Python 3's f-Strings: An Improved String Formatting Syntax (Guide)

Option #1: %-formatting

This is the OG of Python formatting and has been in the language since the very beginning. You can read more in the Python docs. Keep in mind that %-formatting is not recommended by the docs, which contain the following note:

“The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly).

Using the newer formatted string literals or the str.format() interface helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.” (Source)

>>> name = "Eric"
>>> "Hello, %s." % name
'Hello, Eric.'

>>> name = "Eric"
>>> age = 74
>>> "Hello, %s. You are %s." % (name, age)
'Hello Eric. You are 74.'

>>> first_name = "Eric"
>>> last_name = "Idle"
>>> age = 74
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> "Hello, %s %s. You are %s. You are a %s. You were a member of %s." % (first_name, last_name, age, profession, affiliation)
'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'

Option #2: str.format()

This newer way of getting the job done was introduced in Python 2.6. You can check out A Guide to the Newer Python String Format Techniques for more info.

>>> "Hello, {}. You are {}.".format(name, age)
'Hello, Eric. You are 74.'

>>> "Hello, {1}. You are {0}.".format(age, name)
'Hello, Eric. You are 74.'

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age'])
'Hello, Eric. You are 74.'

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(**person)
'Hello, Eric. You are 74.'

>>> first_name = "Eric"
>>> last_name = "Idle"
>>> age = 74
>>> profession = "comedian"
>>> affiliation = "Monty Python"
>>> print(("Hello, {first_name} {last_name}. You are {age}. " + 
>>>        "You are a {profession}. You were a member of {affiliation}.") \
>>>        .format(first_name=first_name, last_name=last_name, age=age, \
>>>                profession=profession, affiliation=affiliation))
'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'

Simple Syntax

The syntax is similar to the one you used with str.format() but less verbose. Look at how easily readable this is:

>>> name = "Eric"
>>> age = 74
>>> f"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

It would also be valid to use a capital letter F:

>>> F"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'

Do you love f-strings yet? I hope that, by the end of this article, you’ll answer >>> F"Yes!".

Arbitrary Expressions

Because f-strings are evaluated at runtime, you can put any and all valid Python expressions in them. This allows you to do some nifty things.

You could do something pretty straightforward, like this:

>>> f"{2 * 37}"
'74'

But you could also call functions. Here’s an example:

>>> def to_lowercase(input):
...     return input.lower()

>>> name = "Eric Idle"
>>> f"{to_lowercase(name)} is funny."
'eric idle is funny.'

You also have the option of calling a method directly:

>>> f"{name.lower()} is funny."
'eric idle is funny.'

You could even use objects created from classes with f-strings. Imagine you had the following class:

class Comedian:
    def __init__(self, first_name, last_name, age):
        self.first_name = first_name
        self.last_name = last_name
        self.age = age

    def __str__(self):
        return f"{self.first_name} {self.last_name} is {self.age}."

    def __repr__(self):
        return f"{self.first_name} {self.last_name} is {self.age}. Surprise!"

You’d be able to do this:

>>> new_comedian = Comedian("Eric", "Idle", "74")
>>> f"{new_comedian}"
'Eric Idle is 74.'

The __str__() and __repr__() methods deal with how objects are presented as strings, so you’ll need to make sure you include at least one of those methods in your class definition. If you have to pick one, go with __repr__() because it can be used in place of __str__().

The string returned by __str__() is the informal string representation of an object and should be readable. The string returned by __repr__() is the official representation and should be unambiguous. Calling str() and repr() is preferable to using __str__() and __repr__() directly.

By default, f-strings will use __str__(), but you can make sure they use __repr__() if you include the conversion flag !r:

>>> f"{new_comedian}"
'Eric Idle is 74.'
>>> f"{new_comedian!r}"
'Eric Idle is 74. Surprise!'

If you’d like to read some of the conversation that resulted in f-strings supporting full Python expressions, you can do so here.