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# Concepts

This page will give an overview of some important concepts to understand when working with PySTAC. If you want to check code examples, see the Tutorials.

## STAC Spec Version Support

The latest version of PySTAC supports STAC Spec 1.1.0 and will automatically update any catalogs to this version. To work with older versions of the STAC Spec, please use an older version of PySTAC:

STAC Spec Version | PySTAC Version
--- | ---
>=1.0 | Latest
0.9 | 0.4.*
0.8 | 0.3.*
<0.8 | Not supported

## Reading STACs

PySTAC can read STAC data from JSON. Generally users read in the root catalog, and then use the python objects to crawl through the data. Once you read in the root of the STAC, you can work with the STAC in memory.

    from pystac import Catalog

    catalog = Catalog.from_file('/some/example/catalog.json')

    for root, catalogs, items in catalog.walk():
        # Do interesting things with the STAC data.

To see how to hook into PySTAC for reading from alternate URIs such as cloud object storage, see I/O in PySTAC.

## Writing STACs

While working with STACs in-memory don’t require setting file paths, in order to save a STAC, you’ll need to give each STAC object a `self` link that describes the location of where it should be saved to. Luckily, PySTAC makes it easy to create a STAC catalog with a canonical layout and with the links that follow the best practices. You simply call `normalize_hrefs` with the root directory of where the STAC will be saved, and then call `save` with the type of catalog (described in the Catalog Types section) that matches your use case.

    from pystac import (Catalog, CatalogType)

    catalog = Catalog.from_file('/some/example/catalog.json')
    catalog.normalize_hrefs('/some/copy/')
    catalog.save(catalog_type=CatalogType.SELF_CONTAINED)

    copycat = Catalog.from_file('/some/copy/catalog.json')

### Normalizing HREFs

The `normalize_hrefs` call sets HREFs for all the links in the STAC according to the Catalog, Collection and Items, all based off of the root URI that is passed in:

    catalog.normalize_hrefs('/some/location')
    catalog.save(catalog_type=CatalogType.SELF_CONTAINED)

This will lay out the HREFs of the STAC according to the best practices document.

#### Layouts

PySTAC provides a few different strategies for laying out the HREFs of a STAC. To use them you can pass in a strategy when instantiating a catalog or when calling normalize_hrefs.

##### Using templates

You can utilize template strings to determine the file paths of HREFs set on Catalogs, Collection or Items. These templates use python format strings, which can name the property or attribute of the item you want to use for replacing the template variable. For example:

    from pystac.layout import TemplateLayoutStrategy

    strategy = TemplateLayoutStrategy(item_template="${collection}/${year}/${month}")
    catalog.normalize_hrefs('/some/location', strategy=strategy)
    catalog.save(catalog_type=CatalogType.SELF_CONTAINED)

The above code will save items in subfolders based on the collection ID, year and month of it’s datetime (or start_datetime if a date range is defined and no datetime is defined). Note that the forward slash (`/`) should be used as path separator in the template string regardless of the system path separator (thus both in POSIX-compliant and Windows environments).

You can use dot notation to specify attributes of objects or keys in dictionaries for template variables. PySTAC will look at the object, it’s `properties` and its `extra_fields` for property names or dictionary keys. Some special cases, like `year`, `month`, `day` and `date` exist for datetime on Items, as well as `collection` for Item’s Collection’s ID.

See the documentation on `LayoutTemplate` for more documentation on how layout templates work.

##### Using custom functions

If you want to build your own strategy, you can subclass `HrefLayoutStrategy` or use `CustomLayoutStrategy` to provide functions that work with Catalogs, Collections or Items. Similar to the templating strategy, you can provide a fallback strategy (which defaults to `BestPracticesLayoutStrategy`) for any stac object type that you don’t supply a function for.

##### Set a default catalog layout strategy

Instead of fixing the HREFs of child objects retrospectively using normalize_hrefs, you can also define a default strategy for a catalog. When instantiating a catalog, pass in a custom strategy and base href. Consequently, the HREFs of all child objects and items added to the catalog tree will be set correctly using that strategy.

    from pystac import Catalog, Collection, Item

    catalog = Catalog(...,
                      href="/some/location/catalog.json",
                      strategy=custom_strategy)
    collection = Collection(...)
    item = Item(...)
    catalog.add_child(collection)
    collection.add_item(item)
    catalog.save()

### Catalog Types

The STAC best practices document lays out different catalog types, and how their links should be formatted. A brief description is below, but check out the document for the official take on these types:

The catalog types will also dictate the asset HREF formats. Asset HREFs in any catalog type can be relative or absolute may be absolute depending on their location; see the section on Relative vs Absolute Asset HREFs below.

