Handling SQL Databases With PyQt: The Basics

Creating a Database Connection

Connecting your applications to a physical SQL database is an important step in the process of developing database applications with PyQt. To perform this step successfully, you need some general information about how your database is set up.

For example, you need to know what database management system your database is built on, and you might also need to have a username, a password, a hostname, and so on.

In this tutorial, you’ll use SQLite 3, which is a well-tested database system with support on all platforms and minimal configuration requirements. SQLite allows you to read and write directly to databases in your local disk without the need for a separate server process. That makes it a user-friendly option for learning database application development.

Another advantage of using SQLite is that the library comes shipped with Python and also with PyQt, so you don’t need to install anything else to start working with it.

In PyQt, you can create a database connection by using the QSqlDatabase class. This class represents a connection and provides an interface for accessing the database. To create a connection, just call .addDatabase() on QSqlDatabase. This static method takes an SQL driver and an optional connection name as arguments and returns a database connection:

QSqlDatabase.addDatabase(
    driver, connectionName=QSqlDatabase.defaultConnection
)

The first argument, driver, is a required argument that holds a string containing the name of a PyQt-supported SQL driver. The second argument, connectionName, is an optional argument that holds a string with the name of the connection. connectionName defaults to QSqlDatabase.defaultConnection, which normally holds the string "qt_sql_default_connection".

If you already have a connection called connectionName, then that connection is removed and replaced with a new connection, and .addDatabase() returns the newly added database connection back to the caller.

A call to .addDatabase() adds a database connection to a list of available connections. This list is a global registry that PyQt maintains behind the scenes to keep track of the available connections in an application. Registering your connections with a meaningful connectionName will allow you to manage several connections in a database application.

Once you’ve created a connection, you might need to set several attributes on it. The specific set of attributes will depend on the driver you’re using. In general, you’ll need to set attributes such as the database name, the username, and the password for accessing the database.

Here’s a summary of the setter methods that you can use to set the more commonly used attributes or properties of a database connection:

Note that the password you pass as an argument to .setPassword() is stored in plain text and can be retrieved later by calling .password(). This is a serious security risk that you should avoid introducing in your database applications. You’ll learn a safer approach in the section Opening a Database Connection later in this tutorial.

To create a connection to an SQLite database using QSqlDatabase, open a Python interactive session and type in the following code:

>>> from PyQt5.QtSql import QSqlDatabase

>>> con = QSqlDatabase.addDatabase("QSQLITE")
>>> con.setDatabaseName("contacts.sqlite")

>>> con
<PyQt5.QtSql.QSqlDatabase object at 0x7f0facec0c10>

>>> con.databaseName()
'contacts.sqlite'

>>> con.connectionName()
'qt_sql_default_connection'

This code will create a database connection object using "QSQLITE" as the connection’s driver and "contacts.sqlite" as the connection’s database name. Since you don’t pass a connection name to .addDatabase(), the newly created one becomes your default connection, whose name is "qt_sql_default_connection".

In the case of SQLite databases, the database name is normally a filename or a path that includes the database filename. You can also use the special name ":memory:" for an in-memory database.

Handling Multiple Connections

There may be situations in which you need to use multiple connections to a single database. For example, you might want to log users’ interactions with the database using a specific connection for each user.

In other situations, you might need to connect your application to several databases. For example, you might want to connect to several remote databases in order to collect data to populate or update a local database.

To handle these situations, you can provide specific names for your different connections and reference each connection by its name. If you want to give your database connection a name, then pass that name as the second argument to .addDatabase():

>>> from PyQt5.QtSql import QSqlDatabase

>>> # First connection
>>> con1 = QSqlDatabase.addDatabase("QSQLITE", "con1")
>>> con1.setDatabaseName("contacts.sqlite")

>>> # Second connection
>>> con2 = QSqlDatabase.addDatabase("QSQLITE", "con2")
>>> con2.setDatabaseName("contacts.sqlite")

>>> con1
<PyQt5.QtSql.QSqlDatabase object at 0x7f367f5fbf90>
>>> con2
<PyQt5.QtSql.QSqlDatabase object at 0x7f3686dd7510>

>>> con1.databaseName()
'contacts.sqlite'
>>> con2.databaseName()
'contacts.sqlite'

