If you run into problems while trying to parse a HTML/XML document, it is more likely because how the parser in use is interpreting the document. To help you locate and correct the problem, Beautiful Soup API provides a dignose() utility.

The diagnose() method in Beautiful Soup is a diagnostic suite for isolating common problems. If you”re facing difficulty in understanding what Beautiful Soup is doing to a document, pass the document as argument to the diagnose() function. A report showing you how different parsers handle the document, and tell you if you”re missing a parser.

The diagnose() method is defined in bs4.diagnose module. Its output starts with a message as follows −

Example

diagnose(markup)

Output

Diagnostic running on Beautiful Soup 4.12.2
Python version 3.11.2 (tags/v3.11.2:878ead1, Feb  7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)]
Found lxml version 4.9.2.0
Found html5lib version 1.1
Trying to parse your markup with html.parser
Here''s what html.parser did with the markup:

If it doesn”t find any of these parsers, a corresponding message also appears.

I noticed that html5lib is not installed. Installing it may help.

If the HTML document fed to diagnose() method is perfectly formed, the parsed tree by any of the parsers will be identical. However if it is not properly formed, then different parser interprets differently. If you don”t get the tree as you anticipate, changing the parser might help.

Sometimes, you may have chosen HTML parser for a XML document. The HTML parsers add all the HTML tags while parsing the document incorrectly. Looking at the output, you will realize the error and can help in correcting.

If Beautiful Soup raises HTMLParser.HTMLParseError, try and change the parser.

parse errors are HTMLParser.HTMLParseError: malformed start tag and HTMLParser.HTMLParseError: bad end tag are both generated by Python”s built-in HTML parser library, and the solution is to install lxml or html5lib.

If you encounter SyntaxError: Invalid syntax (on the line ROOT_TAG_NAME = ”[document]”), it is caused by running an old Python 2 version of Beautiful Soup under Python 3, without converting the code.

The ImportError with message No module named HTMLParser is because of an old Python 2 version of Beautiful Soup under Python 3.

While, ImportError: No module named html.parser – is caused by running the Python 3 version of Beautiful Soup under Python 2.

If you get ImportError: No module named BeautifulSoup – more often than not, it is because of running Beautiful Soup 3 code on a system that doesn”t have BS3 installed. Or, by writing Beautiful Soup 4 code without knowing that the package name has changed to bs4.

Finally, ImportError: No module named bs4 – is due to the fact that you are trying a Beautiful Soup 4 code on a system that doesn”t have BS4 installed.