#### Self-Contained Catalogs

A self-contained catalog (indicated by `catalog_type=CatalogType.SELF_CONTAINED`) applies to STACs that do not have a long term location, and can be moved around. These STACs are useful for copying data to and from locations, without having to change any link metadata.

A self-contained catalog has two important properties:

- It contains only relative links

- It contains no self links.

For a catalog that is the most easy to copy around, it’s recommended that item assets use relative links, and reside in the same directory as the item’s STAC metadata file.

#### Relative Published Catalogs

A relative published catalog (indicated by `catalog_type=CatalogType.RELATIVE_PUBLISHED`) is one that is tied at it’s root to a specific location, but otherwise contains relative links. This is designed so that a self-contained catalog can be ‘published’ online by just adding one field (the self link) to its root catalog.

A relative published catalog has the following properties:

- It contains only one self link: the root of the catalog contains a (necessarily absolute) link to it’s published location.

- All other objects in the STAC contain relative links, and no self links.

#### Absolute Published Catalogs

An absolute published catalog (indicated by `catalog_type=CatalogType.ABSOLUTE_PUBLISHED`) uses absolute links for everything. It is preferable where possible, since it allows for the easiest provenance tracking out of all the catalog types.

An absolute published catalog has the following properties:

- Each STAC object contains only absolute links.

- Each STAC object has a self link.

It is not recommended to have relative asset HREFs in an absolute published catalog.

### Relative vs Absolute HREFs

HREFs inside a STAC for either links or assets can be relative or absolute.

#### Relative vs Absolute Link HREFs

Absolute links point to their file locations in a fully described way. Relative links are relative to the linking object’s file location. For example, if a catalog at `/some/location/catalog.json` has a link to an item that has an HREF set to `item-id/item-id.json`, then that link should resolve to the absolute path `/some/location/item-id/item-id.json`.

Links are set as absolute or relative HREFs at save time, as determine by the root catalog’s catalog_type `catalog_type`. This means that, even if the stored HREF of the link is absolute, if the root `catalog_type=CatalogType.RELATIVE_PUBLISHED` or `catalog_type=CatalogType.SELF_CONTAINED` and subsequent serializing of the any links in the catalog will produce a relative link, based on the self link of the parent object.

You can make all the links of a catalog relative or absolute by setting the `catalog_type()` field then resaving the entire catalog.

#### Relative vs Absolute Asset HREFs

Asset HREFs can also be relative or absolute. If an asset HREF is relative, then it is relative to the Item’s metadata file. For example, if the item at `/some/location/item-id/item-id.json` had an asset with an HREF of `./image.tif`, then the fully resolved path for that image would be `/some/location/item-id/image.tif`

You can make all the asset HREFs of a catalog relative or absolute using the `Catalog.make_all_asset_hrefs_relative` and `Catalog.make_all_asset_hrefs_absolute` methods. Note that these will not move any files around, and if the file location does not share a common parent with the asset’s item’s self HREF, then the asset HREF will remain absolute as no relative path is possible.

### Including a `self` link

Every stac object has a `save_object()` method, that takes as an argument whether or not to include the object’s self link. As noted in the section on Catalog Types, a self link is necessarily absolute; if an object only contains relative links, then it cannot contain the self link. PySTAC uses self links as a way of tracking the object’s file location, either what it was read from or it’s pending save location, so each object can have a self link even if you don’t ever want that self link written (e.g. if you are working with self-contained catalogs).

## I/O in PySTAC

The `pystac.StacIO` class defines fundamental methods for I/O operations within PySTAC, including serialization and deserialization to and from JSON files and conversion to and from Python dictionaries. This is an abstract class and should not be instantiated directly. However, PySTAC provides a `pystac.stac_io.DefaultStacIO` class with minimal implementations of these methods. This default implementation provides support for reading and writing files from the local filesystem as well as HTTP URIs (using `urllib`). This class is created automatically by all of the object-specific I/O methods (e.g. `pystac.Catalog.from_file()`), so most users will not need to instantiate this class themselves.

