How to use f-strings in Python | note.nkmk.me (2024)

Python 3.6 introduced a new feature, known as formatted string literals, or f-strings, to simplify the use of the string method format().

Contents

  • Basic usage of f-strings
  • Format specifications
    • Left-justify, center, right-justify
    • Zero-padding
    • Separator
    • Binary, octal, and hexadecimal representation
    • Decimal places and significant digits
    • Scientific notation
    • Percentage
    • Date and time (datetime)
  • Braces (curly brackets) {}
  • Nested replacement fields
  • Raw strings and f-strings
  • Difference between the format() method and f-strings
    • Expressions
    • How to specify dictionary keys
  • The = specifier in f-strings (Python 3.8 and later)

For a detailed overview of the string method format(), the built-in function format(), and format specification strings, refer to the following article.

  • Format strings and numbers with format() in Python

Basic usage of f-strings

The format() method inserts values into the replacement field {} using positional or keyword arguments.

a = 123b = 'abc'print('{} and {}'.format(a, b))# 123 and abcprint('{first} and {second}'.format(first=a, second=b))# 123 and abc

source: f_strings.py

An f-string is a string literal prefixed with f or F, like f'...' or F'...'. It allows you to specify variables directly in the replacement field {}.

print(f'{a} and {b}')# 123 and abcprint(F'{a} and {b}')# 123 and abc

source: f_strings.py

Like regular string literals, f-strings can use single quotes '', double quotes "", and triple quotes ''', """.

  • Create a string in Python (single/double/triple quotes, str())
print(f"{a} and {b}")# 123 and abcprint(f'''{a} and {b}''')# 123 and abcprint(f"""{a} and {b}""")# 123 and abc

source: f_strings.py

Format specifications

As with the format() method, f-strings can specify a variety of formats by adding a format specification string after the colon : in {}.

Some examples are shown here. For more information, refer to the following article.

  • Format strings and numbers with format() in Python

Left-justify, center, right-justify

s = 'abc'print(f'right : {s:*>8}')print(f'center: {s:*^8}')print(f'left : {s:*<8}')# right : *****abc# center: **abc***# left : abc*****

source: f_strings.py

Zero-padding

i = 1234print(f'zero padding: {i:08}')# zero padding: 00001234

source: f_strings.py

Separator

Binary, octal, and hexadecimal representation

i = 255print(f'bin : {i:b}')print(f'oct : {i:o}')print(f'hex(lower): {i:x}')print(f'hex(upper): {i:X}')# bin : 11111111# oct : 377# hex(lower): ff# hex(upper): FFprint(f'bin : {i:08b}')print(f'oct : {i:08o}')print(f'hex(lower): {i:08x}')print(f'hex(upper): {i:08X}')# bin : 11111111# oct : 00000377# hex(lower): 000000ff# hex(upper): 000000FFprint(f'bin : {i:#010b}')print(f'oct : {i:#010o}')print(f'hex(lower): {i:#010x}')print(f'hex(upper): {i:#010X}')# bin : 0b11111111# oct : 0o00000377# hex(lower): 0x000000ff# hex(upper): 0X000000FF

source: f_strings.py

Decimal places and significant digits

f = 12.3456print(f'decimal places : {f:.3f}')print(f'significant digits: {f:.3g}')# decimal places : 12.346# significant digits: 12.3

source: f_strings.py

Scientific notation

f = 12.3456print(f'exponent(lower): {f:.3e}')print(f'exponent(upper): {f:.3E}')# exponent(lower): 1.235e+01# exponent(upper): 1.235E+01

source: f_strings.py

Percentage

f = 0.123print(f'percent: {f:.2%}')# percent: 12.30%

source: f_strings.py

Date and time (datetime)

import datetimedt = datetime.datetime(2020, 1, 5, 20, 15, 30)print(f'datetime: {dt}')# datetime: 2020-01-05 20:15:30print(f'datetime: {dt:%A, %m/%d/%Y %I:%M:%S %p}')# datetime: Sunday, 01/05/2020 08:15:30 PMprint(f'datetime: {dt.isoformat()}')# datetime: 2020-01-05T20:15:30

source: f_strings.py

For more information on the datetime module, see the following article.

  • Handle date and time with the datetime module in Python

As in the last example, it is easier to use the isoformat() method than to specify format codes when converting to ISO 8601 format.

  • Convert between isoformat string and datetime in Python

Braces (curly brackets) {}

To include braces { and } in f-strings, double them like {{ and }}. Note that the backslash \ cannot be used.

n = 123print(f'{{}}-{n}-{{{n}}}')# {}-123-{123}

source: f_strings.py

Nested replacement fields

Similar to the format() method, f-strings allow replacement fields within other replacement fields. The number in the format specification string can be a variable.

n = 123i = 8print('{n:0{i}}'.format(n=n, i=i))# 00000123print(f'{n:0{i}}')# 00000123

You could also write:

f = 1.2345for i in range(5): print(f'{f:.{i}f}')# 1# 1.2# 1.23# 1.234# 1.2345

source: f_strings.py

Raw strings and f-strings

In regular strings, backslash \ represents special characters. However, when a string is prefixed with r or R, creating a raw string literal, backslash escapes are disregarded.

