F-strings are awesome!

Exploring some neat features I wasn’t aware of

If you know python, you are probably aware of its value. The language is easy to develop in, which allows for quick proto-typing. It is used by many professional companies either as glue to bind compiled languages together, or as a full-stack for their apps. With the advent of python 3.6 and [[pep498][https://www.python.org/dev/peps/pep-0498/]] f-strings arrived. Its longer name is formatted string literals or f-strings for short ;-). This is probably one of my favorite python feature next to dicts being sorted by default.

What are f-strings?

When you are either debugging or writing some logging library, strings (in my opinion) were always a hassle to deal with. In order to write the output of a variable one would either write

some_variable = "hello world!"
print("Some variable = " + some_variable)
print("Some variable = %s".format(some_variable))

The top choice is not that bad, but the bottom one gets complicated when formatting get more complex. With F-strings this procedure can be simplified by writing:

some_variable = "hello world!"
print(f"Some variable = {some_variable}")

Merely by adding the formatting operator “f” in front of the string, the output becomes more readble. This has the advantage of literate programming; in plain langauge one can read this sentence and understand its output in normal plain language. Since the feature became available I quickly replaced the old formatting style with this newer better way: f-strings became the norm.

In my normal debugging routine, I am not used to using full-featured debuggers that can step in and out of functions allowing to see local scopes: very fancy! I usually resort to print statements, and lots of coffee and pacing around my room. F-strings allowed for quicker and clearer debugging. This prompts me to write this post now, what else can F-strings do?

Capabilities of f-strings

F-strings can do a great number of things. One of the major advantages for me is making readable strings as was indicated above.


F-strings allow for formatting opertors to be passed in. Say you have a float with 10 decimes but you only want to plot the rounded number. One option would be to do:

a = 1.23123512351234
print(f"{round(a, 2)}")

This however, quickly makes the f-string “complicated”. Luckliy f-strings allow for format operators to be used

a = 1.23123512351234
print(f"{a:.2f}") # prints up to 2 precision

Next to rouding formats, one can also apply alignment of text

text = "hello world"
print(f"{text: >}") # right align
print(f"{text: <}") # left align [default often]
print(f"{text: ^}") # center align 

There are many more possibilites, including aligning only if the text goes bigger than a certain predetermined number of digits or formatting numbers to hex, binary, leading space for positive numbers and so on. In fact there is a minilanguage that it uses that can be found on the python docs.

Lambda functions

The colon is used for formatting indicator. This prevents lambda functions from being used directly. However, by encapsulating the lambda function one is still able to insert lambda functions inside an f-string.

print(f"This is the output of two times 2 {(lambda x: x*2)(2)}")


In debugging I often revert to writing the output to check the state of some object. This results in boilerplate statements that I wish to prevent. Luckily, f-strings can to this with = formatting.

some_variables = "ERROR!"

will print out

some_variable = "ERROR"

Pretty neat! This reduces the need to write out the variable statement completely.

Casper van Elteren
Casper van Elteren
Computational scientist | Data scientist | Tinkerer

I am a computational scientist interested in data analysis, visualization and software engineering.