If you are dealing with a STAC catalog with URIs that require authentication. It is possible provide auth headers (or any other customer headers) to the `pystac.stac_io.DefaultStacIO`.

    from pystac import Catalog
    from pystac import StacIO

    stac_io = StacIO.default()
    stac_io.headers = {"Authorization": "<some-auth-header>"}

    catalog = Catalog.from_file("<URI-requiring-auth>", stac_io=stac_io)

You can double check that requests PySTAC is making by adjusting logging level so that you see all API calls.

    import logging

    logging.basicConfig()
    logger = logging.getLogger('pystac')
    logger.setLevel(logging.DEBUG)

If you require more custom logic for I/O operations or would like to use a 3rd-party library for I/O operations (e.g. `requests`), you can create a sub-class of `pystac.StacIO` (or `pystac.stac_io.DefaultStacIO`) and customize the methods as you see fit. You can then pass instances of this custom sub-class into the `stac_io` argument of most object-specific I/O methods. You can also use `pystac.StacIO.set_default()` in your client’s `__init__.py` file to make this sub-class the default `pystac.StacIO` implementation throughout the library.

For example, the following code examples will allow for reading from AWS’s S3 cloud object storage using boto3 or Azure Blob Storage using the Azure SDK for Python:

AWS S3

    from urllib.parse import urlparse
    import boto3
    from pystac import Link
    from pystac.stac_io import DefaultStacIO, StacIO
    from typing import Union, Any

    class CustomStacIO(DefaultStacIO):
       def __init__(self):
          self.s3 = boto3.resource("s3")
          super().__init__()

       def read_text(
          self, source: Union[str, Link], *args: Any, **kwargs: Any
       ) -> str:
          parsed = urlparse(source)
          if parsed.scheme == "s3":
             bucket = parsed.netloc
             key = parsed.path[1:]

             obj = self.s3.Object(bucket, key)
             return obj.get()["Body"].read().decode("utf-8")
          else:
             return super().read_text(source, *args, **kwargs)

       def write_text(
          self, dest: Union[str, Link], txt: str, *args: Any, **kwargs: Any
       ) -> None:
          parsed = urlparse(dest)
          if parsed.scheme == "s3":
             bucket = parsed.netloc
             key = parsed.path[1:]
             self.s3.Object(bucket, key).put(Body=txt, ContentEncoding="utf-8")
          else:
             super().write_text(dest, txt, *args, **kwargs)

    StacIO.set_default(CustomStacIO)

Azure Blob Storage

    import os
    import re
    from typing import Any, Dict, Optional, Tuple, Union
    from urllib.parse import urlparse

    from azure.core.credentials import (
       AzureNamedKeyCredential,
       AzureSasCredential,
       TokenCredential,
    )
    from azure.storage.blob import BlobClient, ContentSettings
    from pystac import Link
    from pystac.stac_io import DefaultStacIO

    BLOB_HTTPS_URI_PATTERN = r"https:\/\/(.+?)\.blob\.core\.windows\.net"

    AzureCredentialType = Union[
       str,
       Dict[str, str],
       AzureNamedKeyCredential,
       AzureSasCredential,
       TokenCredential,
    ]

    class BlobStacIO(DefaultStacIO):
       """A custom StacIO class for reading and writing STAC objects
       from/to Azure Blob storage.
       """

       conn_str: Optional[str] = os.getenv("AZURE_STORAGE_CONNECTION_STRING")
       account_url: Optional[str] = None
       credential: Optional[AzureCredentialType] = None
       overwrite: bool = True

       def _is_blob_uri(self, href: str) -> bool:
          """Check if href matches Blob URI pattern."""
          if re.search(
                re.compile(BLOB_HTTPS_URI_PATTERN), href
          ) is not None or href.startswith("abfs://"):
                return True
          else:
                return False

       def _parse_blob_uri(self, uri: str) -> Tuple[str, str]:
          """Parse the container and blob name from a Blob URI.

          Parameters
          ----------
          uri
                An Azure Blob URI.

          Returns
          -------
                The container and blob names.
          """
          if uri.startswith("abfs://"):
                path = uri.replace("abfs://", "/")
          else:
                path = urlparse(uri).path

          parts = path.split("/")
          container = parts[1]
          blob = "/".join(parts[2:])
          return container, blob

       def _get_blob_client(self, uri: str) -> BlobClient:
          """Instantiate a `BlobClient` given a container and blob.

          Parameters
          ----------
          uri
                An Azure Blob URI.

