The Python language is generally more powerful than Vim's scripting language. If you want to evaluate a fragment of Python code in Vim, you can use command:

:py print 2*2

See :help if_pyth.txt.

Below is a more elaborate example. Put the following code in your .vimrc file:

python << EOL
import vim
# Do not say 'from vim import *' because that
# will delete builtin function eval.

def EvaluateCurrentLine(*args):
  cur_str = vim.current.line
  action, symb = None, None
  for i in args:
    if i in ["r","p"]: action = i
    else: symb = i
  try: start = cur_str.rindex(symb)+len(symb)
  except: start = 0
  result = eval(cur_str[start:],globals())
  if action == "r":
    vim.current.line = cur_str[:start]+str(result)
  else:
    print result
EOL
command -narg=* PyEv python EvaluateCurrentLine(<f-args>)

Be careful about identation in the Python part. This code provides command:

:PyEv

This command evaluates expression in line under cursor and prints result in echo area.

With r argument, it will evaluate the expression and be replaced by the result of evaluation:

:PyEv r

You can give one more argument - it will mark from which char in current line it should start evaluation.

For example: Running the following command:

:PyEv r {

on the following line:

\setlength{\textwidth}{450-63

will give:

\setlength{\textwidth}{387

Comments

I tried to modify your script to execute a range of lines, but the :command's -range option did not seem to propagate into vim.current.range. It always just took one line instead of the range I specified. I finally ended up with:

python << EOL
import vim
def EvaluateCurrentRange():
  eval(compile('\n'.join(vim.current.range),'<string>','exec'),globals())
EOL
map H :py EvaluateCurrentRange()<CR>

The command definition using the same function (command -range Pyr python EvaluateCurrentRange()) did not work.