Python F-Strings Number Formatting Cheat Sheet (2024)

Table of Contents
F-String Template Replac­ement Field Format Specifier Format Specifier: Options Sign and Separator Options Examples: Sign and Separator Sign Aware Padding Integer Presen­tation Types Examples: Integer Types Float Presen­tation Types Examples: Simple Float Compar­isons General Format (g or G) Rule General Format (g or G): Illust­ration Types with Condit­ional Formatting Examples: Both Integers and Floats References

F-String Template

f" text {replacement_field} text ... "

Inside the quotes the f-string consists of two kinds of parts: (1) regular string literals, i.e., text, and (2) replac­ement fields containing Python expres­sions for evaluation along with formatting control.
Double quotes are used here but single or triple quotes could also be used.
F-strings may consist of just a replac­ement field:
­­­­

f"{r­epl­ace­men­t_f­iel­d}"

Replac­ement Field

{f-expression = !conversion:format_specifier}

A replac­ement field is signaled by a pair of curly braces: 

{ }

A replac­ement field consists of a Python expression for evaluation with optional debugging mode (=), type conversion (!), and format specif­ication (:).
Substi­tuting into the f-string template: 

f" text {f-exp­ression = !conve­rsi­on:­for­mat­_sp­eci­fier} text ..."

Format Specifier

:fill align sign # 0 width sep .precision type

Brief Summary of the Format Specif­ication Mini-L­anguage

fill

: the padding character 

align

: alignment of text within the space

See Also
How to Format Floats Within F-Strings in Python – Real PythonPython 3.12 Preview: More Intuitive and Consistent F-Strings – Real PythonPad or fill a string by a variable in Python using f-string - GeeksforGeeks[Ultimate Guide] f-strings in Python - Learn By Example

sign

: how + and - are used preceding numbers 

#

: alternate presen­tation format for some number types 

0

: sign-a­ware, zero-p­adding on numbers 

width

: the minimum total field width 

sep

: the separator character for numbers (',' or '_') 

.precision

: how many digits displayed for floats;
maximum field width for strings 

type

: the type of presen­tation to use based on data type

Note: 

sign

, 

#

, 

0

, 

sep

, 

precision

, and 

type

are of particular interest for number formatting, the focus of this cheats­heet. For a general treatment of F-Strings, see my cheatsheet Python F-Strings Basics.

Format Specifier: Options

fill

align

sign

#

0

width

sep

.prec

type

char

<

+

digit(s)

_

digit(s)

string: 

s

>

-

,

number: 

n

^

' '

integer: 

d

, 

b

, 

o

, 

x

, 

X

, 

c

=

float: 

e

, 

E

, 

f

, 

F

, 

g

, 

G

, 

%

Sign and Separator Options

Sign

+

a sign is used for both positive and negative numbers

-

a sign is used only for negative numbers (default)

space

leading space for positive numbers and minus sign for negative

Separator

,

comma. decimal integers and floats: thousands separator

_

unders­core. decimal integers and floats: thousands separator; b, o, x, and X types: separates every four digits
See Also
Python f-String Tutorial – String Formatting in Python Explained with Code Examples

Examples: Sign and Separator

# sign (+): a sign is attached to both positive and negative numbers

 
f"{3:+} or {-3:+}­"

'+3 or -3'

# sign (-): a sign is only used for negative numbers

 
f"{3:-} or {-3:-}­"

'3 or -3'

# no sign is the same as sign (-)

 
f"{3} or {-3}"

'3 or -3'

# sign (space): a leading space for positive numbers and a minus sign for negative

 
f"{3: } or {-3: }"

' 3 or -3'

# sign (+), separator (,)

 
f"{1­000­0:+­,}"

'+10,000'

# fill (?), align (<), sign (+), width (11), sep (,)

 
f"{1­000­0:?­<+1­1,}­"

'+10,0­00????'

Sign Aware Padding

Using the = alignment option forces the padding determined by the fill and width specif­ica­tions to be placed after the sign (if any) but before the digits. This is useful for number repres­ent­ations like '+000042'. This alignment option is only available for numeric types.

The 0-option produces sign aware zero-p­adding just like the = alignment option with a fill of '0'. It is produced by preceding the field width specif­ication with '0'.

# fill (?), align (=), sign (+), width (5)

 
f"{3­:?=­+5}­"

'+???3'

# fill (0), align (=), sign (+), width (5)

 
f"{3­:0=­+5}­"

'+0003'

# sign (+), 0-option, width (5)

 
f"{3­:+0­5}"

'+0003'

# 0-option, width (5)

 
f"{3­:05­}"

'00003'

Integer Presen­tation Types

Decimal Integer Types

 
d

Decimal Integer: the number in base 10

None

Same as 

d
 
n

Number. Same as 

d

, except uses current locale setting for the approp­riate number separator character

Others

 
b

Binary: the number in base 2.

 
o

Octal: the number in base 8.

 
x

, 

X

Hex: the number in base 16. 

x

vs 

X

signals lower vs upper case for digits > 9

 
c

Unicode Character. Converts decimal integer to the corres­ponding unicode character

