Ah okay, it will work via Modules. Perfect, thanks for the reads! 

For those who are interested:

you can create a new Model with your functions:

from odoo import models, fields
from openerp.exceptions import Warning

class ExternalCode(models.Model):
    _name = "external.code"
     
    name = fields.Char(string="Name", required=True)    
    description = fields.Text("Description")

    ##
    # test function to show how to call a function
    # @name - test
    # @parameters - none (self is referencing the model itself)
    # @return - void 
    ##
    def test(self):
        
        # access another model
        recordset = self.env['hr.employee'].search([("name", "<>", "Peter")])

        nameslist = "I have following names: "

        # walk throug data and concatenate model values
        for record in recordset:
            name = record.name
            nameslist += name

        # return String in a native Warning window
        raise Warning(nameslist)

 
    ##
    # test function to show how to call a function with parameters. It returns
    # the given Parameter in a Warning
    # @name - test
    # @parameters - data(String), (self is referencing the model itself)
    # @return - void 
    ##
    def testWithParameters(self, data): 
    raise Warning(data)

.

Install your module in Odoo and now you can create a call with those 2 lines:

tool = env["external.code"] 
tool.test()
# or tool.testWithParameters( "anything you need to pass" )

Any suggestions on that? It works fine for me so far