-started/python below:

Getting started with Spanner in Python

Prepare your local Python environment

Follow the instructions in Setting Up a Python Development Environment.
Clone the sample app repository to your local machine:
```
git clone https://github.com/googleapis/python-spanner
```
Alternatively, you can download the sample as a zip file and extract it.
Change to the directory that contains the Spanner sample code:
```
cd python-spanner/samples/samples
```

Create an isolated Python environment, and install dependencies:

virtualenv env
source env/bin/activate
pip install -r requirements.txt

Create an instance

When you first use Spanner, you must create an instance, which is an allocation of resources that are used by Spanner databases. When you create an instance, you choose an instance configuration, which determines where your data is stored, and also the number of nodes to use, which determines the amount of serving and storage resources in your instance.

Execute the following command to create a Spanner instance in the region us-central1 with 1 node:

gcloud spanner instances create test-instance --config=regional-us-central1 \
    --description="Test Instance" --nodes=1

Note that this creates an instance with the following characteristics:

Instance ID test-instance
Display name Test Instance
Instance configuration regional-us-central1 (Regional configurations store data in one region, while multi-region configurations distribute data across multiple regions. For more information, see About instances.)
Node count of 1 (node_count corresponds to the amount of serving and storage resources available to databases in the instance. Learn more in Nodes and processing units.)

You should see:

Creating instance...done.

Look through sample files

The samples repository contains a sample that shows how to use Spanner with Python.

Take a look through the snippets.py file, which shows how to use Spanner. The code shows how to create and use a new database. The data uses the example schema shown in the Schema and data model page. Create a database GoogleSQL

python snippets.py test-instance --database-id example-db create_database

PostgreSQL

python pg_snippets.py test-instance --database-id example-db create_database

You should see:

Created database example-db on instance test-instance

The following code creates a database and two tables in the database. Note: The subsequent code samples use these two tables. If you don't execute this code, then create the tables by using the Google Cloud console or the gcloud CLI. For more information, see the example schema.

The next step is to write data to your database.

Create a database client Before you can do reads or writes, you must create a Client. You can think of a Client as a database connection: all of your interactions with Spanner must go through a Client. Typically you create a Client when your application starts up, then you re-use that Client to read, write, and execute transactions. The following code shows how to create a client.

Read more in the Client reference.

Write data with DML

You can insert data using Data Manipulation Language (DML) in a read-write transaction.

You use the execute_update() method to execute a DML statement.

Run the sample using the insert_with_dml argument.

python snippets.py test-instance --database-id example-db insert_with_dml

You should see:

4 record(s) inserted.

Note: There are limits to commit size. See CRUD limit for more information. Write data with mutations

You can also insert data using mutations.

You write data using a Batch object. A Batch object is a container for mutation operations. A mutation represents a sequence of inserts, updates, and deletes that Spanner applies atomically to different rows and tables in a Spanner database.

The insert() method in the Batch class adds one or more insert mutations to the batch. All mutations in a single batch are applied atomically.

This code shows how to write the data using mutations:

Run the sample using the insert_data argument.

python snippets.py test-instance --database-id example-db insert_data

You should see:

Inserted data.

Note: There are limits to commit size. See CRUD limit for more information. Query data using SQL

Spanner supports a SQL interface for reading data, which you can access on the command line using the Google Cloud CLI or programmatically using the Spanner client library for Python.

On the command line

Execute the following SQL statement to read the values of all columns from the Albums table:

gcloud spanner databases execute-sql example-db --instance=test-instance \
    --sql='SELECT SingerId, AlbumId, AlbumTitle FROM Albums'

Note: For the GoogleSQL reference, see Query syntax in GoogleSQL and for PostgreSQL reference, see PostgreSQL lexical structure and syntax.

The result should be:

SingerId AlbumId AlbumTitle
1        1       Total Junk
1        2       Go, Go, Go
2        1       Green
2        2       Forever Hold Your Peace
2        3       Terrified

Use the Spanner client library for Python

In addition to executing a SQL statement on the command line, you can issue the same SQL statement programmatically using the Spanner client library for Python.

Use the execute_sql() method of a Snapshot object to run the SQL query. To get a Snapshot object, call the snapshot() method of the Database class in a with statement.

Here's how to issue the query and access the data:

Run the sample using the query_data argument.

python snippets.py test-instance --database-id example-db query_data

You should see the following result:

SingerId: 2, AlbumId: 2, AlbumTitle: Forever Hold Your Peace
SingerId: 1, AlbumId: 2, AlbumTitle: Go, Go, Go
SingerId: 2, AlbumId: 1, AlbumTitle: Green
SingerId: 2, AlbumId: 3, AlbumTitle: Terrified
SingerId: 1, AlbumId: 1, AlbumTitle: Total Junk

Query using a SQL parameter

If your application has a frequently executed query, you can improve its performance by parameterizing it. The resulting parametric query can be cached and reused, which reduces compilation costs. For more information, see Use query parameters to speed up frequently executed queries.

