How do I convert a VB delegate into a python event handler?

Question

I have to rewrite the following VB code that is subscribing to a delegate (event), into python, using python.net.

Imports MtApi

Public Class Form1
    Private apiClient As MtApiClient

    Public Sub New()
        InitializeComponent()
        apiClient = New MtApiClient
        AddHandler apiClient.QuoteUpdated, AddressOf QuoteUpdatedHandler
    End Sub

    Sub QuoteUpdatedHandler(sender As Object, symbol As String, bid As Double, ask As Double)
        Dim quoteSrt As String
        quoteSrt = symbol + ": Bid = " + bid.ToString() + "; Ask = " + ask.ToString()
        ListBoxQuotesUpdate.Invoke(Sub()
                                       ListBoxQuotesUpdate.Items.Add(quoteSrt)
                                   End Sub)
        Console.WriteLine(quoteSrt)
    End Sub

    ' These can be ignored for this discussion
    Private Sub btnConnect_Click(sender As System.Object, e As System.EventArgs) Handles btnConnect.Click
        apiClient.BeginConnect(8222)
    End Sub

    Private Sub btnDisconnect_Click(sender As System.Object, e As System.EventArgs) Handles btnDisconnect.Click
        apiClient.BeginDisconnect()
    End Sub
End Class

_{This VB code is part of a VB app for the mtapi .NET bridge.}

Q: What is the correct way to convert this VB delegate into a python event handler?

---

I have already tried many variation of the following:

...
import MtApi as mt
...
# apiClient_QuoteUpdated(object sender, string symbol, double bid, double ask)
def printTick(symbol, ask, bid):
    print('Tick: Symbol: {}  Ask: {:.5f}  Bid: {:.5f}'.format(symbol, ask, bid))


class OnTick:
    def __init__(self):
        self.listeners = []

    def __iadd__(self, listener):
        # Shortcut for using += to add a listener
        self.listeners.append(listener)
        return self

    def notify(self, *args, **kwargs):
        for listener in self.listeners:
            listener(*args, **kwargs)

mtc = mt.MtApiClient()
res = mtc.BeginConnect('127.0.0.1', 8222);

# This Works!
newTick = OnTick()
newTick += printTick
newTick.notify(SYM, 1.12400, 1.12300)

# This does NOT work!
newTick.notify(mtc.QuoteUpdate())
# TypeError: 'EventBinding' object is not callable

Been looking at answers here:

• What's the correct way to convert this event handler registration from C# to VB.net?
• Does Python classes support events like other languages?

Answer 1

As closely related to this answer in a similar question, the issue was in over-complicating the delegate code. We simply don't need the OnTick class and also realizing that the QuoteUpdatedHandler() need 4 arguments, so we replace the printTick(...) with that.

_{(Of course if you do want to make somethng a little more complicated or elegant, you do want to create this in a class.)}

Then the equivalent Python code to for the VB delegate, become:

...
def QuoteUpdatedHandler(source, sym, bid, ask) :
    qstr = '{}: {:.5f} {:.5f}'.format(sym,bid,ask)
    print(qstr)

...
mtc = mt.MtApiClient()

print('Connecting...')
res = mtc.BeginConnect('127.0.0.1', 8222);

# VB: AddHandler mtc.QuoteUpdated, AddressOf QuoteUpdatedHandler
# Because we want the "AddressOf" of the function, we don't use the invoking "()"
mtc.QuoteUpdated += QuoteUpdatedHandler

print('ok')

# Now run in a loop and wait for the events:
while 1:
    pass
    try: 
        time.sleep(0.1)
    except KeyboardInterrupt:
        print('\n  Break!')
        break

if (mtc.IsConnected()) :
    mtc.PlaySound("tick")
    mtc.BeginDisconnect()
print('\n  Done!')

sys.exit(2)