  • Raw strings in Python
print('x\ty')# x yprint(r'x\ty')# x\ty

source: f_strings.py

If you prefix a string with both r and f, it will be treated as both a raw string and an f-string. The order of r and f doesn't matter, and you can use either lowercase or uppercase letters.

x = 'XXX'y = 'YYY'print(f'{x}\t{y}')# XXX YYYprint(rf'{x}\t{y}')# XXX\tYYYprint(FR'{x}\t{y}')# XXX\tYYY

source: f_strings.py

Difference between the format() method and f-strings

Expressions

With the format() method, expressions cannot be included in the replacement field.

a = 3b = 4# print('{a} + {b} = {a + b}'.format(a=a, b=b))# KeyError: 'a + b'

source: f_strings.py

In contrast, f-strings allow the use of expressions within the replacement field. It is also possible to specify a format specification string for the outcome of the expression.

print(f'{a} + {b} = {a + b}')# 3 + 4 = 7print(f'{a} * {b} = {a * b}')# 3 * 4 = 12print(f'{a} / {b} = {a / b:.2e}')# 3 / 4 = 7.50e-01

source: f_strings.py

How to specify dictionary keys

When using a dictionary (dict) in a replacement field, the format() method does not require the key to be surrounded by ' or ".

d = {'key1': 10, 'key2': 20}print('{0[key1]}, {0[key2]}'.format(d))# 10, 20# print('{0["key1"]}, {0["key2"]}'.format(d))# KeyError: '"key1"'

source: f_strings.py

However, f-strings require the key to be enclosed in ' or ".

print(f'{d["key1"]}, {d["key2"]}')# 10, 20# print(f'{d[key1]}, {d[key2]}')# NameError: name 'key1' is not defined

source: f_strings.py

Like regular strings, you cannot use the same quotation marks within the string as the ones enclosing the entire string. For example, if the whole string is enclosed in ", use ' inside.

# print(f'{d['key1']}, {d['key2']}')# SyntaxError: invalid syntaxprint(f"{d['key1']}, {d['key2']}")# 10, 20

source: f_strings.py

Note that backslash escaping cannot be used within a replacement field.

# print(f'{d[\'key1\']}, {d[\'key2\']}')# SyntaxError: f-string expression part cannot include a backslash

source: f_strings.py

The = specifier in f-strings (Python 3.8 and later)

From Python 3.8, f-strings support an = specifier, which prints both variable names and their corresponding values.

i = 123print(f'{i=}')# i=123

source: f_strings.py

If you place a space before or after the variable name and =, it will be preserved in the output.

print(f'{i = }')# i = 123print(f'{ i = }')# i = 123

source: f_strings.py

You can also specify format specification strings and expressions with the = specifier.

print(f'{i = :#b}')# i = 0b1111011print(f'{i * 2 = }')# i * 2 = 246

source: f_strings.py

This also applies to lists and dictionaries.

l = [0, 10, 20]print(f'{l = }, {l[1] = }')# l = [0, 10, 20], l[1] = 10d = {'key1': 10, 'key2': 20}print(f'{d = }, {d["key1"] = }')# d = {'key1': 10, 'key2': 20}, d["key1"] = 10

source: f_strings.py

How to use f-strings in Python | note.nkmk.me (2024)

FAQs

How does f-string work in Python? ›

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.

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 join strings with F in Python? ›

String Concatenation using f-string

If you are using Python 3.6+, you can use f-string for string concatenation too. It's a new way to format strings and introduced in PEP 498 - Literal String Interpolation. Python f-string is cleaner and easier to write when compared to format() function.

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.

Are F-strings good 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).

How does F write work in Python? ›

The for statement is used to loop over each line of data in the list. The f. write(line + '\n') statement writes each line of data to the file with a newline character (\n) at the end. Finally, the file is automatically closed when the with block ends.

How to use 0.2 f in Python? ›

%f is for floats. 02:01 In addition to the indicator, you can also put formatting information. The 0.2 here tells Python to put as many digits to the left of the decimal as you like, but only 2 significant digits to the right. 02:15 This formatting will work in more than just floats.

How to use .1f in Python? ›

For instance, if we want to display a floating-point number with one decimal place, we can use :. 1f inside the braces and after the expression.

What is 3.2 F in Python? ›

It means print as a floating point at least 3 wide and a precision of 2. This is a format specifier of a floating point number with 2 decimals and at least one digit left of the decimal point.

How do you append F-strings in Python? ›

The f-string was introduced in Python to make string formatting and interpolation easier. But you can also use it to append strings. To use the f-string, you simply write an f followed by quotation marks: f"" . You can then insert strings and variable names between the quotation marks.

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 {}.

Can you use F-strings in logging Python? ›

Using f-strings to format a logging message requires that Python eagerly format the string, even if the logging statement is never executed (e.g., if the log level is above the level of the logging statement), whereas using the extra keyword argument defers formatting until required.

How do you enter 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.

How to round using f-string? ›

Rounding Numbers With F-Strings

F-strings can also be used to round numbers to a specific precision, using the round() function. To round a number using f-strings, simply include the number inside the curly braces, followed by a colon and the number of decimal places to round to.

How to escape in f-string in Python? ›

Python f-string Escaping Characters

For this purpose, we make use of escape characters in f-string. To escape a curly bracket, we double the character. While a single quote is escaped using a backslash.

How does the F function work in Python? ›

Also called formatted string literals, f-strings are string literals that have an f before the opening quotation mark. They can include Python expressions enclosed in curly braces. Python will replace those expressions with their resulting values. So, this behavior turns f-strings into a string interpolation tool.

How does string () work in Python? ›

String literals inside triple quotes, """ or ''', can span multiple lines of text. Python strings are "immutable" which means they cannot be changed after they are created (Java strings also use this immutable style). Since strings can't be changed, we construct *new* strings as we go to represent computed values.

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 does f do in Python print? ›

A string prefixed with 'f' or 'F' and writing expressions as {expression} is a way to format string, which can include the value of Python expressions inside it. f-string in python lets you format data for printing using string templates.

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