Did you know python's vformat()?

published on Sunday, February 26, 2017

It's to str.format() what vprintf() is for printf(). You can get it like this:

import string
vformat = string.Formatter().vformat

It's signature is vformat(spec, args, kwargs) and it does what you would think.

Now you may ponder: The convenient argument unpacking in python generally makes v-type functions unnecessary – so why would you need vformat if you can write spec.format(*args, **kwargs) just as easily without additional import?

This is true if args and kwargs are ordinary tuple and dict. But the great thing about vformat is that it doesn't require a full argument unpacking! This means that you can specify arbitrary objects that support item access without having to preemptively evaluate the values for all the keys. Hell, you need not even provide a method to iterate over the keys!

This can be a superb trait, if you have format strings that typically only access a few properties from a value store with many values – where each of the values may be easy to retrieve, but getting all of them would pose an unnecessary overhead if done frequently, e.g.:

vformat("Mounted {device_presentation} on {mount_path}", (), dbus_device_interface)

(Of course, the same also works for args parameter, but I see no real benefit here.)

vformat allows to pass views onto other objects where the view can perform ad-hoc checks or transformations. For example:

class View:
    def __init__(self, obj, blacklist):
        self.obj = obj
        self.blacklist = blacklist

    def __getitem__(self, key):
        if key in self.blacklist:
            raise KeyError(key)
        try:
            return getattr(self.obj, 'public_' + key)
        except AttributeError:
            raise KeyError(key)

# will access `obj.public_x` and `obj.public_y`:
vformat("{x} {y}", None, View(obj, ATTR_BLACKLIST))

As you see vformat() allows to do all sorts of things and is much more powerful than plain str.format()

Note: from python 3.2 upwards there is also the easier accessible str.format_map(kwargs) that is easier to use.

This entry was tagged gist, programming and python