 

#
-option: using the 

#

-option adds prefixes
­­­­

#b

: adds prefix 

0b

­­­­­­­­­

#o

: adds prefix 

0o

­­­­

#x

: adds prefix 

0x

­­­­­­­­­

#X

: adds prefix 

0X

Examples: Integer Types

i = 13

# repres­enting decimal 13 in binary, octal, and hexade­cimal

 
f"bi­nary: {i:b}; octal: {i:o}; hex: {i:x} or {i:X}"

'binary: 1101; octal: 15; hex: d or D'

# the #-option adds prefixes

 
f"bi­nary: {i:#b}; octal: {i:#o}; hex: {i:#x} or {i:#X}­"

'binary: 0b1101; octal: 0o15; hex: 0xd or 0XD'

# binary with zero padding

 
f"{i­:08­b}"

'00001101'

# binary with prefix and zero padding

 
f"{i­:#0­8b}­"

'0b001101'

# fill (0), align (=), #-option, width (8), type (b)

 
f"{i­:0=­#8b­}"

'0b001101'

# returns the unicode character with decimal repres­ent­ation 36

 
f"{3­6:c­}"

'$'

# printing symbols with unicode decimal repres­ent­ations 33-41

 
print(­''.j­oi­n([­f"{x­:c}­" for x in range(­33,­42)]))

!"#$­%&'()

# separator (,) and type (d)

 
f"{1­000­0:,­d}"

'10,000'

# type d is the default for integers, so unnece­ssary

 
f"{1­000­0:,­}"

'10,000'

# separator (_), type (b)

 
f"{1­200­:_b­}"

'100_1­011­_0000'

# to use locale specific number format­ting, set the locale

 
import locale

locale.se­tlo­cal­e(l­oca­le.L­C_ALL, '')

# then use format type 'n'

 
f"{1­0**­6:n­}"

'1,000­,000'

Float Presen­tation Types

Type

Name

For a given precision 

p
...

 
e

, 

E

Scientific Notation

The coeffi­cient has 1 digit before and 

p

digits after the decimal point.

 
f

, 

F

Fixed-­Point Notation

There are 

p

digits following the decimal point.

 
g

, 

G

General Format

Rounds number to 

p

signif­icant digits, then formats in either fixed-­point or scientific notation depending on magnitude.

 
%

Percentage

Multiplies the number by 100 and displays in fixed f format, followed by a percent sign.

Default for float is p = 6. Default for decima­l.D­ecimal is p large enough to show all coeffi­cient digits.
Lower case versus upper case: Determines whether scientific notation shows 'e' or 'E' and whether to use 'nan' versus 'NAN' and 'inf' versus 'INF'.

Examples: Simple Float Compar­isons

# %-style with precis­ion=1

 
f"{1­/2:.1%­}"

'50.0%'

# Lower case e versus upper case E

 
f"{1­000­:.1e} vs {1000:.1E­}"

'1.0e+03 vs 1.0E+03'

# Lower case f versus upper case F

 
f"{1­000­:.1f} vs {1000:.1F­}"

'1000.0 vs 1000.0'

# Lower case g versus upper case G

 
f"{1­000­:.1g} vs {1000:.1G­}"

'1e+03 vs 1E+03'

# Lower versus upper case NaN repres­ent­ation

 
nan = float(­'nan')

f"{n­an:f} vs {nan:F­}"

'nan vs NAN'

General Format (g or G) Rule

Given the number 

y

and the precision specif­ication 

p

:

(a) Let 

exp

= the exponent of 

y

in scientific notation.

(b) Is 

-4 <= exp < p

?

"­Yes­": Format 

y

using fixed-­point notation and a new precision: 

p' = p - (1 + exp)

where 

p

represents the number of signif­icant digits before any trailing zeros are removed. (In fixed-­point format, the precision, here 

p'

, determines the number of digits following the decimal point. When it is positive, the value 

1 + exp

can be interp­reted as the total number of digits before the decimal; otherwise, it represents the total number of zeros after the decimal point but before the signif­icant digits.)

"­No": Format 

y

using scientific notation and a new precision: 

p' = p - 1

where 

p

represents the total number of signif­icant digits in the coeffi­cient before any trailing zeros are removed. (In scientific format, the precision determines the number of digits displayed after the decimal point of the coeffi­cient. So, the total number of digits displayed will be: precision + 1 = 

p' + 1 = p

.)

(c) With the #-option (

#g

, 

#G

): Keep any trailing zeros and the decimal point.
Without the #-option (

g

, 

G

): If the result has insign­ificant trailing zeros, remove them as well as an unflanked decimal point.

Note: In both cases in (b), the original precision specif­ication 

p

determines the effective number of signif­icant digits up until insign­ificant trailing zeros are removed in (c).

General Format (g or G): Illust­ration

#.4g

.4g

exp

format type

(keep trailing zeros)

(no trailing zeros)

1.000e-06

1e-06

-6

e

1.000e-05

1e-05

-5

e

0.0001000

0.0001

-4

f

0.001000

0.001

-3

f

0.01000

0.01

-2

f

0.1000

0.1

-1

f

1.000

1

f

10.00

10

1

f

100.0

100

2

f

1000.

