I found this vim tip which allows you to evaluate a Python expression and displays the result in the command line. You just have to add

:command! -nargs=+ Calc :py print <args>
:py from math import *

to your .vimrc and then you can do e.g.

:Calc sum([x^2 for x in range(100)])

Which displays 4950in my command line, as if I had executed :echo 4950. This is convenient, but I would like to know how I can either append the result at the location of my cursor, or add it to a certain register.

  • 1
    py vim.current.buffer.append(str(sum([x^2 for x in range(100)])), vim.current.window.cursor[0]) – Alex Kroll Aug 1 '15 at 16:37
  • @AlexKroll This gives me an error: Traceback (most recent call last): File "<string>", line 1 in <module> NameError: name 'vim' is not defined – Pétur Aug 1 '15 at 18:21
  • 1
    'py import vim' of course! – Alex Kroll Aug 1 '15 at 18:23
  • @AlexKroll I was completely unaware that Python had a vim module (or does it?) I prefer your solution to philolo1's but if someone could point me to a command to append the result to a register, that would be great. If you make this an answer I'll probably accept it. – Pétur Aug 1 '15 at 18:34
  • If your Vim compiled with +python or +puthon3 internal Python interpreteur have module vim, :he python-vim for more exploration. – Alex Kroll Aug 1 '15 at 19:02
function! CalcAndAppend(expr)
py << EOF
import vim
evaluated = eval(vim.eval("a:expr"))

vim.current.buffer.append(str(evaluated), vim.current.window.cursor[0])

function! CalcAndAppendRegister(reg, expr)
py << EOF
import vim
evaluated = eval(vim.eval("a:expr"))
evalString = "'" + str(evaluated) + "'"
register = vim.eval("a:reg")

vim.command("let @%s=%s" % (register,evalString))

command! -nargs=+ Calc call CalcAndAppend(<q-args>) 

command! -register -nargs=+ CalcReg call CalcAndAppendRegister("<reg>",<q-args>)

:Calc 2+2
appends 4 to the current buffer on one line below after cursor
:CalcReg a sum[x^2 for x in range(100)] put 4950 to the register named a

  • It looks to me like this uses vim to evaluate the expression and not python. – Pétur Aug 2 '15 at 11:23
  • You're right! Look edited answer! – Alex Kroll Aug 2 '15 at 12:20
  • This is exactly what I had in mind. I added support for floating point numbers to avoid E806: using Float as a String. – Pétur Aug 2 '15 at 15:31

That's what pyeval() is good for. The only (minor) inconvenience is that you have to quote the expression you want to evaluate:

let @a = pyeval('sum([x^2 for x in range(100)])')

It works for all data types, and you can do with it everything you can do with a Vim expression. For example:

echo pyeval('[x^2 for x in range(100)]')

I do the following:

I create somewhere a calculator.py file containing this.

 import sys                                                                  
 print eval(sys.argv[1]) 

Then in vim i can do the command:

read! python calculator.py "[x^2 for x in range(100)]" 
  • I'm curious: why use eval instead of python -c? – muru Aug 1 '15 at 19:22
  • No reason, i did not know the command, so you can also so do python -c "[x^2 for x in range(100)]". – philolo1 Aug 2 '15 at 1:42
  • python -c 'print [x^2 for x in range(100)]', but yes. – muru Aug 2 '15 at 1:42
  1. Open the great vim.
  2. Enter insert mode and add a python3 statement inside your text file buffer:


  3. Go to normal mode (ESC or CTRL+C)

  4. Press !! making sure the cursor is above that line. Your command bar will be filled like:


  5. fill the rest of command with python


  6. press enter end you line will be evaluated:


  • This doesn't appear to answer the question of putting the answer in a register, or inserting it at the cursor. – Herb Jun 2 '17 at 3:08

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