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A module to parse metadata out of urls and html documents

Project description

MetadataParser

Build Status: build_status

MetadataParser is a Python module for pulling metadata out of web documents.

It requires BeautifulSoup , and was largely based on Erik River’s opengraph module. Something more aggressive than Erik’s module was needed, so this project was started.

Installation

pip install metadata_parser

Installation Recommendation

The requests library version 2.4.3 or newer is strongly recommended.

This is not required, but it is better. On earlier versions it is possible to have an uncaught DecodeError exception when there is an underlying redirect/404. Recent fixes to requests improve redirect handling, urllib3 and urllib3 errors.

Features

  • metadata_parser pulls as much metadata out of a document as possible
  • Developers can set a ‘strategy’ for finding metadata (i.e. only accept opengraph or page attributes)
  • Lightweight but functional(!) url validation
  • Verbose logging

Logging

This file has extensive logging to help developers pinpoint problems.

  • log.debug This log level is mostly used to handle library maintenance and troubleshooting, aka “Library Debugging”. Library Debugging is verbose, but is nested under if __debug__: statements, so it is compiled away when PYTHONOPTIMIZE is set. Several sections of logic useful to developers will also emit logging statements at the debug level, regardless of PYTHONOPTIMIZE.
  • log.info Currently unused
  • log.warning Currently unused
  • log.error This log level is mostly used to alert developers of errors that were encountered during url fetching and document parsing, and often emits a log statement just before an Exception is raised. The log statements will contain at least the exception type, and may contain the active URL and additional debugging information, if any of that information is available.
  • log.critical Currently unused

It is STRONGLY recommended to keep Python’s logging at debug.

Optional Integrations

  • tldextract This package will attempt to use the package tldextract for advanced domain and hostname analysis. If tldextract is not found, a fallback is used.

Environment Variables

  • METADATA_PARSER__DISABLE_TLDEXTRACT Default: “0”. If set to “1”, the package will not attempt to load tldextract.
  • METADATA_PARSER__ENCODING_FALLBACK Default: “ISO-8859-1” Used as the fallback when trying to decode a response.
  • METADATA_PARSER__DUMMY_URL Used as the fallback URL when calculating url data.

Notes

  1. This package requires BeautifulSoup 4.
  2. For speed, it will instantiate a BeautifulSoup parser with lxml, and fallback to ‘none’ (the internal pure Python) if it can’t load lxml.
  3. URL Validation is not RFC compliant, but tries to be “Real World” compliant.

It is HIGHLY recommended that you install lxml for usage. lxml is considerably faster. Considerably faster.

Developers should also use a very recent version of lxml. segfaults have been reported on lxml versions < 2.3.x; Using at least the most recent 3.x versions is strongly recommended

The default ‘strategy’ is to look in this order:

og,dc,meta,page

Which stands for the following:

og = OpenGraph
dc = DublinCore
meta = metadata
page = page elements

Developers can specify a strategy as a comma-separated list of the above.

The only 2 page elements currently supported are:

<title>VALUE</title> -> metadata['page']['title']
<link rel="canonical" href="VALUE"> -> metadata['page']['link']

‘metadata’ elements are supported by name and property.

The MetadataParser object also wraps some convenience functions, which can be used otherwise , that are designed to turn alleged urls into well formed urls.

For example, you may pull a page:

http://www.example.com/path/to/file.html

and that file indicates a canonical url which is simple “/file.html”.

This package will try to ‘remount’ the canonical url to the absolute url of “http://www.example.com/file.html”. Tt will return None if the end result is not a valid url.

This all happens under-the-hood, and is honestly really useful when dealing with indexers and spiders.

URL Validation

“Real World” URL validation is enabled by default. This is not RFC compliant.

There are a few gaps in the RFCs that allow for “odd behavior”. Just about any use-case for this package will desire/expect rules that parse URLs “in the wild”, not theoretical.

The differences:

  • If an entirely numeric ip address is encountered, it is assumed to be a dot-notation IPV4 and it is checked to have the right amount of valid octets.

    The default behavior is to invalidate these hosts:

    http://256.256.256.256
    http://999.999.999.999.999
    

    According to RFCs those are valid hostnames that would fail as “IP Addresses” but pass as “Domain Names”. However in the real world, one would never encounter domain names like those.

  • The only non-domain hostname that is allowed, is “localhost”

    The default behavior is to invalidate these hosts

    http://example
    http://examplecom
    

    Those are considered to be valid hosts, and might exist on a local network or custom hosts file. However, they are not part of the public internet.

Although this behavior breaks RFCs, it greatly reduces the number of “False Positives” generated when analyzing internet pages. If you want to include bad data, you can submit a kwarg to MetadataParser.__init__

Handling Bad URLs and Encoded URIs

This library tries to safeguard against a few common situations.

