Python F-strings: a Practical Guide to F-strings in Python (2024)

Summary: in this tutorial, you’ll learn about Python F-strings and how to use them to format strings and make your code more readable.

Introduction to the Python F-strings

Python 3.6 introduced the f-strings that allow you to format text strings faster and more elegant. The f-strings provide a way to embed variables and expressions inside a string literal using a clearer syntax than the format() method.

For example:

name = 'John's = f'Hello, {name}!'print(s)Code language: Python (python)

Output:

Hello, John!Code language: Python (python)

How it works.

  • First, define a variable with the value 'John'.
  • Then, place the name variable inside the curly braces {} in the literal string. Note that you need to prefix the string with the letter f to indicate that it is an f-string. It’s also valid if you use the letter in uppercase (F).
  • Third, print out the string s.

It’s important to note that Python evaluates the expressions in f-string at runtime. It replaces the expressions inside an f-string with their values.

Python f-string examples

The following example calls the upper() method to convert the name to uppercase inside the curly braces of an f-string:

name = 'John's = F'Hello, {name.upper()}!'print(s)Code language: Python (python)

Output:

Hello, JOHN!Code language: Python (python)

The following example uses multiple curly braces inside an f-string:

first_name = 'John'last_name = 'Doe's = F'Hello, {first_name} {last_name}!'print(s)Code language: Python (python)

Output:

Hello, John Doe!Code language: Python (python)

This example is equivalent to the above example but uses the join() method:

first_name = 'John'last_name = 'Doe's = F'Hello, {" ".join((first_name, last_name))}!'print(s)Code language: Python (python)

Output:

Hello, John Doe!Code language: Python (python)

Multiline f-strings

Python allows you to have multiline f-strings. To create a multiline f-string, you place the letter f in each line. For example:

name = 'John'website = 'PythonTutorial.net'message = ( f'Hello {name}. ' f"You're learning Python at {website}." )print(message)Code language: Python (python)

Output:

Hello John. You're learning Python on PythonTutorial.net.Code language: Python (python)

If you want to spread an f-string over multiple lines, you can use a backslash (\) to escape the return character like this:

name = 'John'website = 'PythonTutorial.net'message = f'Hello {name}. ' \ f"You're learning Python at {website}." print(message)Code language: Python (python)

The following example shows how to use triple quotes (""") with an f-string:

name = 'John'website = 'PythonTutorial.net'message = f"""Hello {name}.You're learning Python at {website}."""print(message)Code language: Python (python)

Output:

Hello John.You're learning Python at PythonTutorial.net.Code language: Python (python)

Curly braces

When evaluating an f-string, Python replaces double curly braces with a single curly brace. However, the doubled curly braces do not signify the start of an expression.

Python will not evaluate the expression inside the double curly brace and replace the double curly braces with a single one. For example:

s = f'{{1+2}}'print(s)Code language: Python (python)

Output:

{1+2}Code language: Python (python)

The following shows an f-string with triple curly braces:

s = f'{{{1+2}}}'print(s)Code language: Python (python)

Output:

{3}Code language: Python (python)

In this example, Python evaluates the {1+2} as an expression, which returns 3. Also, it replaces the remaining doubled curly braces with a single one.

To add more curly braces to the result string, you use more than triple curly braces:

s = f'{{{{1+2}}}}'print(s)Code language: Python (python)

Output:

{{1+2}}Code language: Python (python)

In this example, Python replaces each pair of doubled curly braces with a single curly brace.

The evaluation order of expressions in Python f-strings

Python evaluates the expressions in an f-string in the left-to-right order. This is obvious if the expressions have side effects like the following example:

def inc(numbers, value): numbers[0] += value return numbers[0]numbers = [0]s = f'{inc(numbers,1)},{inc(numbers,2)}'print(s)Code language: Python (python)

Output:

1,3Code language: Python (python)

In this example, the following function call increases the first number in the numbers list by one:

inc(numbers,1)Code language: Python (python)

After this call, the numbers[0] is one. And the second call increases the first number in the numbers list by 2, which results in 3.

Format numbers using f-strings

The following example use a f-string to format an integer as hexadecimal:

number = 16s = f'{number:x}'print(s) # 10Code language: PHP (php)

The following example uses the f-string to format a number as a scientific notation:

number = 0.01s = f'{number:e}'print(s) # 1.000000e-02Code language: PHP (php)

If you want to pad zeros at the beginning of the number, you use the f-string format as follows:

number = 200s = f'{number: 06}'print(s) # 00200Code language: PHP (php)

The 06 is the total number of the result numeric string including the leading zeros.

To specify the number of decimal places, you can also use the f-string:

number = 9.98567s = f'{number: .2f}'print(s) # 9.99Code language: PHP (php)

Note that the f-string also performs rounding in this case.

