Chapter 1: Understanding Metadata

Metadata is essentially data that describes other data. It acts as a summary or a descriptor, making it easier to locate and interact with specific data instances. Metadata can be categorized into various types based on its content, function, or application. The most common type of file metadata includes attributes like file type, permissions, tags, and timestamps for creation and last edits. These are classified as standard attributes. For instance, photographs typically contain metadata that provides insights into when, where, and how they were taken, along with technical specifications about the image itself. However, the scope of metadata is extensive and can encompass much more.

We are developing a Python application aimed at aiding the editing process of documents. This application highlights potential spelling and grammar errors, and one feature allows users to disregard suggested corrections (see Figure 2). The challenge we face is ensuring that our program remembers when a user has previously ignored an issue in their document. To achieve this, we need a way to retain this information—enter metadata.

Chapter 2: Exploring macOS Extended Attributes

Our application targets macOS, so we will focus on a solution tailored for that platform. On macOS, extended attributes can store additional details about a file, such as the author, project name, or any other custom data. This metadata exists separately from the file content and can be accessed or modified without changing the file itself.

To interact with extended attributes on macOS, you can use the xattr command-line utility. To view all extended attributes of a file, use the following command:

xattr -l filename

Our application is written in Python, and our specific requirement is to save and retrieve a list of strings as an extended attribute. To utilize extended attributes in Python, you need to install the xattr module.

You can install it using pip:

pip install xattr

Extended attributes are designed for storing string values. To save a list of strings, you need to serialize the list into a string format first. A common approach is using JSON for serialization, which allows you to convert the list into a JSON string before saving it as an extended attribute.

import json

import xattr

# List of strings

my_list = ['string1', 'string2', 'string3']

# Serialize the list to a JSON string

serialized_list = json.dumps(my_list)

# The file you want to attach the extended attribute to

file_path = '/path/to/your/file.txt'

# The name of the extended attribute

attribute_name = 'user.mylist'

# Save the serialized list as an extended attribute

xattr.setxattr(file_path, attribute_name, serialized_list.encode('utf-8'))

To retrieve and deserialize the string back into a list, reverse the process:

# Retrieve the serialized list

serialized_list = xattr.getxattr(file_path, attribute_name)

# Deserialize it back into a Python list

my_list = json.loads(serialized_list.decode('utf-8'))

print(my_list)

This method allows you to associate a complex data structure, such as a list of strings, with a file as metadata, making it easy to retrieve and manipulate later. Be mindful of size limitations on extended attributes, as xattr.h defines a constant called XATTR_MAXSIZE which is set to 64 MiB.

Chapter 3: Cross-Platform Considerations

What about Windows and Linux?

Both platforms offer comparable solutions. In Windows, this capability is referred to as Alternate Data Streams (ADS) for NTFS (New Technology File System). ADS allows files to contain multiple streams of data, which can be used to attach metadata or additional information to a file without altering the primary file content.

You can interact with ADS using command-line tools or access them programmatically via the Windows API. For instance, to add a new stream called metadata to a file named example.txt, you could use:

echo My metadata > example.txt:metadata

To retrieve the data, use:

more < example.txt:metadata

On Linux, extended file attributes (xattrs) serve a similar purpose. Extended attributes allow users to associate key-value pairs with files and are supported by various Linux filesystems, including ext3, ext4, and btrfs. These attributes can be accessed and modified using the getfattr and setfattr command-line tools. For example, to set an attribute, use:

setfattr -n user.description -v "My file description" example.txt

And to retrieve it:

getfattr -n user.description example.txt

The Linux kernel permits extended attribute names of up to 255 bytes and values of up to 64 KiB. However, ext2/3/4 and btrfs impose smaller limits, requiring all attributes to fit within one filesystem block (typically 4 KiB).

All the systems discussed provide mechanisms for storing additional information alongside files, although the tools and methods for interacting with this metadata may vary. Extended attributes in Linux and ADS in Windows function similarly to extended attributes in macOS. If you are developing a cross-platform application, you will need to identify the current platform and employ the appropriate method accordingly.

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The first video titled "PIL: add text as metadata in a png file and recover it" provides insights on how to add and retrieve metadata in PNG files using PIL.

The second video titled "Why add metadata to a MS Word document?" discusses the importance of incorporating metadata into Word documents for better organization and retrieval.