Inserting values into strings

f-strings

In Brisk Python, we claimed that Python f-strings are the usual best way to insert variable values into strings.

The example there was:

shepherd_name = "Mary"
flock_size = 92

f"Shepherd {shepherd_name} is on duty with {flock_size} sheep."

'Shepherd Mary is on duty with 92 sheep.'

There are a couple of other ways you can do the same thing, that may be useful in particular circumstances.

String format method

You can use the string method format method to create new strings with inserted values. Here we insert a string into another string:

"Shepherd {} is on duty.".format(shepherd_name)

'Shepherd Mary is on duty.'

The empty curly braces show where the inserted value should go. shepherd_name is the argument to the format method, and tells Python which value to insert.

You can insert more than one value. As for f-strings, the values do not have to be strings, they can be numbers and other Python objects.

"Shepherd {} is on duty with {} sheep.".format(shepherd_name, flock_size)

'Shepherd Mary is on duty with 92 sheep.'

"Here is a {} floating point number".format(3.33333)

'Here is a 3.33333 floating point number'

You can do more complex formatting of numbers and strings using formatting options within the curly brackets — { and }. See the documentation on curly brace string formatting.

The same formatting rules apply to f-strings.

This system allows us to give formatting instructions for things like numbers, by using a : inside the curly braces, followed by the formatting instructions. Here we ask to print in integer (d) where the number should be prepended with 0 to fill up the field width of 3:

"Number {:03d} is here.".format(11)

'Number 011 is here.'

This prints a floating point value (specified by the f after the : in the string) with exactly 4 digits after the decimal point:

'A formatted number - {:.4f}'.format(0.213)

'A formatted number - 0.2130'

See the Python string formatting documentation linked above for more details and examples.

% formatting

This is the oldest way of doing string interpolation, and you will rarely find a use for it. Here you use the % operator to tell Python the values to put into the string. Just for a quick example, to replicate the example at the top of the page, you could also use:

"Shepherd %s is on duty with %d sheep." % (shepherd_name, flock_size)

'Shepherd Mary is on duty with 92 sheep.'

The %s tells Python you want to insert the first value at that point, and it should be treated as a string (the s in %s). The %d tells Python the second value should be treated as an integer.

As we’ve said, you’ll rarely have need for this % syntax, but you will see it in older code, and rarely, in special cases for recent code.