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Library Module


You can extract a whole set of input files (i.e. templates, overlays, data values, etc.) into a “Library”.

For example:

├── _ytt_lib/
│   └── frontend/
│       ├── schema.yml
│       └── store.yml
└── config.yml


  • config/_ytt_lib/frontend/ and its contents is a library named "frontend"

Libraries are not automatically included in ytt output; one must programmatically load, configure, evaluate, and insert those results into a template that is part of the output.

#! config/config.yml -- example of using a library

#@ load("@ytt:library", "library")
#@ load("@ytt:template", "template")

#! 1. Load an instance of the library
#@ app = library.get("frontend")

#! 2. Create a configured copy of the library (does not mutate original)
#@ app_with_vals = app.with_data_values({"apiDomain": ""})

#! 3. Evaluate the library and include results (a document set) in the output
--- #@ template.replace(app_with_vals.eval())

For a complete working example, see ytt-library-module example.

What is a Library?

A ytt library is a directory tree contained within a specially-named directory: _ytt_lib/.

  • The library’s name is the path relative from the _ytt_lib/ directory.
  • The library’s contents are those of the directory along with subdirectories, recursively.
  • A library may contain libraries as well, if one of its subdirectories is _ytt_lib/.

The root directory of a ytt invocation is itself a library known as the “root library”.

Libraries are evaluated in isolation: each a separate execution of the pipeline described in How it works.

  • Each library has its own data values schema.
  • Overlays within a library only apply over its evaluated document set.
  • The final evaluated result is returned as a YAML Fragment wrapping a document set..


There’s but one function in the @ytt:library module: library.get()


Contructs a new @ytt:library.instance based on the contents from the named library.

instance = library.get(name, [<kwargs>])
  • name (string) — path to the base directory of the desired library: ./_ytt_lib/<name>. Can contain slashes / for sub-directories (e.g.
  • keyword arguments (optional):
    • alias= (string) — unique name for this library instance. See Aliases, below.
    • ignore_unknown_comments= (bool) — equivalent to ytt --ignore-unknown-comments; see File Marks > type detection for YAML files for more details (default: False). (as of v0.31.0)
    • implicit_map_key_overrides= (bool) — equivalent to ytt --implicit-map-key-overrides; see @yaml/map-key-override for more details. (default: False). (as of v0.31.0)
    • strict= (bool) — equivalent to ytt --strict (default: False). (as of v0.31.0)
  • instance (@ytt:library.instance) — a new library instance backed by the contents of the named library.

The file containing this method invocation must be a sibling of the _ytt_lib directory.

Library Instances

Each library returned from a function within this module is a copy: a separate instance.

A library instance (a value of type @ytt:library.instance) is created from source with library.get().

With a library instance:


Calculates and returns just the Data Values configured on this library instance.

dvs = instance.data_values()


Calculates the library’s final data values (i.e. the net result of all configured data values), evaluates its templates into a document set, and applies its overlays on that document set (i.e. executes the pipeline described in How it works for this library instance’s inputs and contents. The output of that execution — rather than rendered automatically — is returned in a variable).

document_set = instance.eval()
  • document_set (yamlfragment) — the YAML document set resulting from the evaluation of this instance.

Note: the resulting Document Set is not automatically included in output. A common way to include this result in the output is to use template.replace():

#@ load("@ytt:template", "template")
#@ load("@ytt:library", "library")

--- #@ template.replace(library.get("cert-manager").eval())

See also, Playground: ytt library module example.


(As of v0.28.0)

Returns the value of an identifier declared within the library instance.

value = instance.export(name, [path=])
  • name (string) — the name of a function or a variable declared within some module/file in the library. (i.e. a file with the extension .lib.yml or .star).
  • path= (string) — the path to the module/file that contains the declaration. Only required when name is not unique within the library.
  • value (any) — a copy of the specified value.
    • if value is a function, it is executed within the context of its library instance. For example, if the function depends on values from the @ytt:data module, the values provided are those of this library instance.


Example 1: Exporting a function from a library.

Assuming some module/file in the “helpers” library contains the definition:

def wrap_name(name):

Can be exported and used from another library:

helpers = library.get("helpers")
wrap_name = helpers.export("wrap_name")

full_name = wrap_name("app")

Example 2: Disambiguating between multiple declarations of function.

Assuming two modules/files in the “helpers” library have the same name:

# main/
def wrap_name(name): ...


# lib/
def wrap_name(name): ...

One of which can be unambiguously referenced:

helpers = library.get("helpers")
wrap_name = helpers.export("wrap_name", path="lib/")

full_name = wrap_name("app")

Note: without the path= keyword argument, helpers.export() would report an error.