Here is an example of using a parameter in the WHERE clause to query records containing a specific value for LastName.

Run the sample using the query_data_with_parameter argument.

python snippets.py test-instance --database-id example-db query_data_with_parameter

You should see the following result:

SingerId: 12, FirstName: Melissa, LastName: Garcia

Read data using the read API

In addition to Spanner's SQL interface, Spanner also supports a read interface.

Use the read() method of a Snapshot object to read rows from the database. To get a Snapshot object, call the snapshot() method of the Database class in a with statement. Use a KeySet object to define a collection of keys and key ranges to read.

Here's how to read the data:

Run the sample using the read_data argument.

python snippets.py test-instance --database-id example-db read_data

You should see output similar to:

SingerId: 1, AlbumId: 1, AlbumTitle: Total Junk
SingerId: 1, AlbumId: 2, AlbumTitle: Go, Go, Go
SingerId: 2, AlbumId: 1, AlbumTitle: Green
SingerId: 2, AlbumId: 2, AlbumTitle: Forever Hold Your Peace
SingerId: 2, AlbumId: 3, AlbumTitle: Terrified

Update the database schema

Assume you need to add a new column called MarketingBudget to the Albums table. Adding a new column to an existing table requires an update to your database schema. Spanner supports schema updates to a database while the database continues to serve traffic. Schema updates don't require taking the database offline and they don't lock entire tables or columns; you can continue writing data to the database during the schema update. Read more about supported schema updates and schema change performance in Make schema updates.

Add a column

You can add a column on the command line using the Google Cloud CLI or programmatically using the Spanner client library for Python.

On the command line

Use the following ALTER TABLE command to add the new column to the table:

GoogleSQL

gcloud spanner databases ddl update example-db --instance=test-instance \
    --ddl='ALTER TABLE Albums ADD COLUMN MarketingBudget INT64'

PostgreSQL

gcloud spanner databases ddl update example-db --instance=test-instance \
    --ddl='ALTER TABLE Albums ADD COLUMN MarketingBudget BIGINT'

You should see:

Schema updating...done.

Use the Spanner client library for Python Use the update_ddl() method of the Database class to modify the schema:

Run the sample using the add_column argument.

python snippets.py test-instance --database-id example-db add_column

You should see:

Added the MarketingBudget column.

Write data to the new column

The following code writes data to the new column. It sets MarketingBudget to 100000 for the row keyed by Albums(1, 1) and to 500000 for the row keyed by Albums(2, 2).

Run the sample using the update_data argument.

python snippets.py test-instance --database-id example-db update_data

You can also execute a SQL query or a read call to fetch the values that you just wrote.

Here's the code to execute the query:

To execute this query, run the sample using the query_data_with_new_column argument.

python snippets.py test-instance --database-id example-db query_data_with_new_column

You should see:

SingerId: 2, AlbumId: 2, MarketingBudget: 500000
SingerId: 1, AlbumId: 2, MarketingBudget: None
SingerId: 2, AlbumId: 1, MarketingBudget: None
SingerId: 2, AlbumId: 3, MarketingBudget: None
SingerId: 1, AlbumId: 1, MarketingBudget: 100000

Update data

You can update data using DML in a read-write transaction.

You use the execute_update() method to execute a DML statement.

Run the sample using the write_with_dml_transaction argument.

python snippets.py test-instance --database-id example-db write_with_dml_transaction

You should see:

Transferred 200000 from Album2's budget to Album1's

Note: You can also update data using mutations. Use a secondary index

Suppose you wanted to fetch all rows of Albums that have AlbumTitle values in a certain range. You could read all values from the AlbumTitle column using a SQL statement or a read call, and then discard the rows that don't meet the criteria, but doing this full table scan is expensive, especially for tables with a lot of rows. Instead you can speed up the retrieval of rows when searching by non-primary key columns by creating a secondary index on the table.

Adding a secondary index to an existing table requires a schema update. Like other schema updates, Spanner supports adding an index while the database continues to serve traffic. Spanner automatically backfills the index with your existing data. Backfills might take a few minutes to complete, but you don't need to take the database offline or avoid writing to the indexed table during this process. For more details, see Add a secondary index.

After you add a secondary index, Spanner automatically uses it for SQL queries that are likely to run faster with the index. If you use the read interface, you must specify the index that you want to use.

Add a secondary index

You can add an index on the command line using the gcloud CLI or programmatically using the Spanner client library for Python.

On the command line

Use the following CREATE INDEX command to add an index to the database:

gcloud spanner databases ddl update example-db --instance=test-instance \
    --ddl='CREATE INDEX AlbumsByAlbumTitle ON Albums(AlbumTitle)'

You should see:

Schema updating...done.

Using the Spanner client library for Python Use the update_ddl() method of the Database class to add an index:

Run the sample using the add_index argument.

python snippets.py test-instance --database-id example-db add_index

Adding an index can take a few minutes. After the index is added, you should see:

Added the AlbumsByAlbumTitle index.

