44 – Format specifier !r

When you’re using f-strings, you can use the format specifier !r to use a value’s debugging representation instead of pretty-printing it.

Some values and types cannot be distinguished from one another if you pretty-print them, but can if you use their debugging representation. For example, if a string represents an integer, you can’t distinguish it from the same integer when printing:

s = "3"
print(f"{s}")  # 3
# !? Was `s` the string "3" or the integer 3?

Using !r makes it clearer what’s being printed:

print(f"{s!r}")  # '3'

Here’s another example:

from fractions import Fraction

one_third = Fraction(1, 3)
print(f"{one_third}, {one_third!r}")
# 1/3, Fraction(1, 3)

Further reading:

• str and repr