fstrings.md

f-strings and string formatting [Readability]

What is this?

We need a way to dynamically combine data from variables and other data structures into a readable string output, or in other words, string interpolation. In Python, the preferred way, according to Brett Slatkin [5] (as of Python 3.6) is to use f-strings. Another option is the str.format() method.

Not Pythonic

This is considered the old "C-style" method of formatting strings. A Pythonic developer should not use this approach.

animal_one = "fox"
animal_two = "dog"

print("The quick brown %s jumped over the lazy sleeping %s" % (animal_one, animal_two))

Pythonic

The following string interpolation method is more readable and more maintainable, as you don't need to compare the order of variables in a tuple -- you can simply read it.

print(f"The quick brown {animal_one} jumped over the lazy sleeping {animal_two}")

Expressions can also be evaluated inside the brackets.

print(f"1+2 is {1+2}")

Another option is the str.format() method. Through this, it is possible to create a reusable template string and format different data into it.

the_string = "The quick brown {one} jumped over the lazy sleeping {two}"

print(the_string.format(one=animal_one, two=animal_two))
print(the_string.format(one="monkey", two="cow"))
print("{0} is {1}".format("foo", "bar"))

References

[1] PEP 498 -- Literal String Interpolation
[2] PEP 3101 -- Advanced String Formatting
[3] str.format documentation

Books that mention this topic:

[4] Python Tricks: A Buffet of Awesome Python Features by Dan Bader
[5] Effective Python: 90 Specific Ways to Write Better Python by Brett Slatkin