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Common YAML Errors

When working with YAML files in Python, you might run into a few common issues. These errors usually mean that your YAML file doesn't follow the proper structure or syntax rules.

Understanding these errors helps you quickly pinpoint and fix problems in your configuration or data files.

1. Syntax Errors​

This is one of the most frequent errors and it means PyYAML got confused while trying to read the basic building blocks (like individual characters or words) of your YAML. Think of it like a spell checker finding a typo or a grammar error.

What it looks like:

YAMLError: while scanning for the next token
found character ' ' that cannot start any token.
  in "config.yaml", line X, column Y

Common Causes:

  • Incorrect Indentation: YAML relies heavily on spaces for defining structure. Mixing tabs and spaces, or inconsistent indentation (e.g., some lines indented with 2 spaces, others with 4) will cause this error.
  • Missing Colons (:): In YAML, key-value pairs are separated by a colon (e.g., key: value). Forgetting a colon or placing it incorrectly is a common mistake.
  • Invalid Characters: Using characters that don't belong in a specific part of the YAML structure.
  • Dashes (-) out of place: Dashes typically signify list items. If a dash appears where a key-value pair is expected, it can lead to a YAMLError.

How to fix it:

  • Check Indentation: The most common culprit! Ensure you are only using spaces for indentation and that indentation levels are consistent. Many text editors have features to show whitespace characters.
  • Verify Syntax: Carefully review the lines mentioned in the error message (line X, column Y) for any missing colons, misplaced characters, or incorrect use of list dashes.
  • Use a YAML Linter/Validator: Online tools or command-line linters (like yamllint) can quickly identify syntax errors before you even run your Python script.

2. Structure Errors​

This error occurs when PyYAML understands the basic elements but struggles to interpret the overall structure of your YAML document. It's like your grammar checker understanding individual words but not how they form a coherent sentence or paragraph.

What it looks like:

YAMLError: while parsing a block mapping
expected <block end>, but found '<block sequence start>'
  in "config.yaml", line X, column Y

Common Causes:

  • Improper Nesting: Your YAML might have a mix-up in how blocks (mappings or sequences) are nested. For example, a list item (-) might be indented under a key where a value is expected, or vice-versa.
  • Inconsistent Block Styles: While YAML supports different ways to write lists and mappings (block vs. flow style), mixing them incorrectly, especially with indentation, can lead to parsing issues.
  • Unexpected End of File: The YAML might abruptly end while a block is still expected to be open.

How to fix it:

  • Review Block Structure: Pay close attention to how your dictionaries (key-value pairs) and lists (items starting with -) are structured and indented.
  • Ensure Proper Termination: Make sure all blocks are properly closed or have the expected subsequent elements.
  • Check for Missing Parent-Child Relationships: Sometimes, a line might be indented as if it's a child of a parent, but the parent itself is missing or malformed.