f-strings For the Win

July, 07 2017

It has been a long time coming, but I am now actively migrating existing projects to Python 3. Python 3.6 specifically, because when I am done I will be able to take advantage of my new favorite feature everywhere!

That feature is f-strings, you can Read PEP-0498 for all the details but here is a basic example:

Python 3.6.1 (...) 
Type "help", "copyright", "credits" or "license" for more...
>>> name = 'Albert'
>>> f'Hello, {name}!'
'Hello, Albert!'

Yes, f-strings or "format strings" are yet another way to format strings. I know how you might feel on first glance, I didn't like f-strings at first myself. Surely the explicitness of string.format() would be better than the syntactic magic of f-strings, but once you use them it is hard not to love them.

Another complaint is that there are too many ways to format strings in Python. That is true, but having one obvious way to do things shouldn't cause a lack of real progress in the language. None of the previous string formatting approaches have felt as natural or as teachable as f-strings.

How do they work?

Initially, you might think about f-strings as having an implicit call to .format() with all of the local and global scope passed in. This isn't the correct mental model and that kind of implementation was specifically rejected in the design process. Instead, you can think about f-strings like this:

>>> f'Hello, {name}!'
# is the same as
>>> 'Hello, ' + format(name) + '!'

The built-in format function calls the underlying __format__ interface which allows types to control how they are formatted. An additional spec can be passed into format as well.

>>> import datetime
>>> today = datetime.date.today()
>>> f'The year {today:%Y}'
'The year 2017'
# is the same as
>>> 'The year ' + format(today, '%Y')

Since what is in the {} is an expression which is evaluated in the same context as the f-string literal appears, what you might feel should work, just does.

>>> data = dict(foo='bar')
>>> f'The answer is {data["foo"]}'
'The answer is bar'
>>> class Circle(object):
...   color = 'red'
>>> circle = Circle()
>>> f'The circle is {circle.color}'
'The circle is red'

The expressions can become quite complicated including literals, operations, and function calls:

>>> i = 10
>>> word = 'grease'
>>> f'Math: {i + 1}'
'Math: 11'
>>> f'Upper case: {word.upper()}'
'Upper case: GREASE'

This offers a huge amount of flexibility with code which just works the way one might expect.

Even more in Python 3.6

First, there is everything in Python 3 including unicode everywhere, all the new async support, and an interactive shell which remembers the commands in your last session!

Second, 3.6 features dictionaries which are even faster and perserve key insertion order. Some claim this is an implementation detail which can change, but as Brandon Rhodes and others have observed there probably isn't any going back!

Python 3.6 isn't available on Ubuntu 16.04 by default. Installing isn't too hard, and you can add it to your provisioning automation in a few simple steps.

Start migrating your projects to Python 3.6, and deprecate support for 2.7 so we can all use f-strings everywhere sooner.

Tweet comments, corrections, or high fives to @amjoconn