Fixing the 'Expected <block end>, but found <block mapping start>' Error: Comprehensive Guide and Solutions

Errors in YAML files can be quite frustrating, especially when the error message isn't helpful. One such error is the 'Expected , but found ' error. This comprehensive guide aims to help you understand and fix this error by providing step-by-step solutions and addressing common questions.


The 'Expected , but found ' error usually occurs when there is an issue with indentation or misplaced characters in your YAML file. YAML relies heavily on proper indentation and formatting to define its data structure.

Follow the steps below to identify and fix the error in your YAML file.

Step 1: Review the Error Message

The error message should provide you with the line number where the error occurred. Open your YAML file in a text editor and navigate to the specified line number. Carefully review the line for any issues related to indentation or misplaced characters.

Step 2: Check for Indentation Issues

YAML relies on proper indentation to define its data structure. Make sure that you're using consistent indentation throughout your file. It's common to use two spaces for each level of indentation, but you can use a different number of spaces if you prefer. Just make sure to be consistent.

# Example with consistent indentation:
    key1: value1
    key2: value2
    key1: value1
    key2: value2

Step 3: Check for Misplaced Characters

Characters like colons, dashes, and quotes can cause problems if they're not used correctly in your YAML file. Make sure that you're using these characters correctly and that they're properly placed within your file.

  • Colons are used to separate keys and values, and there should be a space after the colon.
  • Dashes are used to denote list items, and there should be a space after the dash.
  • Quotes should be used consistently for strings, and make sure to close any open quotes.
# Example with correct character placement:
  - child1:
      key1: "value1"
      key2: 'value2'
  - child2:
      key1: "value1"
      key2: 'value2'

Step 4: Validate Your YAML File

After fixing any issues found in steps 2 and 3, you should validate your YAML file to ensure there are no remaining errors. There are several online tools available to validate your YAML file, such as YAML Validator or YAML Lint. Copy and paste your YAML content into one of these validators to check for any remaining errors.


1. What is YAML, and why is it used?

YAML (short for "YAML Ain't Markup Language") is a human-readable data serialization format. It is commonly used for configuration files, data exchange between languages with different data structures, and as a language for writing playbooks in Ansible. The main advantage of YAML is its simplicity and readability, which makes it easy to understand and edit.

2. What is the difference between YAML and JSON?

YAML and JSON are both data serialization formats, but they have some differences in syntax and readability. YAML uses indentation to define its data structure, while JSON uses curly braces and square brackets. YAML is considered more human-readable due to its use of indentation and lack of punctuation, whereas JSON is more compact and often used when data size is a concern.

3. How do I properly format YAML lists and dictionaries?

In YAML, lists are denoted by a dash followed by a space, and dictionaries are denoted by a colon followed by a space. Here's an example of a YAML file containing both a list and a dictionary:

  - item1
  - item2
  - item3

  key1: value1
  key2: value2
  key3: value3

4. Can I use tabs instead of spaces for indentation in YAML?

Using tabs for indentation in YAML is not recommended, as it can lead to parsing errors. It's best to use spaces for indentation, as this ensures consistency and compatibility with YAML parsers.

5. How do I add comments to my YAML file?

You can add comments to your YAML file using the # symbol. Any text following the # symbol on a line will be treated as a comment and will not be parsed as part of the YAML data structure.

# This is a comment in a YAML file
  key: value # This is an inline comment

By following this comprehensive guide, you should be able to identify and fix the 'Expected , but found ' error in your YAML file. Remember to always validate your YAML file after making changes to ensure that there are no remaining errors.

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