>>> con1.connectionName()
'con1'
>>> con2.connectionName()
'con2'

Here, you create two different connections to the same database, contacts.sqlite. Each connection has its own connection name. You can use the connection name to get a reference to a specific connection any time later in your code according to your needs. To do this, you can call .database() with a connection name:

>>> from PyQt5.QtSql import QSqlDatabase

>>> db = QSqlDatabase.database("con1", open=False)

>>> db.databaseName()
'contacts.sqlite'
>>> db.connectionName()
'con1'

In this example, you see that .database() takes two arguments:

1. connectionName holds the connection name that you need to use. If you don’t pass a connection name, then the default connection will be used.
2. open holds a Boolean value that tells .database() if you want to automatically open the connection or not. If open is True (the default) and the connection isn’t open, then the connection is opened automatically.

The return value of .database() is a reference to the connection object called connectionName. You can use different connection names to get references to specific connection objects and then use them to manage your database.

Using Different SQL Divers

So far, you’ve learned how to create a database connection using the SQLite driver. This isn’t the only driver available in PyQt. The library provides a rich set of SQL drivers that allow you to use different types of database management systems according to your specific needs:

The Driver Name column holds the identifier strings that you need to pass to .addDatabase() as its first argument to use the associated driver. Unlike with the SQLite driver, when you use a different driver, you might need to set several attributes, such as databaseName, hostName , userName, and password, for the connection to work properly.

Database drivers are derived from QSqlDriver. You can create your own database drivers by subclassing QSqlDriver, but that topic goes beyond the scope of this tutorial. If you’re interested in creating your own database drivers, then check out How to Write Your Own Database Driver for more details.

Opening a Database Connection

Once you have a database connection, you need to open that connection to be able to interact with your database. To do that, you call .open() on the connection object. .open() has the following two variations:

1. .open() opens a database connection using the current connection values.
2. .open(username, password) opens a database connection using the provided username and password.

Both variations return True if the connection is successful. Otherwise, they return False. If the connection can’t be established, then you can call .lastError() to get information about what happened. This function returns information about the last error reported by the database.

Here’s an example of how to open an SQLite database connection using the first variation of .open():

>>> from PyQt5.QtSql import QSqlDatabase

>>> # Create the connection
>>> con = QSqlDatabase.addDatabase("QSQLITE")
>>> con.setDatabaseName("contacts.sqlite")

>>> # Open the connection
>>> con.open()
True
>>> con.isOpen()
True

In the above example, you first create a connection to your SQLite database and open that connection using .open(). Since .open() returns True, the connection is successful. At this point, you can check the connection using .isOpen(), which returns True if the connection is open and False otherwise.

In real-world applications, you need to make sure that you have a valid connection to your database before you try to do any operations on your data. Otherwise, your application can break down and fail. For example, what if you don’t have write permissions for the directory in which you’re trying to create that database file? You need to make sure that you’re handling any error that can occur while opening a connection.

A common way of calling .open() is to wrap it in a conditional statement. This allows you to handle errors that can occur when opening the connection:

>>> import sys
>>> from PyQt5.QtSql import QSqlDatabase

>>> # Create the connection
>>> con = QSqlDatabase.addDatabase("QSQLITE")
>>> con.setDatabaseName("contacts.sqlite")

>>> # Open the connection and handle errors
>>> if not con.open():
...     print("Unable to connect to the database")
...     sys.exit(1)

Wrapping the call to .open() in a conditional statement allows you to handle any error that happens when you open the connection. This way, you can inform your users about any problems before the application runs. Note that the application exits with an exit status of 1, which is customarily used to indicate a program failure.

In the above example, you use .open() in an interactive session, so you use print() to present error messages to the users. However, in GUI applications, instead of using print(), you normally use a QMessageBox object. With QMessageBox, you can create small dialogs to present information to your users.

