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
Alternatives	{empty|title}	Hello World
Element Access	{user[name]}	John Doe
Attribute Access	{extractor.url}	https://example.org/
Environment Variable	{_env[FOO]}	BAR

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
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
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:?[/]/}
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
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

All special format specifiers (?, L, J, R, D) can be chained and combined with one another, but must always come 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

Special Type Format Strings

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

Type	Description	Usage
T	A template file containing the actual format string	\fT ~/.templates/booru.txt
F	An f-string literal	\fF '{title.strip()}' by {artist.capitalize()}
E	An arbitrary Python expression	\fE title.upper().replace(' ', '-')
M	Name of a Python module followed by 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

Note:

\f is the Form Feed character. (ASCII code 12 or 0xc)

Writing it as \f is native to JSON, but will not get interpreted as such by most shells. To use this character there:

• hold Ctrl, then press v followed by l, resulting in ^L or
• use echo or printf (e.g. gallery-dl -f "$(echo -ne \\fM) my_module:generate_text")