View source for Formatted String Literals


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

Questions to [mailto:davrot@uni-bremen.de 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:<syntaxhighlight lang="python">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)</syntaxhighlight><syntaxhighlight lang="python">A: Hello B: 1 C: 3.141592653589793</syntaxhighlight>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:<syntaxhighlight lang="python">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)</syntaxhighlight><syntaxhighlight lang="python">Variable A: Hello Blablablablabla Variable B: 1 Variable C: 3.141592653589793</syntaxhighlight>

== [https://docs.python.org/3/library/string.html#formatspec Formats] ==
<syntaxhighlight lang="python">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" | "%"</syntaxhighlight>

=== Integer formats ===
{| class="wikitable"
|‘b’
|Binary format.
|-
|‘c’
|Character.
|-
|‘d’
|Decimal Integer.
|-
|‘o’
|Octal format.
|-
|‘x’
|Hex format.
|-
|‘X’
|Hex format.
|-
|‘n’
|Number.
|-
|'''None'''
|'''‘d’'''
|}
{| class="wikitable"
|‘+’
|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.
|}
{| class="wikitable"
|‘&#x3C;’
|Forces the field to be left-aligned within the available space (this is the default for most objects).
|-
|‘&#x3E;’
|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 ===
{| class="wikitable"
|‘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’
|}
{| class="wikitable"
|‘+’
|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.
|}
<syntaxhighlight lang="python">import numpy as np

value: float = np.pi

print(f"{value}")
print(f"{value:.3f}")
print(f"{value:+.3f}")
print(f"{-value:+.3f}")</syntaxhighlight><syntaxhighlight lang="python">3.141592653589793
3.142
+3.142
-3.142</syntaxhighlight><syntaxhighlight lang="python">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}")</syntaxhighlight><syntaxhighlight lang="python">                          3.14
3.14                          
                          3.14
             3.14             </syntaxhighlight>

=== String formats ===
{| class="wikitable"
|‘s’
|String format.
|-
|'''None'''
|'''‘s’'''
|}

=== Dates ===
<syntaxhighlight lang="python"># %%
from datetime import datetime

d = datetime.now()
print(f"{d:%Y-%m-%d %H:%M:%S}")</syntaxhighlight><syntaxhighlight lang="python">2022-04-03 01:18:45</syntaxhighlight>
{| class="wikitable"
!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 [https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes here] for the date formats 

== [https://docs.python.org/3/reference/lexical_analysis.html#escape-sequences Escape Sequence] ==
{| class="wikitable"
!Escape Sequence
!Meaning
|-
|&#x3C;newline&#x3E;
|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
|}
{| class="wikitable"
!Escape Sequence
!Meaning
|-
|\N{name}
|Character named name in the Unicode database&#x3E;
|-
|\uxxxx
|Character with 16-bit hex value xxxx
|-
|\Uxxxxxxxx
|Character with 32-bit hex value xxxxxxxx
|}

== Reference ==

* [https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals Formatted String Literals]
* [https://docs.python.org/3/library/string.html#formatspec Format Specification Mini-Language]