How to implement a new Python command that takes values from tty?

Hi there,

I have written a few gdb pretty-printers using Python but this time I'm 
trying to write a new command in Python. I want it to take arguments 
from gdb and somewhat works like gdb's print. (It's a special printer 
for some kind of std::vector).

I know I should start by inheriting `gdb.Command` and call `__init__`, 
but I'm definitely stuck on `invoke`. The arguments passed in are in 
string, which, I would like to convert them to gdb.Value, by looking 
them up somehow. I have a partially working version that uses 
"gdb.lookup_symbol", but it does not work for expressions (of course, 
expected).

For example, say the command was called `ppack` which accepts an 
argument just as native `print` do. Desired syntax would be like "pprint 
some_var" or "pprint some_other_var.field1" or "pprint *some_ptr".

How can I let gdb evaluates the expression for me so that I don't have 
to parse it, and then give me the value instead of the name?

Thanks,
Yifei