          Returns
          -------
                A `BlobClient` for interacting with `blob` in `container`.
          """
          container, blob = self._parse_blob_uri(uri)

          if self.conn_str:
                return BlobClient.from_connection_string(
                   self.conn_str,
                   container_name=container,
                   blob_name=blob,
                )
          elif self.account_url:
                return BlobClient(
                   account_url=self.account_url,
                   container_name=container,
                   blob_name=blob,
                   credential=self.credential,
                )
          else:
                raise ValueError(
                   "Must set conn_str or account_url (and credential if required)"
                )

       def read_text(self, source: Union[str, Link], *args: Any, **kwargs: Any) -> str:
          if isinstance(source, Link):
                source = source.href
          if self._is_blob_uri(source):
                blob_client = self._get_blob_client(source)
                obj = blob_client.download_blob().readall().decode()
                return obj
          else:
                return super().read_text(source, *args, **kwargs)

       def write_text(
          self, dest: Union[str, Link], txt: str, *args: Any, **kwargs: Any
       ) -> None:
          """Write STAC Objects to Blob storage. Note: overwrites by default."""
          if isinstance(dest, Link):
                dest = dest.href
          if self._is_blob_uri(dest):
                blob_client = self._get_blob_client(dest)
                blob_client.upload_blob(
                   txt,
                   overwrite=self.overwrite,
                   content_settings=ContentSettings(content_type="application/json"),
                )
          else:
                super().write_text(dest, txt, *args, **kwargs)

    # set Blob storage connection string
    BlobStacIO.conn_str = "my-storage-connection-string"

    # OR set Blob account URL, credential
    BlobStacIO.account_url = "https://myblobstorageaccount.blob.core.windows.net"
    BlobStacIO.credential = AzureSasCredential("my-sas-token")

    # modify overwrite behavior
    BlobStacIO.overwrite = False

    # set BlobStacIO as default StacIO
    StacIO.set_default(BlobStacIO)

If you only need to customize read operations you can inherit from `DefaultStacIO` and only overwrite the read method. For example, to take advantage of connection pooling using a requests.Session:

    from urllib.parse import urlparse
    import requests
    from pystac.stac_io import DefaultStacIO, StacIO
    from typing import Union, Any

    class ConnectionPoolingIO(DefaultStacIO):
       def __init__(self):
          self.session = requests.Session()

       def read_text(
          self, source: Union[str, Link], *args: Any, **kwargs: Any
       ) -> str:
          parsed = urlparse(uri)
          if parsed.scheme.startswith("http"):
             return self.session.get(uri).text
          else:
             return super().read_text(source, *args, **kwargs)

    StacIO.set_default(ConnectionPoolingIO)

## Validation

PySTAC includes validation functionality that allows users to validate PySTAC objects as well JSON-encoded STAC objects from STAC versions 0.8.0 and later.

### Enabling validation

To enable the validation feature you’ll need to have installed PySTAC with the optional dependency via:

    > pip install pystac[validation]

This installs the `jsonschema` package which is used with the default validator. If you define your own validation class as described below, you are not required to have this extra dependency.

### Validating PySTAC objects

You can validate any `Catalog`, `Collection` or `Item` by calling the `validate()` method:

    item.validate()

This validates against the latest set of JSON schemas (which are included with the PySTAC package) or older versions (which are hosted at https://schemas.stacspec.org). This validation includes any extensions that the object extends (these are always accessed remotely based on their URIs).

If there are validation errors, a `STACValidationError` is raised.

You can also call `validate_all()` on a Catalog or Collection to recursively walk through a catalog and validate all objects within it.

    catalog.validate_all()

### Validating STAC JSON

You can validate STAC JSON represented as a `dict` using the `pystac.validation.validate_dict()` method:

    import json
    from pystac.validation import validate_dict

    with open('/path/to/item.json') as f:
        js = json.load(f)
    validate_dict(js)

You can also recursively validate all of the catalogs, collections and items across STAC versions using the `pystac.validation.validate_all()` method:

    import json
    from pystac.validation import validate_all

    with open('/path/to/catalog.json') as f:
        js = json.load(f)
    validate_all(js)

### Using your own validator

By default PySTAC uses the `JsonSchemaSTACValidator` implementation for validation. Users can define their own implementations of `STACValidator` and register it with pystac using `pystac.validation.set_validator()`.

The `JsonSchemaSTACValidator` takes a `SchemaUriMap`, which by default uses the `DefaultSchemaUriMap`. If desirable, users can create their own implementation of `SchemaUriMap` and register a new instance of `JsonSchemaSTACValidator` using that schema map with `pystac.validation.set_validator()`.

## Extensions

From the documentation on STAC Spec Extensions:

> Extensions to the core STAC specification provide additional JSON fields that can be used to better describe the data. Most tend to be about describing a particular domain or type of data, but some imply functionality.

