Missing Arguments

Optional arguments which are not supplied and have no defaults are initialized with the VBMissingArgument object. This object can be detected by the vbfunctions.IsMissing function to provide initialization of missing arguments within the body of the subroutine. This functionality is transparent under normal conditions, but if the subroutine manually assigns a value to the missing parameter prior to the IsMissing call then the behaviour may not match that of VB, since the vbfunctions.IsMissing function has no way to detect that the parameter was not supplied.

Dim moduleGlobal1, moduleGlobal2

Sub MySub(X, Optional Y, Optional Z=20)
    Dim subLocal
    If IsMissing(Y) Then Y = 12
    subLocal = X + Y + Z + moduleGlobal
    moduleGlobal2 = moduleGlobal2 + 1
End Sub

MySub 1, 2
MySub 1, Z:=10

    moduleGlobal1 = Variant()
    moduleGlobal2 = Variant()

    def MySub(X, Y=VBMissingArgument, Z=20):
        global moduleGlobal2
        if IsMissing(Y):
            Y = 12
        subLocal = X + Y + Z + moduleGlobal
        moduleGlobal2 = moduleGlobal2 + 1

    MySub(1, 2)
    MySub(1, Z=10)

Argument Passing

VB has two argument passing schemes,

1. ByRef (Default) - arguments are passed by reference. Changes to the value inside the subroutine are reflected in the corresponding parameter in the calling scope.
2. ByVal - arguments are passed by value. Changes to the value inside the subroutine do not affect the parameter in the calling scope.

Although Python's argument passing semantics are often refered to as pass-by-reference, the actual behaviour does not always match VB's ByRef because of immutable object types and name re-binding. Although there are technical solutions for these issues, the current version of vb2Py does not make any attempt to match behaviours.

This means that the following code, although converted, does not behave the same in the Python version

Sub DoIt(x, ByVal y)
    x = x + 1
    y = y + 1
End Sub

x = 0
y = 0
DoIt x, y
' x is now 1, y is still 0

    def DoIt(x, y):
        x = x + 1
        y = y + 1

    x = 0
    y = 0
    DoIt(x, y)
    # x is now 1, y is still 0