Here’s a sample GUI application that illustrates a way of handling connection errors:

 1import sys
 2
 3from PyQt5.QtSql import QSqlDatabase
 4from PyQt5.QtWidgets import QApplication, QMessageBox, QLabel
 5
 6# Create the connection
 7con = QSqlDatabase.addDatabase("QSQLITE")
 8con.setDatabaseName("/home/contacts.sqlite")
 9
10# Create the application
11app = QApplication(sys.argv)
12
13# Try to open the connection and handle possible errors
14if not con.open():
15    QMessageBox.critical(
16        None,
17        "App Name - Error!",
18        "Database Error: %s" % con.lastError().databaseText(),
19    )
20    sys.exit(1)
21
22# Create the application's window
23win = QLabel("Connection Successfully Opened!")
24win.setWindowTitle("App Name")
25win.resize(200, 100)
26win.show()
27sys.exit(app.exec_())

The if statement in line 14 checks if the connection was unsuccessful. If the /home/ directory doesn’t exist or if you don’t have permission to write in it, then the call to .open() fails because the database file can’t be created. In this situation, the flow of execution enters the if statement code block and shows a message on the screen.

If you change the path to any other directory in which you can write, then the call to .open() will succeed and you’ll see a window displaying the message Connection Successfully Opened! You’ll also have a new database file called contacts.sqlite in the selected directory.

Note that you pass None as the message’s parent because, at the time of showing the message, you haven’t created a window yet, so you don’t have a viable parent for the message box.

Executing Static SQL Queries

To start creating and executing queries with PyQt, you’re going to fire up your favorite code editor or IDE and create a Python script called queries.py. Save the script and add the following code to it:

 1import sys
 2
 3from PyQt5.QtSql import QSqlDatabase, QSqlQuery
 4
 5# Create the connection
 6con = QSqlDatabase.addDatabase("QSQLITE")
 7con.setDatabaseName("contacts.sqlite")
 8
 9# Open the connection
10if not con.open():
11    print("Database Error: %s" % con.lastError().databaseText())
12    sys.exit(1)
13
14# Create a query and execute it right away using .exec()
15createTableQuery = QSqlQuery()
16createTableQuery.exec(
17    """
18    CREATE TABLE contacts (
19        id INTEGER PRIMARY KEY AUTOINCREMENT UNIQUE NOT NULL,
20        name VARCHAR(40) NOT NULL,
21        job VARCHAR(50),
22        email VARCHAR(40) NOT NULL
23    )
24    """
25)
26
27print(con.tables())

In this script, you start by importing the modules and classes that you’re going to work with. Then you create a database connection using .addDatabase() with the SQLite driver. You set the database name to "contacts.sqlite" and open the connection.

To create your first query, you instantiate QSqlQuery without any arguments. With the query object in place, you call .exec(), passing a string-based SQL query as an argument. This kind of query is known as a static query because it doesn’t get any parameters from outside the query.

The above SQL query creates a new table called contacts in your database. That table will have the following four columns:

The last line in the above script prints the list of tables contained in your database. If you run the script, then you’ll note that a new database file called contacts.sqlite is created in your current directory. You’ll also get something like ['contacts', 'sqlite_sequence'] printed on your screen. This list contains the names of the tables in your database.

Calling .exec() on a QSqlQuery object is a common way of immediately executing string-based SQL queries on your databases, but what if you want to prepare your queries beforehand for later execution? That’s the topic of the next section.

Executing Dynamic Queries: String Formatting

So far, you’ve learned how to execute static queries on a database. Static queries are those that don’t accept parameters, so the query runs as it is. Even though these queries are fairly useful, sometimes you need to create queries that retrieve data in response to certain input parameters.

Queries that accept parameters at execution time are known as dynamic queries. Using parameters allows you to fine-tune the query and retrieve data in response to specific parameter values. Different values will produce different results. You can take input parameters in a query by using one of the following two approaches:

1. Build the query dynamically, using string formatting to interpolate parameter values.
2. Prepare the query using placeholder parameters and then bind specific values to parameters.

The first approach allows you to create dynamic queries quickly. However, to safely use this approach, you need to be sure that your parameter values come from a trusted source. Otherwise, you might face SQL injection attacks.

Here’s an example of how to use string formatting to create dynamic queries in PyQt:

>>> from PyQt5.QtSql import QSqlQuery, QSqlDatabase

>>> con = QSqlDatabase.addDatabase("QSQLITE")
>>> con.setDatabaseName("contacts.sqlite")
>>> con.open()
True

>>> name = "Linda"
>>> job = "Technical Lead"
>>> email = "linda@example.com"

>>> query = QSqlQuery()
>>> query.exec(
...     f"""INSERT INTO contacts (name, job, email)
...     VALUES ('{name}', '{job}', '{email}')"""
... )
True

In this example, you use an f-string to create a dynamic query by interpolating specific values into a string-based SQL query. The final query inserts data into your contacts table, which now contains data about Linda.

