Computed and constant hidden fields

Computed and constant hidden fields#

In this tutorial you will add fields that are never shown to the user but appear in the final answers dict. TUI Forms resolves them automatically after all user-facing questions have been answered.

Two kinds of hidden field exist:

Kind

Schema marker

Behaviour

Constant

format: constant

Returns the default value unchanged. No rendering.

Computed

format: computed

Evaluates the default as a Jinja2 template against the collected answers.

What you will build#

A package scaffolding form that:

  1. Asks for a project name (free text).

  2. Derives a package name automatically by lower-casing the project name and replacing spaces and hyphens with underscores (computed).

  3. Embeds a fixed schema version string that is always "1" (constant).

Prerequisites#

Step 1—Add a constant field#

A constant field stores a fixed value that never changes regardless of what the user types. Set format to "constant" and provide a default:

schema = {
    "title": "Package scaffold",
    "properties": {
        "schema_version": {
            "type": "string",
            "format": "constant",
            "title": "Schema version",
            "default": "1",
        },
    },
}

schema_version will not be asked. The returned dict will always contain {"schema_version": "1"}.

Step 2—Add a user-facing question#

Add the question whose answer the computed field will use:

schema = {
    "title": "Package scaffold",
    "properties": {
        "project_name": {
            "type": "string",
            "title": "Project name",
            "default": "My Library",
        },
        "schema_version": {
            "type": "string",
            "format": "constant",
            "title": "Schema version",
            "default": "1",
        },
    },
}

Step 3—Add a computed field#

A computed field evaluates its default as a Jinja2 template. The template context is the answers collected so far, so you can reference any key that was asked before the hidden fields are resolved.

Add package_name after project_name:

schema = {
    "title": "Package scaffold",
    "properties": {
        "project_name": {
            "type": "string",
            "title": "Project name",
            "default": "My Library",
        },
        "package_name": {
            "type": "string",
            "format": "computed",
            "title": "Package name",
            "default": "{{ project_name | lower | replace('-', '_') | replace(' ', '_') }}",
        },
        "schema_version": {
            "type": "string",
            "format": "constant",
            "title": "Schema version",
            "default": "1",
        },
    },
}

The Jinja2 template {{ project_name | lower | replace('-', '_') | replace(' ', '_') }} takes the project_name answer and transforms it into a valid Python identifier.

Step 4—Run the form#

from tui_forms import create_renderer

renderer = create_renderer("stdlib", schema)
answers = renderer.render()
print(answers)

Run the script. Only project_name is asked. After the user answers, TUI Forms resolves the hidden fields and returns:

{'project_name': 'My Library', 'package_name': 'my_library', 'schema_version': '1'}

Try entering a different name, for example "Data Pipeline":

{'project_name': 'Data Pipeline', 'package_name': 'data_pipeline', 'schema_version': '1'}

Constant list values#

A constant field can also hold a list by using type: array:

"supported_locales": {
    "type": "array",
    "format": "constant",
    "title": "Supported locales",
    "default": ["en", "pt-br"],
},

The list is returned as-is without any rendering.

Computed list and dict values#

A computed field supports list and dict defaults in addition to strings. Each element (or value) is rendered as a Jinja2 template independently:

"author_info": {
    "type": "object",
    "format": "computed",
    "title": "Author info",
    "default": {
        "name":  "{{ author_name }}",
        "email": "{{ author_email | lower }}",
    },
},

Order matters#

Hidden fields are resolved after all user-facing questions have been answered, so a computed field can reference any answer—regardless of where it appears in properties.

Next steps#

  • Conditional questions: combine hidden fields with conditions so that a computed field is only resolved when a specific branch is active.

  • JSONSchema support: full reference for format: constant, format: computed, and Jinja2 defaults.

Contents