1000

3

f

1.000e+04

1e+04

4

e

In this illust­ration, the precision specif­ication is 

p = 4

; so, for 

exp

from -4 to 3, fixed-­point notation (

f

) is used; otherwise, scientific notation (

e

) is used.

Types with Condit­ional Formatting

Type

Interp­ret­ation

 
g

, 

G

= 

e

or 

f

(

E

or 

F

) according to the General Format Rule

none

= 

s

for strings

= 

d

for integers

= 

g

for floats

When fixed-­point notation is used, it always includes at least one digit past the decimal point. The precision varies to represent the given value faithf­ully.

 
n

= 

d

for integers

= 

g

for floats

Uses the current locale setting for separator format­ting.

Examples: Both Integers and Floats

si = 55

­# small integer 

li = 666666

­# large integer 

sf = 7.77777

­# small float 

lf = 8888.88

­# large float 

sc = 9e6

­# scientific notation

# none -- no type specified

f"{si} {li} {sf} {lf} {sc}"

'55 ­­666666 ­­7.77777 ­­8888.88 ­­900­0000.0'

# n -- number type 

f"{si:n} {li:n} {sf:n} {lf:n} {sc:n}­"

'55 ­­666666 ­­7.77777 ­­8888.88 ­­9e+06'

# n with #-option 

f"{s­i:#n} {li:#n} {sf:#n} {lf:#n} {sc:#n­}"

'55 ­­666666 ­­7.77777 ­­8888.88 ­­9.0­000­0e+06'

# n with precision = 3 (precision not allowed for integers) 

f"{s­i:.3n} {li:.3n} {sf:.3n} {lf:.3n} {sc:.3­n}"

# e with precision = 3 (forces 3 digits after decimal point) 

f"{s­i:.3e} {li:.3e} {sf:.3e} {lf:.3e} {sc:.3­e}"

# including the #-option gives the same result here 

f"{s­i:#.3e} {li:#.3e} {sf:#.3e} {lf:#.3e} 
{sc:#.3e}"

'5.500e+01 ­­6.6­67e+05 ­­7.7­78e+00 ­­8.8­89e+03 ­­9.0­00e+06'

# f with precision = 3 (forces 3 digits after decimal point) 

f"{s­i:.3f} {li:.3f} {sf:.3f} {lf:.3f} {sc:.3­f}"

# including the #-option gives the same result here 

f"{s­i:#.3f} {li:#.3f} {sf:#.3f} {lf:#.3f} 
{sc:#.3f}"

'55.000 ­­666­666.000 ­­7.778 ­­888­8.880 ­­900­000­0.000'

# g with precision = 3 (shows no unnecesary zeros or decimal points) 

f"{s­i:.3g} {li:.3g} {sf:.3g} {lf:.3g} {sc:.3­g}"

'55 ­­6.6­7e+05 ­7.78 ­­8.8­9e+03 ­­9e+06'

# g with #-option (forces showing 3 signif­icant digits) 

f"{s­i:#.3g} {li:#.3g} {sf:#.3g} {lf:#.3g} 
{sc:#.3g}"

'55.0 ­­6.6­7e+05 ­7.78 ­­8.8­9e+03 ­­9.0­0e+06'

# In float types generally, the #-option forces a decimal point occurrence even when no digits follow it 

f"{1­0:#.0e} {10:#.0f} {10:#.0­g}­"

'1.e+01 ­10. ­­1.e+01'

References

"A Guide to the Newer Python String Format Techni­que­s" by John Sturtz at Real Python: https:­//r­eal­pyt­hon.co­m/p­yth­on-­for­mat­ted­-ou­tpu­t/#­the­-py­tho­n-f­orm­att­ed-­str­ing­-li­ter­al-­f-s­tring

"­Python 3's f-Strings: An Improved String Formatting Syntax (Guide­)" by Joanna Jablonski at Real Python: https:­//r­eal­pyt­hon.co­m/p­yth­on-­f-s­trings/

"­Format String Syntax­" including "­Format Specif­ication Mini-L­ang­uag­e" from the page "

string

-- Common string operat­ion­s": https:­//d­ocs.py­tho­n.o­rg/­3/l­ibr­ary­/st­rin­g.h­tml­#fo­rma­t-s­tri­ng-­syntax

"­2.4.3. Formatted string litera­ls" from the page "2. Lexical analys­is": https:­//d­ocs.py­tho­n.o­rg/­3/r­efe­ren­ce/­lex­ica­l_a­nal­ysi­s.h­tml­#fo­rma­tte­d-s­tri­ng-­lit­erals

"PEP 498 -- Literal String Interp­ola­tio­n": https:­//w­ww.p­yt­hon.or­g/d­ev/­pep­s/p­ep-­0498/

"­Python String Format Cookbo­ok": https:­//m­kaz.bl­og/­cod­e/p­yth­on-­str­ing­-fo­rma­t-c­ook­book/
Python F-Strings Number Formatting Cheat Sheet (2024)
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