Note that for this kind of dynamic query to work, you need to make sure that the values to be inserted have the right data type. So, you use single quotes around the placeholder in the f-string because those values need to be strings.

Executing Dynamic Queries: Placeholder Parameters

The second approach for executing dynamic queries requires you to prepare your queries beforehand using a template with placeholders for parameters. PyQt supports two parameter placeholder styles:

1. Oracle style uses named placeholders such as :name or :email.
2. ODBC style uses a question mark (?) as a positional placeholder.

Note that these styles can’t be mixed in the same query. You can check out Approaches to Binding Values for extra examples on how to use placeholders.

To create this kind of dynamic query in PyQt, you first create a template with a placeholder for each query parameter and then pass that template as an argument to .prepare(), which parses, compiles, and prepares the query template for execution. If the template has any problems, such as an SQL syntax error, then .prepare() fails to compile the template and returns False.

If the preparation process succeeds, then prepare() returns True. After that, you can pass a specific value to each parameter using .bindValue() with named or positional parameters or using .addBindValue() with positional parameters. .bindValue() has the following two variations:

1. .bindValue(placeholder, val)
2. .bindValue(pos, val)

In the first variation, placeholder represents an Oracle-style placeholder. In the second variation, pos represents a zero-based integer number with the position of a parameter in the query. In both variations, val holds the value to be bound to a specific parameter.

.addBindValue() adds a value to the list of placeholders using positional binding. This means that the order of the calls to .addBindValue() determines which value will be bound to each placeholder parameter in the prepared query.

To start using prepared queries, you can prepare an INSERT INTO SQL statement to populate your database with some sample data. Go back to the script that you created in the section Executing Static SQL Queries and add the following code right after the call to print():

28# Creating a query for later execution using .prepare()
29insertDataQuery = QSqlQuery()
30insertDataQuery.prepare(
31    """
32    INSERT INTO contacts (
33        name,
34        job,
35        email
36    )
37    VALUES (?, ?, ?)
38    """
39)
40
41# Sample data
42data = [
43    ("Joe", "Senior Web Developer", "joe@example.com"),
44    ("Lara", "Project Manager", "lara@example.com"),
45    ("David", "Data Analyst", "david@example.com"),
46    ("Jane", "Senior Python Developer", "jane@example.com"),
47]
48
49# Use .addBindValue() to insert data
50for name, job, email in data:
51    insertDataQuery.addBindValue(name)
52    insertDataQuery.addBindValue(job)
53    insertDataQuery.addBindValue(email)
54    insertDataQuery.exec()

The first step is to create a QSqlQuery object. Then you call .prepare() on the query object. In this case, you use the ODBC style for the placeholders. Your query will take values for your contact’s name, job, and email, so you need three placeholders. Since the id column is an autoincremented integer number, you don’t need to supply values for it.

Then you create some sample data to populate the database. data holds a list of tuples, and each tuple contains three items: the name, job, and email of each contact.

The final step is to bind the values that you want to pass to each placeholder and then call .exec() to execute the query. To do that, you use a for loop. The loop header unpacks each tuple in data into three separate variables with convenient names. Then you call .addBindValue() on the query object to bind the values to the placeholders.

Note that you’re using positional placeholders, so the order in which you call .addBindValue() will define the order in which each value is passed to the corresponding placeholder.

This approach for creating dynamic queries is handy when you want to customize your queries using values that come from your user’s input. Anytime you take the user’s input to complete a query on a database, you face the security risk of SQL injection.

In PyQt, combining .prepare(), .bindValue(), and .addBindValue() fully protects you against SQL injection attacks, so this is the way to go when you’re taking untrusted input to complete your queries.

Navigating the Records in a Query

If you execute a SELECT statement, then your QSqlQuery object will retrieve zero or more records from one or more tables in your database. The query will hold records containing data that matches the query’s criteria. If no data matches the criteria, then your query will be empty.

QSqlQuery provides a set of navigation methods that you can use to move throughout the records in a query result:

All these methods position the query object on the retrieved record if that record is available. Most of these methods have specific rules that apply when using them. With these methods, you can move forward, backward, or arbitrarily through the records in a query result. Since they all return either True or False, you can use them in a while loop to navigate all the records in one go.