This library makes an effort to support all extensions that are part of the stac-extensions GitHub org, and we are committed to supporting all STAC Extensions at the “Candidate” maturity level or above (see the Extension Maturity documentation for details).

### Accessing Extension Functionality

Extension functionality is encapsulated in classes that are specific to the STAC Extension (e.g. Electro-Optical, Projection, etc.) and STAC Object (`Collection`, `pystac.Item`, or `pystac.Asset`). All classes that extend these objects inherit from `pystac.extensions.base.PropertiesExtension`, and you can use the `ext` accessor on the object to access the extension fields.

For instance, if you have an item that implements the Electro-Optical Extension, you can access the fields associated with that extension using `Item.ext`:

    import pystac

    item = pystac.Item.from_file("tests/data-files/eo/eo-landsat-example.json")

    # As long as the Item implements the EO Extension you can access all the
    # EO properties directly
    bands = item.ext.eo.bands
    cloud_cover = item.ext.eo.cloud_cover
    ...

Note

`ext` will raise an `ExtensionNotImplemented` exception if the object does not implement that extension (e.g. if the extension URI is not in that object’s `stac_extensions` list). See the Adding an Extension section below for details on adding an extension to an object.

If you don’t want to raise an error you can use `Item.ext.has` to first check if the extension is implemented on your pystac object:

    if item.ext.has("eo"):
       bands = item.ext.eo.bands

See the documentation for each extension implementation for details on the supported properties and other functionality.

Extensions have access to the properties of the object. This attribute is a reference to the properties of the `Collection`, `Item` or `Asset` being extended and can therefore mutate those properties. For instance:

    item = pystac.Item.from_file("tests/data-files/eo/eo-landsat-example.json")
    print(item.properties["eo:cloud_cover"])
    # 78

    print(item.ext.eo.cloud_cover)
    # 78

    item.ext.eo.cloud_cover = 45
    print(item.properties["eo:cloud_cover"])
    # 45

There is also a `additional_read_properties` attribute that, if present, gives read-only access to properties of any objects that own the extended object. For instance, an extended `pystac.Asset` instance would have read access to the properties of the `pystac.Item` that owns it (if there is one). If a property exists in both additional_read_properties and properties, the value in additional_read_properties will take precedence.

An `apply` method is available on extended objects. This allows you to pass in property values pertaining to the extension. Properties that are required by the extension will be required arguments to the `apply` method. Optional properties will have a default value of `None`:

    # Can also omit cloud_cover entirely...
    item.ext.eo.apply(0.5, bands, cloud_cover=None)

### Adding an Extension

You can add an extension to a STAC object that does not already implement that extension using the `Item.ext.add` method. The `Item.ext.add` method adds the correct schema URI to the `stac_extensions` list for the STAC object.

    # Load a basic item without any extensions
    item = pystac.Item.from_file("tests/data-files/item/sample-item.json")
    print(item.stac_extensions)
    # []

    # Add the Electro-Optical extension
    item.ext.add("eo")
    print(item.stac_extensions)
    # ['https://stac-extensions.github.io/eo/v1.1.0/schema.json']

### Extended Summaries

Extension classes like `ProjectionExtension` may also provide a `summaries` static method that can be used to extend the Collection summaries. This method returns a class inheriting from `pystac.extensions.base.SummariesExtension` that provides tools for summarizing the properties defined by that extension. These classes also hold a reference to the Collection’s `pystac.Summaries` instance in the `summaries` attribute.

    import pystac
    from pystac.extensions.projection import ProjectionExtension

    # Load a collection that does not implement the Projection extension
    collection = pystac.Collection.from_file(
       "tests/data-files/examples/1.0.0/collection.json"
    )

    # Add Projection extension summaries to the collection
    proj = ProjectionExtension.summaries(collection, add_if_missing=True)
    print(collection.stac_extensions)
    # [
    #     ....,
    #     'https://stac-extensions.github.io/projection/v1.1.0/schema.json'
    # ]

    # Set the values for various extension fields
    proj.epsg = [4326]
    collection_as_dict = collection.to_dict()
    collection_as_dict["summaries"]["proj:epsg"]
    # [4326]

## Item Asset properties

Properties that apply to Items can be found in two places: the Item’s properties or in any of an Item’s Assets. If the property is on an Asset, it applies only to that specific asset. For example, gsd defined for an Item represents the best Ground Sample Distance (resolution) for the data within the Item. However, some assets may be lower resolution and thus have a higher gsd. In that case, the gsd can be found on the Asset.

