Python : Formatted String Literals

The goal

Using formated string literals is an easy way to produce formated strings from data / variables.

Basic Formatted String Literals

A formatted string literals starts with an f and the variables that you want to print are placed at their intended position embeded into { }. For example:

import numpy as np

a: str = "Hello"
b: int = 1
c: float = np.pi

mystring: str = f"A: {a} B: {b} C: {c}"
print(mystring)

A: Hello B: 1 C: 3.141592653589793

In the case the string is to long (i.e. longer than the 80 character limit), you can use this construct to break the line into several lines:

import numpy as np

a: str = "Hello"
b: int = 1
c: float = np.pi


mystring: str = (
    f"Variable A: {a} " 
    "Blablablablabla " 
    f"Variable B: {b} " 
    f"Variable C: {c} "
)

print(mystring)

Variable A: Hello Blablablablabla Variable B: 1 Variable C: 3.141592653589793

Formats

format_spec     ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill            ::=  <any character>
align           ::=  "<" | ">" | "=" | "^"
sign            ::=  "+" | "-" | " "
width           ::=  digit+
grouping_option ::=  "_" | ","
precision       ::=  digit+
type            ::=  "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

Integer formats


‘b’	Binary format.
‘c’	Character.
‘d’	Decimal Integer.
‘o’	Octal format.
‘x’	Hex format.
‘X’	Hex format.
‘n’	Number.
None	‘d’


’+’	indicates that a sign should be used for both positive as well as negative numbers.
’-‘	indicates that a sign should be used only for negative numbers (this is the default behavior).
space	indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.


’<’	Forces the field to be left-aligned within the available space (this is the default for most objects).
’>’	Forces the field to be right-aligned within the available space (this is the default for numbers).
’=’	Forces the padding to be placed after the sign (if any) but before the digits.
’^’	Forces the field to be centered within the available space.

Float formats


‘e’	Scientific notation.
‘E’	Scientific notation.
‘f’	Fixed-point notation.
‘F’	Fixed-point notation.
‘g’	General format.
‘G’	General format.
‘n’	Number.
’%’	Percentage.
None	‘g’


’+’	indicates that a sign should be used for both positive as well as negative numbers.
’-‘	indicates that a sign should be used only for negative numbers (this is the default behavior).
space	indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.

import numpy as np

value: float = np.pi

print(f"{value}")
print(f"{value:.3f}")
print(f"{value:+.3f}")
print(f"{-value:+.3f}")

3.141592653589793
3.142
+3.142
-3.142

import numpy as np

value: float = np.pi

print(f"{value:>30.2f}")
print(f"{value:<30.2f}")
print(f"{value:=30.2f}")
print(f"{value:^30.2f}")

String formats


’s’	String format.
None	’s’

Dates

# %%
from datetime import datetime

d = datetime.now()
print(f"{d:%Y-%m-%d %H:%M:%S}")

2022-04-03 01:18:45

Directive	Meaning	Example
%a	Weekday as locale’s abbreviated name.	Sun, Mon, …, Sat (en_US); So, Mo, …, Sa (de_DE)
%A	Weekday as locale’s full name.	Sunday, Monday, …, Saturday (en_US);Sonntag, Montag, …, Samstag (de_DE)
%w	Weekday as a decimal number, where 0 is Sunday and 6 is Saturday.	0, 1, …, 6
%d	Day of the month as a zero-padded decimal number.	01, 02, …, 31
%b	Month as locale’s abbreviated name.	Jan, Feb, …, Dec (en_US); Jan, Feb, …, Dez (de_DE)
%B	Month as locale’s full name.	January, February, …, December (en_US); Januar, Februar, …, Dezember (de_DE)
%m	Month as a zero-padded decimal number.	01, 02, …, 12
%y	Year without century as a zero-padded decimal number.	00, 01, …, 99
%Y	Year with century as a decimal number.	0001, 0002, …, 2013, 2014, …, 9998, 9999
%H	Hour (24-hour clock) as a zero-padded decimal number.	00, 01, …, 23
%I	Hour (12-hour clock) as a zero-padded decimal number.	01, 02, …, 12
%p	Locale’s equivalent of either AM or PM.	AM, PM (en_US); am, pm (de_DE)
%M	Minute as a zero-padded decimal number.	00, 01, …, 59
%S	Second as a zero-padded decimal number.	00, 01, …, 59
%f	Microsecond as a decimal number, zero-padded to 6 digits.	000000, 000001, …, 999999
%z	UTC offset in the form ±HHMM[SS[.ffffff]] (empty string if the object is naive).	(empty), +0000, -0400, +1030, +063415, -030712.345216
%Z	Time zone name (empty string if the object is naive).	(empty), UTC, GMT
%j	Day of the year as a zero-padded decimal number.	001, 002, …, 366
%U	Week number of the year (Sunday as the first day of the week) as a zero-padded decimal number. All days in a new year preceding the first Sunday are considered to be in week 0.	00, 01, …, 53
%W	Week number of the year (Monday as the first day of the week) as a zero-padded decimal number. All days in a new year preceding the first Monday are considered to be in week 0.	00, 01, …, 53
%c	Locale’s appropriate date and time representation.	Tue Aug 16 21:30:00 1988 (en_US); Di 16 Aug 21:30:00 1988 (de_DE)
%x	Locale’s appropriate date representation.	08/16/88 (None); 08/16/1988 (en_US); 16.08.1988 (de_DE)
%X	Locale’s appropriate time representation.	21:30:00 (en_US); 21:30:00 (de_DE)
%%	A literal ‘%’ character.	%

see here for the date formats

Escape Sequence

Escape Sequence	Meaning
<newline>	Backslash and newline ignored
\ \	Backslash (\)
\’	Single quote (‘)
\”	Double quote (“)
\a	ASCII Bell (BEL)
\b	ASCII Backspace (BS)
\f	ASCII Formfeed (FF)
\n	ASCII Linefeed (LF)
\r	ASCII Carriage Return (CR)
\t	ASCII Horizontal Tab (TAB)
\v	ASCII Vertical Tab (VT)
\ooo	Character with octal value ooo
\xhh	Character with hex value hh

Escape Sequence	Meaning
\N{name}	Character named name in the Unicode database>
\uxxxx	Character with 16-bit hex value xxxx
\Uxxxxxxxx	Character with 32-bit hex value xxxxxxxx

Reference

The source code is Open Source and can be found on GitHub.