If the number is too large, you can use the number separator to make it easier to read:

number = 400000000000s = f'{number: ,}' # also can use _print(s) # 400,000,000,000Code language: PHP (php)

To format a number as a percentage, you use the following f-string format:

number = 0.1259s = f'{number: .2%}'print(s) # 12.59%s = f'{number: .1%}'print(s) # 12.5%Code language: PHP (php)

Python has more sophisticated format rules that you can reference via the following link.

Summary

  • Python f-strings provide an elegant way to format text strings.
  • Python replaces the result of an expression embedded inside the curly braces {} in an f-string at runtime.

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Python F-strings: a Practical Guide to F-strings in Python (2024)

FAQs

How to f-string in Python? ›

To use formatted string literals, begin a string with f or F before the opening quotation mark or triple quotation mark. Inside this string, you can write a Python expression between { and } characters that can refer to variables or literal values.

Should you use f strings in Python? ›

Using f-strings, your code will not only be cleaner but also faster to write. With f-strings you are not only able to format strings but also print identifiers along with a value (a feature that was introduced in Python 3.8). Before we get into this, it's important to understand what string formatting is.

What can I use instead of F-string in Python? ›

Python has several tools for string interpolation that support many formatting features. In modern Python, you'll use f-strings or the .format() method most of the time. However, you'll see the modulo operator ( % ) being used in legacy code.

What does F stand for in a string Python? ›

The release of Python version 3.6 introduced formatted string literals, simply called “f-strings.” They are called f-strings because you need to prefix a string with the letter 'f' to create an f- string. The letter 'f' also indicates that these strings are used for formatting.

What is the F-string symbol in Python? ›

The basic syntax of an F-string involves the following components: Prefix 'f' or 'F': An F-string begins with an 'f' or 'F' character. This tells Python that the string contains expressions that should be evaluated. Curly Braces '{}': Inside the string, you enclose expressions within curly braces {}.

What is the F-string error in Python? ›

What Causes IndexError: string index out of range. This error occurs when an attempt is made to access a character in a string at an index that does not exist in the string. The range of a string in Python is [0, len(str) - 1] , where len(str) is the length of the string.

What are the disadvantages of F-strings? ›

From the user's perspective, the current f-string implementation imposes some limitations:
  • Reusing quotes or string delimiters isn't possible.
  • Embedding backslashes isn't possible, which means you can't use escape characters.
  • Adding inline comments is forbidden.
Jul 26, 2023

Why are F-strings faster? ›

F-strings are faster than str. format() because f-strings are evaluated at compile-time rather than at runtime. When you use an f-string, the expression inside the curly braces is evaluated at compile-time, and the resulting value is inserted into the string.

When did Python get f-strings? ›

F-String was introduced in Python 3.6, and is now the preferred way of formatting strings. Before Python 3.6 we had to use the format() method.

Can I return an F-string in Python? ›

Yes, we can. It's the same as when we show the output of the variables. class Sample: def __init__(self, name, age): self.name = name self. age = age ## this method will be called when we print the object def __str__(self): return f'{self.name} is {self.

What version of Python is the F-string? ›

F-string is a way to format strings in Python. It was introduced in Python 3.6 and aims to make it easier for users to add variables, comma separators, do padding with zeros and date format. F-string was introduced in Python 3.6 and provides a better way to format strings.

How do you evaluate an F-string in Python? ›

We can also evaluate expressions with f-strings in Python. To do so we have to write the expression inside the curly braces in f-string and the evaluated result will be printed as shown in the below code's output.

When should I use F-string in Python? ›

Python f-strings provide a quick way to interpolate and format strings. They're readable, concise, and less prone to error than traditional string interpolation and formatting tools, such as the . format() method and the modulo operator ( % ). An f-string is also a bit faster than those tools!

What is f-string in Python with example? ›

In Python source code, an f-string is a literal string, prefixed with 'f', which contains expressions inside braces. The expressions are replaced with their values. Some examples are: >>> import datetime >>> name = 'Fred' >>> age = 50 >>> anniversary = datetime.

Why not use F-string in Python? ›

It would make sense to use variable placeholders within brackets, e.g. {variable_name} , since it is used in f-strings and . format() . The problem with using f-strings is that variables are instantly replaced by the variable in the current scope, thus it becomes more complex to store the template.

How do I use .2f in Python? ›

Explanation:
  1. format() method is called on the float value, specifying '{:.2f}' as the format specifier.
  2. {:.2f} format specifier indicates that the float should be formatted with two decimal places.
  3. The formatted value is stored in formatted_value.
  4. Finally, print() function is used to display the formatted value.

How do you quote an F-string in Python? ›

When using quotation marks inside an f-string, you can use either single quotes ('') or double quotes (“”). This allows you to include quotes within your strings without causing syntax errors.

How to escape {} in f-string in Python? ›

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

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