These methods work with active queries. A query is active when you’ve successfully run .exec() on it, but the query isn’t finished yet. Once an active query is on a valid record, you can retrieve data from that record using .value(index). This method takes a zero-based integer number, index, and returns the value at that index (column) in the current record.

You’ll look at a few examples of how to use some of the navigation methods to move throughout a query below. But first, you need to create a connection to your database:

>>> from PyQt5.QtSql import QSqlDatabase, QSqlQuery

>>> con = QSqlDatabase.addDatabase("QSQLITE")
>>> con.setDatabaseName("contacts.sqlite")
>>> con.open()
True

Here, you create and open a new connection to contacts.sqlite. If you’ve been following along with this tutorial so far, then this database already contains some sample data. Now you can create a QSqlQuery object and execute it on that data:

>>> # Create and execute a query
>>> query = QSqlQuery()
>>> query.exec("SELECT name, job, email FROM contacts")
True

This query retrieves data about the name, job, and email of all the contacts stored in the contacts table. Since .exec() returned True, the query was successful and is now an active query. You can navigate the records in this query using any of the navigation methods you saw before. You can also retrieve the data at any column in a record using .value():

>>> # First record
>>> query.first()
True

>>> # Named indices for readability
>>> name, job, email = range(3)

>>> # Retrieve data from the first record
>>> query.value(name)
'Linda'

>>> # Next record
>>> query.next()
True
>>> query.value(job)
'Senior Web Developer'

>>> # Last record
>>> query.last()
True
>>> query.value(email)
'jane@example.com'

With the navigation methods, you can move around the query result. With .value(), you can retrieve the data at any column in a given record.

You can also iterate through all the records in your query using a while loop along with .next():

>>> query.exec()
True

>>> while query.next():
...     print(query.value(name), query.value(job), query.value(email))
...
Linda Technical Lead linda@example.com
Joe Senior Web Developer joe@example.com
...

With .next(), you navigate all the records in a query result. .next() works similar to the iterator protocol in Python. Once you’ve iterated over the records in a query result, .next() starts returning False until you run .exec() again. A call to .exec() retrieves data from a database and places the query object’s internal pointer one position before the first record, so when you call .next(), you get the first record again.

You can also loop in reverse order using .previous():

>>> while query.previous():
...     print(query.value(name), query.value(job), query.value(email))
...
Jane Senior Python Developer jane@example.com
David Data Analyst david@example.com
...

.previous() works similar to .next(), but the iteration is done in reverse order. In other words, the loop goes from the query pointer’s position back to the first record.

Sometimes you might want to get the index that identifies a given column in a table by using the name of that column. To do that, you can call .indexOf() on the return value of .record():

>>> query.first()
True

>>> # Get the index of name
>>> name = query.record().indexOf("name")

>>> query.value(name)
'Linda'

>>> # Finish the query object if unneeded
>>> query.finish()
>>> query.isActive()
False

The call to .indexOf() on the result of .record() returns the index of the "name" column. If "name" doesn’t exist, then .indexOf() returns -1. This is handy when you use a SELECT * statement in which the order of columns is unknown. Finally, if you’re done with a query object, then you can turn it inactive by calling .finish(). This will free the system memory associated with the query object at hand.

Understanding PyQt’s Model-View Architecture

The Model-View-Controller (MVC) design pattern is a general software pattern intended to divide an application’s code into three general layers, each with a different role.

The model takes care of the business logic of the application, the view provides on-screen representations, and the controller connects the model and the view to make the application work.

Qt provides a custom variation of MVC. They call it the Model-View architecture, and it’s available for PyQt as well. The pattern also separates the logic into three components:

1. Models communicate with and access the data. They also define an interface that’s used by views and delegates to access the data. All models are based on QAbstractItemModel. Some commonly used models include QStandardItemModel, QFileSystemModel, and SQL-related models.

2. Views are responsible for displaying the data to the user. They also have similar functionality to the controller in the MVC pattern. All views are based on QAbstractItemView. Some commonly used views are QListView, QTableView, and QTreeView.

3. Delegates paint view items and provide editor widgets for modifying items. They also communicate back with the model if an item has been modified. The base class is QAbstractItemDelegate.

