Python F-strings Tricks You Should Know

PYTHON

Stop doing print(f”variable = {variable}”)

Python 3.6, all the way back in 2016, introduced a new string formatting method called f-strings, which stands for formatted string literal.

Today they’re pretty much ubiquitous for printing variables in Python.

F-strings are faster than the other string formatting methods and are easier to read and use.

Here are some tricks you may not have known existed.

Code for this article 👇

1. Debug mode

The first one is the debug feature with f-strings.

Instead of writing out “variable = “, do {variable = } instead.

This saves a lot of time and effort, as well as makes your code look cleaner.

Within f-strings debugging, you can also perform math operations as you see in the last line.

x = 10
y = 20

print(f"x = {x}, y = {y}") 
print(f"{x = }, {y = }") 

# math operations
print(f"{x * y = }")

x = 10, y = 20
x = 10, y = 20
x * y = 200

2. Number formatting

There are various formatting/conversions you can perform with strings.

The following are done below

set decimal places :.nf where n is the number of decimal places
hex conversion
binary conversion
octal conversion
scientific notation
pad number with leading zeros :0n where n is the total number of characters

number = 420

# decimal places
print(f"number: {number:.2f}")

# hex conversion
print(f"hex: {number:#0x}")

# binary conversion
print(f"binary: {number:b}")

# octal conversion
print(f"octal: {number:o}")

# scientific notation
print(f"scientific: {number:e}") 

# total number of characters
print(f"Number: {number:09}")

number: 420.00
hex: 0x1a4
binary: 110100100
octal: 644
scientific: 4.200000e+02
Number: 000000420

What about large numbers and percentages?

Let’s say you have a large number, as large as Apple’s market cap; you can use :, where , is the separator

Or, if you want f string to print out a percentage value, you can use :.2% telling Python to set 2 decimal places and add a percentage sign to the end of the string.

apple_marketcap = 2.626 * 10e12

print(f"{apple_marketcap = :,}") # comma separator

percentage = 10.394394

print(f"{percentage = :.2%}") # percentage

apple_marketcap = 26,260,000,000,000.0
percentage = 1039.44%

3. Date formatting

Now, what if you want to format your dates?

Let’s create a sample date time value.

Just like how you would format dates with pandas or in your application, you’d define the format you want in the f-string by doing :<date_format>

Below we format a UTC DateTime to:

no microseconds
date only
time only
time with AM/PM
24-hour format

import datetime

today = datetime.datetime.utcnow()
print(f"datetime : {today}\n")

print(f"date time: {today:%m/%d/%Y %H:%M:%S}")
print(f"date: {today:%m/%d/%Y}")
print(f"time: {today:%H:%M:%S.%f}") 
print(f"time: {today:%H:%M:%S %p}") 
print(f"time: {today:%H:%M}")

datetime : 2022-09-13 05:44:17.546036

date time: 09/13/2022 05:44:17
date: 09/13/2022
time: 05:44:17.546036
time: 05:44:17 AM
time: 05:44

You could also do a lot more with the formatting options.

Here’s how you can get the weekday from the date and the day of the year.

You could also do something fun like calculating how far we are into the year.

It’s 70%, time flies!

# Locale’s appropriate date and time representation
print(f"locale appropriate: {today:%c}")

# weekday
print(f"weekday: {today:%A}")

# day of the year
print(f"day of year: {today:%j}")

# how far are we into the year?
day_of_year = f"{today:%j}"
print(f"progress % year: {int(day_of_year)/365 * 100:.2f}%")

locale appropriate: Tue Sep 13 05:44:17 2022
weekday: Tuesday
day of year: 256
progress % year: 70.14%

You can find more formatting options at https://strftime.org/

4. Repr & str

If you write OOP in Python, you’d be familiar with the dunder methods __repr__ and __str__

The basic idea is:

__repr__= developer friendly
__str__ = user friendly

Below is an example of a dataclass Person with name and age attributes.

For a dataclass, by default (without a str method defined), printing the object will give you the output of repr.

With a str the method defined, you’d need to write !r to tell Python to print out the repr method instead.

from dataclasses import dataclass

@dataclass
class Person:
    name : str
    age : int

    def __str__(self) -> str:
        return f"{self.name} is {self.age} years old"

Elon = Person("Elon Musk", 51)
print(f"{Elon}") # str
print(f"{Elon!r}") # repr

Elon Musk is 51 years old
Person(name='Elon Musk', age=51)

5. Alignment

If you want your variables to be printed at a specific position, alignments are the way to go!

Notice in the first line number:n . Here n stands for the width of space to print the variable number starting from the string “is” (inclusive of the variable itself)

You also have options to do a left, center, or right alignment.

Here left:>20 means given a width of 20 characters print out the string “left text” starting from the left.

For center:^20 that means to leave whatever space is left on the left and right. Since the string “center text!” is 12 characters, the left and right would have four characters of white space.

If we put all three strings together with their formatting options, we would have a width of 60 to place the left, center, and right string variables.

number = 4
print(f"number is {number:4}") # width of 10

# numbers
for number in range(1, 5):
    print(f"the number is {number:{number}}")

left = "left text"
center = "center text!"
right = "right text"

print(f"{left:>20}") # left align
print(f"{center:^20}") # center align
print(f"{right:<20}") # right align

print(f"{left : <20}{center : ^20}{right : >20}")

number is    4
the number is 1
the number is  2
the number is   3
the number is    4
           left text
   center text!    
right text          
left text               center text!              right text

6. Multi-line f-string

Last but not least, you can have multiple lines within an f-string.

Just use triple quotes ''' and then define anything you want within the f-string.

Here’s an example below.

company_name = "Tesla"
employee_count = 100000
mission = "To accelerate the world's transition to sustainable energy"

print(f"""
Company: {company_name}
# of employees: {employee_count:,}
Mission: {mission}
""")

Company: Tesla
# of employees: 100,000
Mission: To accelerate the world's transition to sustainable energy

Thanks for reading!

More about f-strings from the docs

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