Contributed by Yonel Ceruto and Maxime Steinhausser in #23694 , #24185 and #24208

One of the best features of Symfony is the amount of debugging tools available so you can quickly fix any problem. We're continuously improving those tools and that's why in Symfony 3.4 we added a new command called debug:form.

When executed without arguments, the command lists all built-in types, services types, type extensions and guessers available in your application. For example, if you execute it in the Symfony Demo application:

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        $ ./bin/console debug:form

Built-in form types (Symfony\Component\Form\Extension\Core\Type)
----------------------------------------------------------------
 BirthdayType, ButtonType, CheckboxType, ChoiceType, CollectionType
 ColorType, CountryType, CurrencyType, DateIntervalType, DateTimeType
 ...
 TextareaType, TimeType, TimezoneType, UrlType

Service form types
------------------
 * App\Form\CommentType
 * App\Form\PostType
 * App\Form\Type\DateTimePickerType
 * App\Form\Type\TagsInputType
 * Symfony\Bridge\Doctrine\Form\Type\EntityType

Type extensions
---------------
 * Symfony\Component\Form\Extension\Csrf\Type\FormTypeCsrfExtension
 * Symfony\Component\Form\Extension\DataCollector\Type\DataCollectorTypeExtension
 * ...
 * Symfony\Component\Form\Extension\Validator\Type\UploadValidatorExtension

Type guessers
-------------
 * Symfony\Bridge\Doctrine\Form\DoctrineOrmTypeGuesser
 * Symfony\Component\Form\Extension\Validator\ValidatorTypeGuesser
    

If you pass a form type as the first argument, the command shows the options defined for that type, its parents and its extensions:

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        $ ./bin/console debug:form 'App\Form\PostType'

# if the type class is unambiguous, you don't need to pass the FQCN:
# ./bin/console debug:form PostType

App\Form\PostType (Block prefix: "post")
========================================

 -------------------- ------------------------- ----------------------------
  Overridden options   Parent options            Extension options
 -------------------- ------------------------- ----------------------------
  FormType             FormType                  FormTypeValidatorExtension
 -------------------- ------------------------- ----------------------------
  data_class           action                    allow_extra_fields
                       attr                      constraints
                       auto_initialize           error_mapping
                       block_name                extra_fields_message
                       by_reference              invalid_message
                       compound                  invalid_message_parameters
                       data                      validation_groups
                       disabled
                       empty_data                FormTypeCsrfExtension
                       error_bubbling           ----------------------------
                       inherit_data              csrf_field_name
                       label                     csrf_message
                       label_attr                csrf_protection
                       label_format              csrf_token_id
                       mapped                    csrf_token_manager
                       method
                       post_max_size_message
                       property_path
                       required
                       translation_domain
                       trim
                       upload_max_size_message
 -------------------- ------------------------- ----------------------------

Parent types
------------

 * Symfony\Component\Form\Extension\Core\Type\FormType

Type extensions
---------------

 * Symfony\Component\Form\Extension\HttpFoundation\Type\FormTypeHttpFoundationExtension
 * Symfony\Component\Form\Extension\Validator\Type\FormTypeValidatorExtension
 * Symfony\Component\Form\Extension\Validator\Type\UploadValidatorExtension
 * Symfony\Component\Form\Extension\Csrf\Type\FormTypeCsrfExtension
 * Symfony\Component\Form\Extension\DataCollector\Type\DataCollectorTypeExtension
    

If you pass both a form type and an option name, the command displays the full definition of the given option:

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        $ ./bin/console debug:form 'App\Form\PostType' label_attr

App\Form\PostType (label_attr)
==============================

 ---------------- -----------
  Required         false
 ---------------- -----------
  Default          []
 ---------------- -----------
  Allowed types    ["array"]
 ---------------- -----------
  Allowed values   -
 ---------------- -----------
  Normalizer       -
 ---------------- -----------
    

Lastly, all this debug information is also available in JSON format just by adding the --format=json option when executing the command.