Notes on Flask, WTForms, Protocol Buffers and more…#

These notes are a part of Flask based Web Server I build for CVP Protobuf Encoded Communication Testing Server.

Protocol Buffers#

Compiling large number of .proto files.#

Suppose you have a folder with ~100 .proto files. You wont do protoc --python_out=compile_pb/ myProtoFile.proto for each one of them. Instead a simple shell script makes job easier.

for file in *.proto; do
    echo "Compiling $file"
    protoc --python_out=pyi_out:$1 $file

The .pyi files to help in IDEs and code editors like VsCode or PyCharm.

Looping over all fields in protobuf messages.#

The python protobuf provides an api ListField, however does not lists empty fields. This causes fields specifically set to values like 0 to be skipped.

Another way is to use message.DESCRIPTOR.fields which will list all the field descriptors for that message type. This includes empty ones. This creates a list of names of all fields in the message, where field is of type google.protobuf.descriptor.FieldDescriptor

[f.name for f in a.DESCRIPTOR.fields]

See protoserver.utils.MessageToTable() for entire code, but here is the gist

gen = message.ListFields()
## To loop over fields which are not shown in ListFields
if show_empty:
    gen = ((f, getattr(message, f.name)) for f in message.DESCRIPTOR.fields)

for field_descriptor, value in gen:
    ...

Getting string name of Enum in protobuf message#

Suppose we have an enum class ExampleEnum we can get name for corresponding int type as ExampleEnum.Name(1). In case of enum field inside a message we can use field.enum_type.values_by_number

for field_descriptor, value in message.ListFields():
    field_descriptor.enum_type.values_by_number[int(sub_message)].name

WTForms#

I have used this for creating dynamic flask forms. See protoserver.app.generateDynamicForm() for more deatils.

Basic Stuff#

class MyFLaskForm(FlaskForm):
    intField = IntegerField("Integer Field", render_kw={"class":"form-control"})
    stringField = StringField("String Field", render_kw={"placeholder": "Enter text"})
    selectField = SelectField("Select Field", choices=[(e.number, e.name) for e in enum_type.values] render_kw={"class":"form-select"})

Creating Dynamic Flask Forms based on Protobuf Payload#

def generateDynamicForm(message):
    class DynamicForm(FlaskForm):
        pass

    for field in message.DESCRIPTOR.fields:
        # {ENUM: "SelectField", INTEGER: "IntegerField", STRING: "StringField"}[field.type]
        setattr(DynamicForm, field.name, {{TYPE}}Field(field.name, **kwrgs, render_kw=...))

        if field.label == field.LABEL_REPEATED:
            nested_form = FieldList(FormField({{TYPE}}Field(field.name), **kwargs), min_entries=..., max_entries=... **kwargs)
            setattr(DynamicForm, field.name, nested_form)

        if field.type == TYPE_MESSAGE:
            # Recursion
            setattr(DynamicForm, field.name, generateDynamicForm(message[field.name]))

    return DynamicForm

This enabled me to create form based fields in protobuf payload.

Flask#

Best place to learn#

The best place to start in my opinion is CS50x’s Web Track (This is from CS50 2020, I don’t know about current versions)

Another good tutorial is the Flask’s own docs.
There may be some youtube resources, but I am not fond of video lectures. I’d rather grind my way through the docs. RTFM baby.

Ecosystem#

Another wonderful thing about Flask is the eco-sysmtem around it.

For WTForms: we have Flask-WTF
For MQTT: we have Flask-MQTT
For SocketIO: we have Flask-SocketIO
And I never saw this one coming, there is literally Flask-Rich for Rich based text formatting and logging in Flask.
This one’s popular: Flask-SQLAlchemy

Adding attributes to variables in `render_template`#

So adding CSS class is as simple as: {{ field(class="form-control") }}

You can also add custom JavaScript function like this:

<label for="{{ form.numEntries.id }}" class="form-label">{{ form.numEntries.label }}</label>
{{
    form.numEntries(
        onfocus="document.getElementById('numEntriesInfo').style.display='block'",
        onblur="document.getElementById('numEntriesInfo').style.display='none'"
    )

}}
<label id="numEntriesInfo" class="text-muted small" style="display: none;">This field will not be included in the form.</label>

Using `render_template` outside of registered URL Route#

This is just to solve an error which occured in :func:decode_response <protoserver.app.decode_response>.

RuntimeError: Working outside of application context.

To solve this I did:

@mqtt.on_message()
def decode_response():
    # ...
    with app.app_context():
        response_table = render_template('response_table.html', **kwargs)
    # ...

SocketIO#

To send message from backend to frontend#

This is what ChatGPT said when I asked about how do I pass MQTT response from background thread to front html.

If you’re receiving MQTT responses in another thread in your Flask application and you want to update the HTML template with this data, you need a mechanism to communicate between threads. Flask-SocketIO is one way to achieve real-time communication between the server and the client using WebSockets.

So this is what I needed SocketIO for

Python

socketio.emit('mqtt_message',
    {
        'topic': message.topic,
        'message': mqtt_response,
        'messageHex': message.payload.hex(" ").upper()
    })

JavaSricpt

const socket = io.connect('http://' + document.domain + ':' + location.port);
socket.on('mqtt_message', function(data) {
    showToast({
        header: 'MQTT Response received on',
        data: data.topic
    });
    $('#responseHex').html(data.messageHex);
    $('#response').html(data.message);
});