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QNetworkAccessManager in PyQt

last modified July 9, 2020

QNetworkAccessManager in PyQt tutorial shows how to shows how to use QNetworkAccessManager to send requests and receive responses.

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QNetworkAccessManager

QNetworkAccessManager allows the application to send network requests and receive replies. The QNetworkRequest holds a request to be sent with the network manager and the QNetworkReply contains the data and headers returned for a response.

QNetworkAccessManager has an asynchronous API which means that its methods always return immediately and do not wait until they finish. Instead, a signal is emitted when the request is done. We handle the response in the method attached to the finished signal.

HTTP GET request

The HTTP GET method requests a representation of the specified resource.

get_request.py
#!/usr/bin/python

'''
QNetworkAccessManager in PyQt

In this example we get a web page.

Author: Jan Bodnar
Website: zetcode.com
'''

from PyQt5 import QtNetwork
from PyQt5.QtCore import QCoreApplication, QUrl
import sys

class Example:

    def __init__(self):

        self.doRequest()


    def doRequest(self):

        url = 'http://webcode.me'
        req = QtNetwork.QNetworkRequest(QUrl(url))

        self.nam = QtNetwork.QNetworkAccessManager()
        self.nam.finished.connect(self.handleResponse)
        self.nam.get(req)


    def handleResponse(self, reply):

        er = reply.error()

        if er == QtNetwork.QNetworkReply.NoError:

            bytes_string = reply.readAll()
            print(str(bytes_string, 'utf-8'))

        else:
            print("Error occured: ", er)
            print(reply.errorString())

        QCoreApplication.quit()


def main():

    app = QCoreApplication([])
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

The example retrieves the HTML code of the specified web page. 

url = 'http://webcode.me'
req = QtNetwork.QNetworkRequest(QUrl(url))

With the QNetworkRequest we send a request to the specified URL. 

self.nam = QtNetwork.QNetworkAccessManager()
self.nam.finished.connect(self.handleResponse)
self.nam.get(req)

A QNetworkAccessManager object is created. When the request is finished, the handleResponse method is called. The request is fired with the get method. 

def handleResponse(self, reply):

    er = reply.error()

    if er == QtNetwork.QNetworkReply.NoError:

        bytes_string = reply.readAll()
        print(str(bytes_string, 'utf-8'))

    else:
        print("Error occured: ", er)
        print(reply.errorString())

    QCoreApplication.quit()

The handleResponse receives a QNetworkReply object. It contains data and headers for the request that was sent. If there is no error in the network reply, we read all data using the readAll method; otherwise we print an error message. The errorString returns a human-readable description of the last error that occurred. The readAll returns the data in QByteArray that has to be decoded. 

$ ./get_request.py 
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>My html page</title>
</head>
<body>

    <p>
        Today is a beautiful day. We go swimming and fishing.
    </p>
    
    <p>
         Hello there. How are you?
    </p>
    
</body>
</html>

This is the output.

HTTP POST request

The HTTP POST method sends data to the server. The type of the body of the request is indicated by the Content-Type header. A POST request is typically sent via an HTML form. The data sent in the request can be encoded in different ways; in application/x-www-form-urlencoded the values are encoded in key-value tuples separated by '&', with a '=' between the key and the value. Non-alphanumeric characters are percent encoded. The multipart/form-data is used for binary data and file uploads.

post_request.py
#!/usr/bin/python

'''
QNetworkAccessManager in PyQt

In this example we post data to a web page.

Author: Jan Bodnar
Website: zetcode.com
'''

from PyQt5 import QtNetwork
from PyQt5 import QtCore
import sys, json

class Example:

    def __init__(self):

        self.doRequest()


    def doRequest(self):

        data = QtCore.QByteArray()
        data.append('name=Peter&')
        data.append('age=34')

        url = 'https://httpbin.org/post'
        req = QtNetwork.QNetworkRequest(QtCore.QUrl(url))
        req.setHeader(QtNetwork.QNetworkRequest.ContentTypeHeader,
            'application/x-www-form-urlencoded')

        self.nam = QtNetwork.QNetworkAccessManager()
        self.nam.finished.connect(self.handleResponse)
        self.nam.post(req, data)


    def handleResponse(self, reply):

        er = reply.error()

        if er == QtNetwork.QNetworkReply.NoError:

            bytes_string = reply.readAll()

            json_ar = json.loads(str(bytes_string, 'utf-8'))
            data = json_ar['form']

            print(f"Name: {data['name']}")
            print(f"Age: {data['age']}")
            print()

        else:
            print('Error occurred: ', er)
            print(reply.errorString())

        QtCore.QCoreApplication.quit()


def main():

    app = QtCore.QCoreApplication([])
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

The example sends a post request to the https://httpbin.org/post testing site, which sends the data back in JSON format. 

data = QtCore.QByteArray()
data.append('name=Peter&')
data.append('age=34')

As per specification, we encode the data sent in the QByteArray. 

url = 'https://httpbin.org/post'
req = QtNetwork.QNetworkRequest(QtCore.QUrl(url))
req.setHeader(QtNetwork.QNetworkRequest.ContentTypeHeader,
    'application/x-www-form-urlencoded')

We specify the application/x-www-form-urlencoded encoding type. 

bytes_string = reply.readAll()

json_ar = json.loads(str(bytes_string, 'utf-8'))
data = json_ar['form']

print(f"Name: {data['name']}")
print(f"Age: {data['age']}")
print()

In the handler method, we read the response data and decode it. With the built-in json module, we extract the posted data. 

