How to pass dynamic values such as username to a galaxy tool using dynamic_options

1 How to pass dynamic values such as username to a galaxy tool using dynamic_options

Tried a lot of different approaches to pass the username or user id to a galaxy tool configuration, nothing seemed to work,
e.g. the __user_id__ or user_name could not be passed to a function referenced by dynamic_options, e.g.:

<param name="mytool" type="select" dynamic_options="mystuff(userid=__user_id__)" />
 or
 <param name="mytool" type="select" dynamic_options="mystuff(username=__user_name__)" />
 
 did not work, neither other approaches such as username='$__user_name__' or ${__user_name__} or '${__user_name__}'
 
 it seems that the reserved variables from this document here cannot be accessed by a dynamic_options method:
 https://wiki.galaxyproject.org/Admin/Tools/ToolConfigSyntax?action=show&redirect=Admin%2FTools%2FTool+Config+Syntax#A.3Ccode.3E_tag_set
 
 The only solution or working approach is to submit the __trans__ dictionary, which is accessible in the referencing
 dynamics_option function.
 
 Because this feature is not well documented, here is some example code to get you started.
 

my-ftp-downloader.xml

<tool id="my-ftp-downloader" name="Tool to upload ftp stuff to a specific dir" version="0.0.1">
    <description>download all files from official a public server</description>
        <command><![CDATA[
                ## bash command to be executed
                ## comments with double hash
        /software/galaxy_ftp_uploader/uploader.py "$ftp_dynamic_upload_dir" 
        ]]> 
        </command>
        <stdio>
                <exit_code range="1:" />
                <exit_code range=":-1" />
        </stdio>
        <inputs>
          <param name="ftp_dynamic_upload_dir" type="select" label="FTP upload dir" dynamic_options="generate_ftp_download_dir( trans=__trans__ )"/>
        </inputs>
        <outputs>
        </outputs>
    <help>
        <![CDATA[
                This tool does whatever it can.
        ]]>
        </help>
        <options refresh="True"/>
        <code file="my_ftp_uploader.py" />
</tool>

my_ftp_uploader.py

#!/usr/bin/env python
import time

# needs to return a list of (Name, Value, Bool)
def generate_ftp_download_dir(trans = None):
    if trans:
       username = trans.user.username
       unix_timestamp = str(int(time.time()))
       upload_path = "galaxy_" + username + "_" + unix_timestamp
       return_obj = []
       return_obj.append( (upload_path, upload_path, True) )
       return return_obj
    return []