Evaluate Python expression {expr} and return its result converted to Vim data structures. Numbers and strings are returned as they are (strings are copied though). Lists are represented as Vim |List| type. Dictionaries are represented as Vim |Dictionary| type, non-string keys result in error. Note that in a :def function local variables are not visible to {expr}. Can also be used as a |method|: GetExpr()->pyeval() {only available when compiled with the |+python| feature}