Loading Packages with Parameters

Question

I have some packages that uses database connections inside it, let's call it ConnPackage. The connection IP is a public package variable, and has a default value:

$connectionIP="127.0.0.1".

This variable is used to read database values when the package is loading. So change it value after package load don't solve the problem. The problem is similar to pass args in shell wolfram script, but must be done in the notebook.

The question is:

How could I read the ConnPackage, using Needs[], but setting $connectionIP to be another value?

UPDATE

I can solve the problem defining a variable in System. But I don't feel it's a good pattern. Here is my attempt. Inside ConnPackage I can do:

$connectionIP=If[StringQ[#],#,"127.0.0.1"]&[System`$argConnectionIP]

So, if there is no value in System`$argConnectionIP, the standard IP will be load, otherwise it will use System`$argConnectionIP.

Answer 1

You can catch next assignment with TagSetDelayed:

$connectionIP /: Set[$connectionIP, _] := (ClearAll[$connectionIP]; 
                                           $connectionIP = "whatever");
Needs["ConnPackage`"]

You should make this TagSetDelayed statement in proper context so use Begin or provide explicit context.

Does it fit your needs?

---

Different approach.

If you know what is done inside the package, and you know that $connectionIP is a constant you can just do:

$connectionIP = "whatever";                (*keep in mind the context*)

SetAttributes[$connectionIP, Protected];

Needs["ConnPackage`"];