Python String Formatting

Python 3.6 gave us the f-string, which is now the favoured way to format text.

In earlier versions, the format() method filled that role.

An f-string lets you control how chosen parts of a string are formatted.

All it takes is an f written just before the string literal:

A first, very simple f-string:

Values get into an f-string through placeholders, written as {}. A placeholder can hold variables, operations and functions.

It can also carry a modifier, which decides how the value looks.

Write a modifier after a colon :, using a valid formatting type. For example, .2f asks for a fixed-point number showing 2 decimal places:

Drop the price variable into a placeholder:

Show the same price with 2 decimal places:

The value doesn't have to live in a variable. You can format it right in the placeholder:

Format the number 95 with 2 decimal places:

Placeholders can also run Python operations. That covers plain math, math on variables, and even if...else statements, as the next three examples show:

Multiply two numbers inside the placeholder and show the answer:

Work out the price with tax included, right in the placeholder:

Show "Expensive" when the price is above 50, and "Cheap" in any other case:

A placeholder can call functions as well. Python's built-in methods work, and so do functions you write yourself:

Call the string method upper() to turn the value into capital letters:

Write your own function that turns feet into meters, then call it in the placeholder:

Earlier in this lesson, the .2f modifier turned a number into a fixed-point value with 2 decimal places.

That is just one option, and other modifiers format values in different ways.

Add a comma to separate the thousands:

Before Python 3.6, formatting a string meant calling the format() method.

That method still works, although f-strings run faster and are the recommended choice.

The remaining examples on this page use format() so you can see how it works.

It also marks placeholders with curly brackets {}, but its syntax differs a little:

To control how a value is converted, add parameters between the curly brackets:

Put a placeholder at the spot where the price should appear:

Need more values? Pass them all to format():

Then give each one its own placeholder:

A number inside the curly brackets, such as {0}, is an index. It guarantees that each value lands in the right placeholder:

It also lets you use one value several times, simply by repeating its index:

Placeholders can have names as well. Write a name between the curly brackets, as in {carname}, and then supply the value under that same name when you call the method, as in txt.format(carname = "Ford"):