When to use vim.eval and when to use vim.command in python-vim?

Question

From vim doc:

vim.command(str)                    *python-command*
    Executes the vim (ex-mode) command str.  Returns None.



vim.eval(str)                       *python-eval*
    Evaluates the expression str using the vim internal expression
    evaluator (see |expression|).  Returns the expression result as:
    - a string if the Vim expression evaluates to a string or number
    - a list if the Vim expression evaluates to a Vim list
    - a dictionary if the Vim expression evaluates to a Vim dictionary
    Dictionaries and lists are recursively expanded.

I am having trouble to distinguish these two, perhaps it's because I don't understand the difference between expression and command in the first place. An explanation with some examples is very very welcome.

expression in substitute commands, expression-commands, vim.command, yeah i also stuck at that, when i started with vim... If you really plan to work with vim, you should read the whole help document, which can be found here vimdoc.sourceforge.net/htmldoc/help.html — CodeFanatic
– CodeFanatic, Commented Feb 28, 2014 at 12:39

Ingo Karkat · Accepted Answer · 2014-02-28 12:40:24Z

TL;DR: Use command() to execute a Vim command for its side effects, and eval() to get back a value computed by a Vimscript function.

Like other programming languages, Vim(script) distinguishes between a procedure (which you just invoke, but get nothing back; the interesting thing is the actions it performs), and a function (that returns a value, and can optionally also perform actions like a procedure).

Example

With :split foo.txt, you invoke this to open a file in a window split. The command returns nothing, but its effects can be easily observed (another window opens, a file being edited in there). You'd use vim.command() for that.

With :echo winnr('$'), you query the number of open windows. The :echo prints that value, but if you wanted that in Python, you'd use vim.eval("winnr('$')"). (But note that certain Vim properties are already exposed by the Python integration in Vim; you'd only use this for stuff that's not available in Python yet.)

Kent · Accepted Answer · 2014-02-28 12:34:48Z

1

assume you have two strings:

"5d"

and

"5+5"

if you call command() on those, it is same as in vim press :, then input command.

"5d" -> remove the 5th line
"5+5" -> move cursor to 10th line

if you call eval(), vim evaluates the string as vim expression

"5d" -> error
"5+5" -> 10

answered Feb 28, 2014 at 12:34

Kent

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1 Comment

alpha_989 Over a year ago

I actually had a issue.. where vim.eval works generally find for most strings such as vim.eval('a:items'), but if a:items happens to contain BOM headers, vim.eval crashes.. Other people such as @ludovicchabant has also found it here:ludovic.chabant.com/devblog/2017/02/25/aaa-gamedev-with-vim. I wonder if you know of alternate way of evaluating/transfering a variable from the vimscript namespace to the python namespace? vi.stackexchange.com/questions/16756/…

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