Using Python interpolation in UltiSnips snippets

The basics of Python interpolation in UltiSnips

Python scripts can be embedded inside UltiSnips snippets using `!p`. In this context, UltiSnips predefines a few Python objects and variables for us. For example, the snip.rv variable stands for return value. It represents the text that will be interpolated into the document when our snippet is expanded. The t variable is a list representing the values of tabstops numbered one, two, three, and so on. For more details, look up :help UltiSnips-python.

A snippet to calculate average typing speed

The snippet that I demonstrated in the video appeared in text.snippets like this:

global !p
def fraction(a, b):
    try:
        return "%.1f" % (float(a)/float(b))
    except (ValueError, ZeroDivisionError):
        return "ERR"
endglobal

snippet 75wrds "Log a 750words.com session" b
750words.com: ${1:750} words in ${2:20} minutes - `!p
snip.rv = fraction(t[1], t[2])
` wpm.
endsnippet

The global block allows us to create Python functions, which we can then call from any snippet in the same snippet file. This fraction() function attempts to cast two arguments to floats and divide one by the other, rounding the result to a single decimal place. If it encounters an exception, it returns the string “ERR”.

When using this snippet, the words-per-minute calculation is updated in real-time as the values for tabstops one and two are changed. It’s almost like working with a spreadsheet!

Sharing helper functions between snippet files

The fraction() function defined above would be available only to snippets defined in the text.snippets file. We could make that function available to other snippet files by moving it into a ~/.vim/pythonx/snippet_helpers.py file:

def fraction(a, b):
    try:
        return "%.1f" % (float(a)/float(b))
    except (ValueError, ZeroDivisionError):
        return "ERR"

We could then import this into a snippet file by including this:

global p!
from snippet_helpers.py import *
endglobal

One potential gotcha to watch out for here is that Vim will load the snippet_helpers.py one time only. That means that any changes you make to the helper functions in snippet_helpers.py won’t be seen by your snippet files until you reboot Vim. Despite that limitation, this mechanism is still useful for sharing helper functions between your snippet files.

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