Python can read the graph parameter values but they cannot be written (they are effectively treated as constants when the graph is run).
You can access the values within the script using textual substitution of the parameter.
So if you have a graph parameter named 'FolderName' and it has a value set to be '/home/Bob/data' (without the apostrophes) then your script could access the value using the usual textual substitution mechanism, e.g.
fName = r"{{^FolderName^}}"
which would result in the 'fName' variable having a value of "/home/Bob/data"
Note the presence of the r before the opening quote character - this is useful if you are dealing with a mix of Linux and Windows style paths as the backslash characters in the Windows paths would result in errors. It is best to replace any backslash characters with forward slash characters if possible e.g. using
fName = fName.replace("\\", "/")
Values resulting from textual substitution should be considered as strings - you can then, if required, cast them to an appropriate data type within your script using the relevant Python operator (e.g. float() etc).
See the Parameters Overview topic in the Help for additional information on textual substitution.