pytutorial/python_basics/formatted_string_literals

Python : Formatted String Literals

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The goal

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

Questions to David Rotermund

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}")

                          3.14
3.14                          
                          3.14
             3.14             

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 

Reference

• Formatted String Literals
• Format Specification Mini-Language