Separating classes into these three components implies that changes on models will be reflected on associated views or widgets automatically, and changes on views or widgets through delegates will update the underlying model automatically.

In addition, you can display the same data in different views without the need for multiple models.

Using Standard Widget Classes

PyQt provides a bunch of standard widgets for displaying and editing data in your GUI applications. These standard widgets provide views such as tables, trees, and lists. They also provide an internal container for storing data and convenient delegates for editing the data. All these features are grouped into a single class.

Here are three of these standard classes:

QTableWidget is arguably the most popular widget when it comes to displaying and editing data. It creates a 2D array of QTableWidgetItem objects. Each item holds an individual value as a string. All these values are displayed and organized in a table of rows and columns.

You can perform at least the following operations on a QTableWidget object:

• Editing the content of its items using delegate objects
• Adding new items using .setItem()
• Setting the number of rows and columns using .setRowCount() and .setColumnCount()
• Adding vertical and horizontal header labels using setHorizontalHeaderLabels() and .setVerticalHeaderLabels

Here’s a sample application that shows how to use a QTableWidget object to display data in a GUI. The application uses the database you created and populated in previous sections, so if you want to run it, then you need to save the code into the same directory in which you have the contacts.sqlite database:

If you double-click any cell of the table, then you’ll be able to edit the content of the cell. However, your changes won’t be saved to your database.

Here’s the code for your application:

 1import sys
 2
 3from PyQt5.QtSql import QSqlDatabase, QSqlQuery
 4from PyQt5.QtWidgets import (
 5    QApplication,
 6    QMainWindow,
 7    QMessageBox,
 8    QTableWidget,
 9    QTableWidgetItem,
10)
11
12class Contacts(QMainWindow):
13    def __init__(self, parent=None):
14        super().__init__(parent)
15        self.setWindowTitle("QTableView Example")
16        self.resize(450, 250)
17        # Set up the view and load the data
18        self.view = QTableWidget()
19        self.view.setColumnCount(4)
20        self.view.setHorizontalHeaderLabels(["ID", "Name", "Job", "Email"])
21        query = QSqlQuery("SELECT id, name, job, email FROM contacts")
22        while query.next():
23            rows = self.view.rowCount()
24            self.view.setRowCount(rows + 1)
25            self.view.setItem(rows, 0, QTableWidgetItem(str(query.value(0))))
26            self.view.setItem(rows, 1, QTableWidgetItem(query.value(1)))
27            self.view.setItem(rows, 2, QTableWidgetItem(query.value(2)))
28            self.view.setItem(rows, 3, QTableWidgetItem(query.value(3)))
29        self.view.resizeColumnsToContents()
30        self.setCentralWidget(self.view)
31
32def createConnection():
33    con = QSqlDatabase.addDatabase("QSQLITE")
34    con.setDatabaseName("contacts.sqlite")
35    if not con.open():
36        QMessageBox.critical(
37            None,
38            "QTableView Example - Error!",
39            "Database Error: %s" % con.lastError().databaseText(),
40        )
41        return False
42    return True
43
44app = QApplication(sys.argv)
45if not createConnection():
46    sys.exit(1)
47win = Contacts()
48win.show()
49sys.exit(app.exec_())

Here’s what’s happening in this example:

• Lines 18 to 20 create a QTableWidget object, set the number of columns to 4, and set user-friendly labels for each column’s header.
• Line 21 creates and executes a SELECT SQL query on your database to get all the data in the contacts table.
• Line 22 starts a while loop to navigate the records in the query result using .next().
• Line 24 increments the number of rows in the table by 1 using .setRowCount().
• Lines 25 to 28 add items of data to your table using .setItem(). Note that since the values in the id columns are integer numbers, you need to convert them into strings to be able to store them in a QTableWidgetItem object.

.setItem() takes three arguments:

1. row holds a zero-based integer that represents the index of a given row in the table.
2. column holds a zero-based integer that represents the index of a given column in the table.
3. item holds the QTableWidgetItem object that you need to place at a given cell in the table.

Finally, you call .resizeColumnsToContents() on your view to adjust the size of the columns to their content and provide a better rendering of the data.

