Evaluate parenthesis expression for correct nesting and matching parens
Use snake_case (matchParens) 
You are checking a property, so I would name your function has_correct_parens to make it clearer that this function return True or False
[i for i in iterable] is a longer way of saying list(iterable) so plist = [l for l in pstring] should become plist = list(pstring)
The preventive check should be at the start for clarity and efficiency
What if you will want to use square / curly / triangular parens in the future? I would use constants:




OPEN_PAREN = '('
CLOSE_PAREN = ')'