Returns a copy of the library instance with data values overlayed with those given.

new_instance = instance.with_data_values(dvs, [plain=])
  • dvs (struct | yamlfragment) — data values with which to overlay (or set, if none exist).
    • only yamlfragments wrapping a map or an array are supported (i.e. yamlfragments wrapping document sets are not supported).
    • yamlfragment values can contain overlay annotations for fine-grained overlay control.
  • plain= (bool) — when True indicates that dvs should be “plain merged” over existing data values (i.e. the exact same behavior as --data-values-file).
    • dvs must be plain YAML (i.e. a struct or a yamlfragment with no annotations).
  • new_instance (@ytt:library.instance) — a copy of instance with dvs overlayed on its data values; instance remains unchanged.


(As of v0.35.0)

Returns a copy of the library instance with data values schema overlayed with that given.

new_instance = instance.with_data_values_schema(schema)
  • schema (struct | yamlfragment) — schema for data values with which to overlay on existing schema (or set if none exist).
    • only yamlfragments wrapping a map or an array are supported (i.e. yamlfragments wrapping document sets are not supported)
    • yamlfragment values can contain overlay annotations for fine-grained overlay control.
  • new_instance (@ytt:library.instance) — a copy of instance with a schema updated with schema; instance remains unchanged.


Example 1: Declaring a new data value (and setting it).

#@ def app_schema():
name: ""
#@overlay/match missing_ok=True
  custom_key: ""
#@ end

#@ app1_with_schema = app1.with_data_values_schema(app_schema())
#@ def app_vals():
name: app1
  custom_key: some_val
#@ end

#@ app1_with_vals = app1.with_data_values(app_vals())



(As of v0.28.0)

Attaches a YAML document to the specified library. When the library is evaluated, the annotated document is included. Only supported on documents annotated with @data/values and @data/values-schema.

@library/ref library_name
  • library_name (string) — @-prefixed path to the base directory of the desired library: ./_ytt_lib/<name>. Can contain slashes / for sub-directories (e.g. Can also be an alias for specific library instance(s).


Example 1: Change schema default for a data value in a library.

#@library/ref "@frontend"
name: "custom"

Overlays the default value for name in the “frontend” library to be “custom”.

Example 2: Target a data value overlay to a library.

#@library/ref "@backend"

Sets the “backend” library’s domains data value to be exactly the values given.

See also: Data Values > Setting Library Values via Files.

Note: data values may also be attached to libraries via command line flags.


To facilitate configuring specific library instances, one can mark them with an alias.

An alias:

  • is defined in a library.get() call, using the optional alias= keyword argument.
  • is added to a library reference by prefixing it with a tilde, ~:
    • @~<alias-name> refers to any library instance with the alias.
    • @<library-name>~<alias-name> refers to any instance of the named library that also has the alias.

For example, given a library known as “fruit”:

├── apple-values.yml
├── config.yml
├── orange-values.yml
└── _ytt_lib
    └── fruit
        ├── doc.yml
        └── values.yml


#! _ytt_lib/fruit/doc.yml

#@ load("@ytt:data", "data")
--- #@ data.values

the template in the library simply returns its data values as a document, and …

#! _ytt_lib/fruit/values.yml

variety: ordinary
poisoned: false

… those are the data values in the library.

The root library can assign aliases to library instances:

#! ./config.yml

#@ load("@ytt:library", "library")

#@ apple1 = library.get("fruit", alias="apple")
#@ apple2 = apple1.with_data_values({"variety": "jonamac"})

#@ orange = library.get("fruit", alias="orange")

  1: #@ apple1.eval()[0]
  2: #@ apple2.eval()[0]
orange: #@ orange.eval()[0]


  • apple1 has the alias “apple”
  • apple2 also has the alias “apple” (part of being a copy of apple1)
  • orange has the alias “orange”

These aliases can be used to target changes to specific library instance(s).

For example, our root library has these two data values overlays:

#! ./apple-values.yml

#@library/ref "@~apple"
variety: red delicious
poisoned: true

… which will affect all library instances with the alias “apple”, and …

#! ./orange-values.yml

#@library/ref "@~orange"
variety: valencia

… overlays on top of library instance with the alias “orange”.

When the whole fileset is evaluated, the result is:

    variety: red delicious
    poisoned: true
    variety: jonamac
    poisoned: true
  variety: valencia
  poisoned: false


  • only the “@~orange” instance has the variety = “valencia”
  • both “@~apple” library instances are poisoned; while the “orange” instance is not.

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