gallery-dl/docs/formatting.md

String Formatting

Format strings in gallery-dl follow the general rules of str.format() (PEP 3101) plus several extras.

The syntax for replacement fields is {<field-name>!<conversion>:<format-specifiers>}, where !<conversion> and :<format-specifiers> are both optional and can be used to specify how the value selected by <field-name> should be transformed.

Field Names

Field names select the metadata value to use in a replacement field.

While simple names are usually enough, more complex forms like accessing values by attribute, element index, or slicing are also supported.

	Example	Result
Name	{title}	Hello World
Element Index	{title[6]}	W
Slicing	{title[3:8]}	lo Wo
Slicing (Bytes)	{title_ja[b3:18]}	ロー・ワー
Alternatives	{empty|title}	Hello World
Attribute Access	{extractor.url}	https://example.org/
Element Access	{user[name]}	John Doe
	{user['name']}	John Doe

All of these methods can be combined as needed. For example {title[24]|empty|extractor.url[15:-1]} would result in .org.

Conversions

Conversion specifiers allow to convert the value to a different form or type. Such a specifier must only consist of 1 character. gallery-dl supports the default three (s, r, a) as well as several others:

Conversion	Description	Example	Result
l	Convert a string to lowercase	{foo!l}	foo bar
u	Convert a string to uppercase	{foo!u}	FOO BAR
c	Capitalize a string, i.e. convert the first character to uppercase and all others to lowercase	{foo!c}	Foo bar
C	Capitalize each word in a string	{foo!C}	Foo Bar
g	Slugify a value	{foo!g}	foo-bar
j	Serialize value to a JSON formatted string	{tags!j}	["sun", "tree", "water"]
t	Trim a string, i.e. remove leading and trailing whitespace characters	{bar!t}	FooBar
T	Convert a datetime object to a unix timestamp	{date!T}	1262304000
d	Convert a unix timestamp to a datetime object	{created!d}	2010-01-01 00:00:00
U	Convert HTML entities	{html!U}	<p>foo & bar</p>
H	Convert HTML entities & remove HTML tags	{html!H}	foo & bar
s	Convert value to str	{tags!s}	['sun', 'tree', 'water']
S	Convert value to str while providing a human-readable representation for lists	{tags!S}	sun, tree, water
r	Convert value to str using repr()
a	Convert value to str using ascii()

Format Specifiers

Format specifiers can be used for advanced formatting by using the options provided by Python (see Format Specification Mini-Language) like zero-filling a number ({num:>03}) or formatting a datetime object ({date:%Y%m%d}), or with gallery-dl's extra formatting specifiers:

Format Specifier	Description	Example	Result
?<start>/<end>/	Adds <start> and <end> to the actual value if it evaluates to True. Otherwise the whole replacement field becomes an empty string.	{foo:?[/]/}	[Foo Bar]
		{empty:?[/]/}
[<start>:<stop>]	Applies a Slicing operation to the current value, similar to Field Names	{foo:[1:-1]}	oo Ba
[b<start>:<stop>]	Same as above, but applies to the bytes() representation of a string in filesystem encoding	{foo_ja:[b3:-1]}	ー・バ
L<maxlen>/<repl>/	Replaces the entire output with <repl> if its length exceeds <maxlen>	{foo:L15/long/}	Foo Bar
		{foo:L3/long/}	long
J<separator>/	Concatenates elements of a list with <separator> using str.join()	{tags:J - /}	sun - tree - water
R<old>/<new>/	Replaces all occurrences of <old> with <new> using str.replace()	{foo:Ro/()/}	F()() Bar
S<order>/	Sort a list. <order> can be either ascending or descending/reverse. (default: a)	{tags:Sd}	['water', 'tree', 'sun']
D<format>/	Parse a string value to a datetime object according to <format>	{updated:D%b %d %Y %I:%M %p/}	2010-01-01 00:00:00
O<offset>/	Apply <offset> to a datetime object, either as ±HH:MM or local for local UTC offset	{date:O-06:30/}	2009-12-31 17:30:00

All special format specifiers (?, L, J, R, D, O, etc) can be chained and combined with one another, but must always appear before any standard format specifiers:

For example {foo:?//RF/B/Ro/e/> 10} ->    Bee Bar

• ?// - Tests if foo has a value
• RF/B/ - Replaces F with B
• Ro/e/ - Replaces o with e
• > 10 - Left-fills the string with spaces until it is 10 characters long

Global Replacement Fields

Replacement field names that are available in all format strings.

Field Name	Description	Example	Result
_env	Environment variables	{_env[HOME]}	/home/john
_now	Current local date and time	{_now:%Y-%m}	2022-08
_lit	String literals	{_lit[foo]}	foo
		{'bar'}	bar

Special Type Format Strings

Starting a format string with \f<Type> allows to set a different format string type than the default. Available ones are:

Type	Description	Usage
F	An f-string literal	\fF '{title.strip()}' by {artist.capitalize()}
E	An arbitrary Python expression	\fE title.upper().replace(' ', '-')
T	Path to a template file containing a regular format string	\fT ~/.templates/booru.txt
TF	Path to a template file containing an f-string literal	\fTF ~/.templates/fstr.txt
M	Path or name of a Python module followed by the name of one of its functions. This function gets called with the current metadata dict as argument and should return a string.	\fM my_module:generate_text