3.3.5. Including variable contents in strings using f-strings

We often use strings to report results. For instance, we could print a calculated ankle moment to the console using two print calls:

ankle_moment = 100.1  # (for instance)

print("The calculated moment at the ankle in Nm is:")
print(ankle_moment)

The calculated moment at the ankle in Nm is:
100.1

A single string that includes both the definition and the result would be more readable:

“The calculated ankle moment is 100.1 Nm.”

Including variables into strings is easily done using f-strings. The name “f-strings” comes from their f prefix. With f-strings, Python evaluates the result of any instruction placed between curly braces {}:

# Here, we create the sentence above.
the_string = (
    f"The calculated ankle moment is {ankle_moment} Nm."
)

# And here, we print it.
print(the_string)

The calculated ankle moment is 100.1 Nm.

When an f-string contains a float, we can print it to a given precision, using :.xf just after the expression in braces, where x is the number of digits after the decimal point. For instance:

print(f"The calculated ankle moment is {ankle_moment:.0f} Nm.")
print(f"The calculated ankle moment is {ankle_moment:.2f} Nm.")
print(f"The calculated ankle moment is {ankle_moment:.5f} Nm.")

The calculated ankle moment is 100 Nm.
The calculated ankle moment is 100.10 Nm.
The calculated ankle moment is 100.10000 Nm.