$ ./post_request.py
Name: Peter
Age: 34

This is the output.

QNetworkAccessManager authentication

The authenticationRequired signal is emitted whenever a final server requests authentication before it delivers the requested contents.

authentication.py
#!/usr/bin/python

'''
QNetworkAccessManager in PyQt

In this example, we show how to authenticate
to a web page.

Author: Jan Bodnar
Website: zetcode.com
'''

from PyQt5 import QtCore, QtGui, QtNetwork
import sys, json


class Example:

    def __init__(self):

        self.doRequest()


    def doRequest(self):

        self.auth = 0

        url = 'https://httpbin.org/basic-auth/user7/passwd7'
        req = QtNetwork.QNetworkRequest(QtCore.QUrl(url))

        self.nam = QtNetwork.QNetworkAccessManager()
        self.nam.authenticationRequired.connect(self.authenticate)
        self.nam.finished.connect(self.handleResponse)
        self.nam.get(req)


    def authenticate(self, reply, auth):

        print('Authenticating')

        self.auth += 1

        if self.auth >= 3:
            reply.abort()

        auth.setUser('user7')
        auth.setPassword('passwd7')


    def handleResponse(self, reply):

        er = reply.error()

        if er == QtNetwork.QNetworkReply.NoError:

            bytes_string = reply.readAll()

            data = json.loads(str(bytes_string, 'utf-8'))

            print(f"Authenticated: {data['authenticated']}")
            print(f"User: {data['user']}")

            print()

        else:
            print('Error occurred: ', er)
            print(reply.errorString())

        QtCore.QCoreApplication.quit()


def main():

    app = QtCore.QCoreApplication([])
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In the example se use the https://httpbin.org website to show how authentication is done with QNetworkAccessManager. 

self.nam.authenticationRequired.connect(self.authenticate)

We connect the authenticationRequired signal to the authenticate method. 

def authenticate(self, reply, auth):

    print('Authenticating')
    ...

The third parameter of the authenticate method is the QAuthenticator, which is used to pass the required authentication information. 

self.auth += 1

if self.auth >= 3:
    reply.abort()

The QNetworkAccessManager keeps emitting the authenticationRequired signal if the authentication fails. We abort the process after three failed attempts. 

auth.setUser('user7')
auth.setPassword('passwd7')

We set the user and the password to the QAuthenticator. 

bytes_string = reply.readAll()

data = json.loads(str(bytes_string, 'utf-8'))

print(f"Authenticated: {data['authenticated']}")
print(f"User: {data['user']}")

print()

The https://httpbin.org responds with JSON data, which contains the user name and a boolean value indicating authentication success. 

$ ./authentication.py
Authenticating
Authenticated: True
User: user7

This is the output.

QNetworkAccessManager fetch favicon

A favicon is a small icon associated with a particular website. In the following example we are going to download a favicon from a website.

fetch_icon.py
#!/usr/bin/python

'''
QNetworkAccessManager in PyQt

In this example we fetch a favicon from
a website.

Author: Jan Bodnar
Website: zetcode.com
'''

from PyQt5 import QtCore, QtGui, QtNetwork
import sys

class Example:

    def __init__(self):

        self.doRequest()


    def doRequest(self):

        url = 'http://webcode.me/favicon.ico'
        req = QtNetwork.QNetworkRequest(QtCore.QUrl(url))

        self.nam = QtNetwork.QNetworkAccessManager()
        self.nam.finished.connect(self.handleResponse)
        self.nam.get(req)


    def handleResponse(self, reply):

        er = reply.error()

        if er == QtNetwork.QNetworkReply.NoError:

            data = reply.readAll()
            self.saveFile(data)

        else:
            print('Error occured: ', er)
            print(reply.errorString())

        QtCore.QCoreApplication.quit()


    def saveFile(self, data):

        f = open('favicon.ico', 'wb')

        with f:

            f.write(data)


def main():

    app = QtCore.QCoreApplication([])
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

The code example downloads a Google's favicon. 

self.nam.get(req)

We download the icon with the get method. 

data = reply.readAll()
self.saveFile(data)

In the handleResponse method we read the data and save it to the file. 

def saveFile(self, data):

    f = open('favicon.ico', 'wb')

    with f:

        f.write(data)

The image data is saved on the disk in the saveFile method.

In this tutorial, we have worked with QNetworkAccessManager.

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