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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"]
L Return the length of a value {foo!L} 7
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
X<maxlen>/<ext>/ Limit output to <maxlen> characters. Cut output and add <ext> to its end if its length exceeds <maxlen> {foo:X15/ .../} Foo Bar
{foo:L6/ .../} Fo ...
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
A<op><value>/ Apply arithmetic operation <op> (+, -, *) to the current value {num:A+1/} "2"
C<conversion(s)>/ Apply Conversions to the current value {tags:CSgc/} "Sun-tree-water"
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
_nul Universal null value {date|_nul:%Y-%m} None
_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