Displaying and editing database tables using standard widgets can become a challenging task. That’s because you’ll have two copies of the same data. In other words you’ll have a copy of the data in two locations:

1. Outside the widget, in your database
2. Inside the widget, in the widget’s internal containers

You’re responsible for synchronizing both copies of your data manually, which can be an annoying and error-prone operation. Luckily, you can use PyQt’s Model-View architecture to avoid most of these problems, as you’ll see in the following section.

Using View and Model Classes

PyQt’s Model-View classes eliminate the problems of data duplication and synchronization that may occur when you use standard widget classes to build database applications. The Model-View architecture allows you to use several views to display the same data because you can pass one model to many views.

Model classes provide an application programming interface (API) that you can use to manipulate data. View classes provide convenient delegate objects that you can use to edit data in the view directly. To connect a view with a given module, you need to call .setModel() on the view object.

PyQt offers a set of view classes that support the Model-View architecture:

You can use these view classes along with model classes to create your database applications. This will make your applications more robust, faster to code, and less error-prone.

Here are some of the model classes that PyQt provides for working with SQL databases:

Once you’ve connected one of these models to a physical database table or query, you can use them to populate your views. Views provide delegate objects that allow you to modify the data directly in the view. The model connected to the view will update the data in your database to reflect any change in the view. Note that you don’t have to update the data in the database manually. The model will do that for you.

Here’s an example that shows the basics of how to use a QTableView object and a QSqlTableModel object together to build a database application using PyQt’s Model-View architecture:

To edit the data in a cell of the table, you can double-click the cell. A convenient delegate widget will show in the cell, allowing you to edit its content. Then you can hit Enter to save the changes.

The ability to automatically handle and save changes in the data is one of the more important advantages of using PyQt’s Model-View classes. The Model-View architecture will improve your productivity and reduce the errors that can appear when you have to write data manipulation code by yourself.

Here’s the code to create the application:

 1import sys
 2
 3from PyQt5.QtCore import Qt
 4from PyQt5.QtSql import QSqlDatabase, QSqlTableModel
 5from PyQt5.QtWidgets import (
 6    QApplication,
 7    QMainWindow,
 8    QMessageBox,
 9    QTableView,
10)
11
12class Contacts(QMainWindow):
13    def __init__(self, parent=None):
14        super().__init__(parent)
15        self.setWindowTitle("QTableView Example")
16        self.resize(415, 200)
17        # Set up the model
18        self.model = QSqlTableModel(self)
19        self.model.setTable("contacts")
20        self.model.setEditStrategy(QSqlTableModel.OnFieldChange)
21        self.model.setHeaderData(0, Qt.Horizontal, "ID")
22        self.model.setHeaderData(1, Qt.Horizontal, "Name")
23        self.model.setHeaderData(2, Qt.Horizontal, "Job")
24        self.model.setHeaderData(3, Qt.Horizontal, "Email")
25        self.model.select()
26        # Set up the view
27        self.view = QTableView()
28        self.view.setModel(self.model)
29        self.view.resizeColumnsToContents()
30        self.setCentralWidget(self.view)
31
32def createConnection():
33    con = QSqlDatabase.addDatabase("QSQLITE")
34    con.setDatabaseName("contacts.sqlite")
35    if not con.open():
36        QMessageBox.critical(
37            None,
38            "QTableView Example - Error!",
39            "Database Error: %s" % con.lastError().databaseText(),
40        )
41        return False
42    return True
43
44app = QApplication(sys.argv)
45if not createConnection():
46    sys.exit(1)
47win = Contacts()
48win.show()
49sys.exit(app.exec_())

Here’s what’s happening in this code:

• Line 18 creates an editable QSqlTableModel object.
• Line 19 connects your model with the contacts table in your database using .setTable().
• Line 20 sets the edit strategy of the model to OnFieldChange. This strategy allows the model to automatically update the data in your database if the user modifies any of the data directly in the view.
• Lines 21 to 24 set some user-friendly labels to the horizontal headers of the model using .setHeaderData().
• Line 25 loads the data from your database and populates the model by calling .select().
• Line 27 creates the table view object to display the data contained in the model.
• Line 28 connects the view with the model by calling .setModel() on the view with your data model as an argument.
• Line 29 calls .resizeColumnsToContents() on the view object to adjust the table to its content.

That’s it! You now have a fully-functional database application.