Encoded URIs and relative urls

Most website publishers will define an image as a URL:

<meta property="og:image" content="http://example.com/image.jpg" />

Some will define an image as an encoded URI:

<meta property="og:image" content="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNM+Q8AAc0BZX6f84gAAAAASUVORK5CYII=" />

By default, the get_metadata_link() method can be used to ensure a valid link is extracted from the metadata payload:

>>> import metadata_parser
>>> page = metadata_parser.MetadataParser(url="http://www.example.com")
>>> print page.get_metadata_link('image')

This method accepts a kwarg allow_encoded_uri (default False) which will return the image without further processing:

>>> print page.get_metadata_link('image', allow_encoded_uri=True)

Similarly, if a url is local:

<meta property="og:image" content="/image.jpg" />

The get_metadata_link method will automatically upgrade it onto the domain:

>>> print page.get_metadata_link('image')
http://example.com/image.jpg

Poorly Constructed Canonical URLs

Many website publishers implement canonical URLs incorrectly. This package tries to fix that.

By default MetadataParser is constructed with require_public_netloc=True and allow_localhosts=True.

This will require somewhat valid ‘public’ network locations in the url.

For example, these will all be valid URLs:

http://example.com
http://1.2.3.4
http://localhost
http://127.0.0.1
http://0.0.0.0

If these known ‘localhost’ urls are not wanted, they can be filtered out with allow_localhosts=False:

http://localhost
http://127.0.0.1
http://0.0.0.0

There are two convenience methods that can be used to get a canonical url or calculate the effective url:

* MetadataParser.get_discrete_url
* MetadataParser.get_metadata_link

These both accept an argument require_public_global, which defaults to True.

Assuming we have the following content on the url http://example.com/path/to/foo:

<link rel="canonical" href="http://localhost:8000/alt-path/to/foo">

By default, versions 0.9.0 and later will detect ‘localhost:8000’ as an improper canonical url, and remount the local part “/alt-path/to/foo” onto the domain that served the file. The vast majority of times this ‘behavior’ has been encountered, this is the intended canonical:

print page.get_discrete_url()
>>> http://example.com/alt-path/to/foo

In contrast, versions 0.8.3 and earlier will not catch this situation:

print page.get_discrete_url()
>>> http://localhost:8000/alt-path/to/foo

In order to preserve the earlier behavior, just submit require_public_global=False:

print page.get_discrete_url(require_public_global=False)
>>> http://localhost:8000/alt-path/to/foo

Handling Bad Data

Many CMS systems (and developers) create malformed content or incorrect document identifiers. When this happens, the BeautifulSoup parser will lose data or move it into an unexpected place.

There are two arguments that can help you analyze this data:

  • force_doctype:

    ``MetadataParser(..., force_doctype=True, ...)``
    

force_doctype=True will try to replace the identified doctype with “html” via regex. This will often make the input data usable by BS4.

  • search_head_only:

    ``MetadataParser(..., search_head_only=False, ...)``
    

search_head_only=False will not limit the search path to the “<head>” element. This will have a slight performance hit and will incorporate data from CMS/User content, not just templates/Site-Operators.

WARNING

1.0 will be a complete API overhaul. pin your releases to avoid sadness.

Version 0.9.19 Breaking Changes

Issue #12 exposed some flaws in the existing package

1. MetadataParser.get_metadatas replaces MetadataParser.get_metadata

Until version 0.9.19, the recommended way to get metadata was to use get_metadata which will either return a string (or None).

Starting with version 0.9.19, the recommended way to get metadata is to use get_metadatas which will always return a list (or None).

This change was made because the library incorrectly stored a single metadata key value when there were duplicates.

2. The ParsedResult payload stores mixed content and tracks it’s version ==————————————————————————–

Many users (including the maintainer) archive the parsed metadata. After testing a variety of payloads with an all-list format and a mixed format (string or list), a mixed format had a much smaller payload size with a negligible performance hit. A new _v attribute tracks the payload version. In the future, payloads without a _v attribute will be interpreted as the pre-versioning format.

3. DublinCore payloads might be a dict

Tests were added to handle dublincore data. An extra attribute may be needed to properly represent the payload, so always returning a dict with at least a name+content (and possibly lang or scheme is the best approach.

Usage

Until version 0.9.19, the recommended way to get metadata was to use get_metadata which will return a string (or None):

From an URL:

>>> import metadata_parser
>>> page = metadata_parser.MetadataParser(url="http://www.example.com")
>>> print page.metadata
>>> print page.get_metadatas('title')
>>> print page.get_metadatas('title', strategy=['og',])
>>> print page.get_metadatas('title', strategy=['page', 'og', 'dc',])

From HTML:

>>> HTML = """<here>"""
>>> page = metadata_parser.MetadataParser(html=HTML)
>>> print page.metadata
>>> print page.get_metadatas('title')
>>> print page.get_metadatas('title', strategy=['og',])
>>> print page.get_metadatas('title', strategy=['page', 'og', 'dc',])

Malformed Data

It is very common to find malformed data. As of version 0.9.20 the following methods should be used to allow malformed presentation:

>>> page = metadata_parser.MetadataParser(html=HTML, support_malformed=True)

or:

>>> parsed = page.parse(html=html, support_malformed=True)
>>> parsed = page.parse(html=html, support_malformed=False)

The above options will support parsing common malformed options. Currently this only looks at alternate (improper) ways of producing twitter tags, but may be expanded.

Notes

when building on Python3, a static toplevel directory may be needed

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