Read using the index

For SQL queries, Spanner automatically uses an appropriate index. In the read interface, you must specify the index in your request.

To use the index in the read interface, provide an Index argument to the read() method of a Snapshot object. To get a Snapshot object, call the snapshot() method of the Database class in a with statement.

Run the sample using the read_data_with_index argument.

python snippets.py test-instance --database-id example-db read_data_with_index

You should see:

AlbumId: 2, AlbumTitle: Forever Hold Your Peace
AlbumId: 2, AlbumTitle: Go, Go, Go
AlbumId: 1, AlbumTitle: Green
AlbumId: 3, AlbumTitle: Terrified
AlbumId: 1, AlbumTitle: Total Junk

Add an index for index-only reads

You might have noticed that the previous read example doesn't include reading the MarketingBudget column. This is because Spanner's read interface doesn't support the ability to join an index with a data table to look up values that are not stored in the index.

Create an alternate definition of AlbumsByAlbumTitle that stores a copy of MarketingBudget in the index.

On the command line GoogleSQL

gcloud spanner databases ddl update example-db --instance=test-instance \
    --ddl='CREATE INDEX AlbumsByAlbumTitle2 ON Albums(AlbumTitle) STORING (MarketingBudget)

PostgreSQL

gcloud spanner databases ddl update example-db --instance=test-instance \
    --ddl='CREATE INDEX AlbumsByAlbumTitle2 ON Albums(AlbumTitle) INCLUDE (MarketingBudget)

Adding an index can take a few minutes. After the index is added, you should see:

Schema updating...done.

Using the Spanner client library for Python Use the update_ddl() method of the Database class to add an index with a STORING clause:

Run the sample using the add_storing_index argument.

python snippets.py test-instance --database-id example-db add_storing_index

You should see:

Added the AlbumsByAlbumTitle2 index.

Now you can execute a read that fetches all AlbumId, AlbumTitle, and MarketingBudget columns from the AlbumsByAlbumTitle2 index:

Run the sample using the read_data_with_storing_index argument.

python snippets.py test-instance --database-id example-db read_data_with_storing_index

You should see output similar to:

AlbumId: 2, AlbumTitle: Forever Hold Your Peace, MarketingBudget: 300000
AlbumId: 2, AlbumTitle: Go, Go, Go, MarketingBudget: None
AlbumId: 1, AlbumTitle: Green, MarketingBudget: None
AlbumId: 3, AlbumTitle: Terrified, MarketingBudget: None
AlbumId: 1, AlbumTitle: Total Junk, MarketingBudget: 300000

Retrieve data using read-only transactions

Suppose you want to execute more than one read at the same timestamp. Read-only transactions observe a consistent prefix of the transaction commit history, so your application always gets consistent data. Use a Snapshot object for executing read-only transactions. To get a Snapshot object, call the snapshot() method of the Database class in a with statement.

The following shows how to run a query and perform a read in the same read-only transaction:

Run the sample using the read_only_transaction argument.

python snippets.py test-instance --database-id example-db read_only_transaction

You should see output similar to:

Results from first read:
SingerId: 2, AlbumId: 2, AlbumTitle: Forever Hold Your Peace
SingerId: 1, AlbumId: 2, AlbumTitle: Go, Go, Go
SingerId: 2, AlbumId: 1, AlbumTitle: Green
SingerId: 2, AlbumId: 3, AlbumTitle: Terrified
SingerId: 1, AlbumId: 1, AlbumTitle: Total Junk
Results from second read:
SingerId: 1, AlbumId: 1, AlbumTitle: Total Junk
SingerId: 1, AlbumId: 2, AlbumTitle: Go, Go, Go
SingerId: 2, AlbumId: 1, AlbumTitle: Green
SingerId: 2, AlbumId: 2, AlbumTitle: Forever Hold Your Peace
SingerId: 2, AlbumId: 3, AlbumTitle: Terrified

Cleanup

To avoid incurring additional charges to your Cloud Billing account for the resources used in this tutorial, drop the database and delete the instance that you created.

Delete the database

If you delete an instance, all databases within it are automatically deleted. This step shows how to delete a database without deleting an instance (you would still incur charges for the instance).

On the command line

gcloud spanner databases delete example-db --instance=test-instance

Using the Google Cloud console

Go to the Spanner Instances page in the Google Cloud console.

Go to the Instances page
Click the instance.
Click the database that you want to delete.
In the Database details page, click Delete.
Confirm that you want to delete the database and click Delete.

Delete the instance

Deleting an instance automatically drops all databases created in that instance.

On the command line

gcloud spanner instances delete test-instance

Using the Google Cloud console

Go to the Spanner Instances page in the Google Cloud console.

Go to the Instances page
Click your instance.
Click Delete.
Confirm that you want to delete the instance and click Delete.

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