Why does my scripted parameter fail, when I use a User Parameter as input ?

I need to check the encoding of several files.

For this I can use a scripted parameter like so:

featureType =open('F:\\PGV\\Projektarbejdsmapper\\P4\\SCSV_til_kontrol\\scsv_2021e1_2021e3\\\\2021e1_lev_k_fortidsminde.scsv')

t = str(featureType)

return t

With a Creator and a ParameterFetcher i get the following attribute:

<_io.TextIOWrapper name='F:\\\\PGV\\\\Projektarbejdsmapper\\\\P4\\\\SCSV_til_kontrol\\\\scsv_2021e1_2021e3\\\\2021e1_lev_k_fortidsminde.scsv' mode='r' encoding='cp1252'>

When I try to do this with a useparameter like so:

file = FME_MacroValues['SCSV_file']

featureType =open(file)

t = str(featureType)

return t

This returns the following log:

Starting translation...

INFORM: Using Python interpreter from `C:\\Program Files\\FME\\fmepython38\\python38.dll' with PYTHONHOME `C:\\Program Files\\FME\\fmepython38'

INFORM: Python version 3.8 loaded successfully

ERROR : Python Exception <KeyError>: 'SCSV_file'

INCLUDE -- failed to evaluate Python script `def ParamFunc():

file = FME_MacroValues['SCSV_file']

featureType =open(file)

t = str(featureType)

return t

value = ParamFunc()

macroName = 'test_script'

if value == None:

return { macroName : u'' }

else:

import six

try:

value = six.text_type(value)

except UnicodeDecodeError:

value = six.text_type(value, 'utf-8')

return { macroName : value }

'

Program Terminating

Translation FAILED.

Why does my scripted parameter fail, when I use a User Parameter (text) as input ?