See the STAC documentation on Additional Fields for Assets and the relevant Best Practices for more information.

The implementation of this feature in PySTAC uses the method described here and is consistent across Item and ItemExtensions. The bare property names represent values for the Item only, but for each property where it is possible to set on both the Item or the Asset there is a `get_` and `set_` methods that optionally take an Asset. For the `get_` methods, if the property is found on the Asset, the Asset’s value is used; otherwise the Item’s value will be used. For the `set_` method, if an Asset is passed in the value will be applied to the Asset and not the Item.

For example, if we have an Item with a `gsd` of 10 with three bands, and only asset “band3” having a `gsd` of 20, the `get_gsd` method will behave in the following way:

>     assert item.common_metadata.gsd == 10
>     assert item.common_metadata.get_gsd() == 10
>     assert item.common_metadata.get_gsd(item.asset['band1']) == 10
>     assert item.common_metadata.get_gsd(item.asset['band3']) == 20

Similarly, if we set the asset at ‘band2’ to have a `gsd` of 30, it will only affect that asset:

>     item.common_metadata.set_gsd(30, item.assets['band2']
>     assert item.common_metadata.gsd == 10
>     assert item.common_metadata.get_gsd(item.asset['band2']) == 30

## Manipulating STACs

PySTAC is designed to allow for STACs to be manipulated in-memory. This includes Copying STACs in-memory, walking over all objects in a STAC and mutating their properties, or using collection-style map methods for mapping over items.

### Walking over a STAC

You can walk through all sub-catalogs and items of a catalog with a method inspired by the Python Standard Library os.walk() method: `Catalog.walk()`:

    for root, subcats, items in catalog.walk():
        # Root represents a catalog currently being walked in the tree
        root.title = '{} has been walked!'.format(root.id)

        # subcats represents any catalogs or collections owned by root
        for cat in subcats:
            cat.title = 'About to be walked!'

        # items represent all items that are contained by root
        for item in items:
            item.title = '{} - owned by {}'.format(item.id, root.id)

### Mapping over Items

The `Catalog.map_items` method is useful for into smaller chunks (e.g. tiling out large image items). item, you can return multiple items, in case you are generating new objects, or splitting items manipulating items in a STAC. This will create a full copy of the STAC, so will leave the original catalog unmodified. In the method that manipulates and returns the modified

    def modify_item_title(item):
        item.title = 'Some new title'
        return item

    def duplicate_item(item):
        duplicated_item = item.clone()
        duplicated_item.id += "-duplicated"
        return [item, duplicated_item]

    c = catalog.map_items(modify_item_title)
    c = c.map_items(duplicate_item)
    new_catalog = c

### Copying STACs in-memory

The in-memory copying of STACs to create new ones is crucial to correct manipulations and mutations of STAC data. The `STACObject.full_copy` mechanism handles this in a way that ties the elements of the copies STAC together correctly. This includes situations where there might be cycles in the graph of connected objects of the STAC (which otherwise would be a tree).

## Resolving STAC objects

PySTAC tries to only “resolve” STAC Objects - that is, load the metadata contained by STAC files pointed to by links into Python objects in-memory - when necessary. It also ensures that two links that point to the same object resolve to the same in-memory object.

### Lazy resolution of STAC objects

Links are read only when they need to be. For instance, when you load a catalog using `Catalog.from_file`, the catalog and all of its links are read into a `Catalog` instance. If you iterate through `Catalog.links`, you’ll see the `target` of the `Link` will refer to a string - that is the HREF of the link. However, if you call `Catalog.get_items`, for instance, you’ll get back the actual `Item` instances that are referred to by each item link in the Catalog. That’s because at the time you call `get_items`, PySTAC is “resolving” the links for any link that represents an item in the catalog.

The resolution mechanism is accomplished through `Link.resolve_stac_object`. Though this method is used extensively internally to PySTAC, ideally this is completely transparent to users of PySTAC, and you won’t have to worry about how and when links get resolved. However, one important aspect to understand is how object resolution caching happens.

### Resolution Caching

The root `Catalog` instance of a STAC (the Catalog which is linked to by every associated object’s `root` link) contains a cache of resolved objects. This cache points to in-memory instances of `STACObject` s that have already been resolved through PySTAC crawling links associated with that root catalog. The cache works off of the stac object’s ID, which is why it is necessary for every STAC object in the catalog to have a unique identifier, which is unique across the entire STAC.

When a link is being resolved from a STACObject that has it’s root set, that root is passed into the `Link.resolve_stac_object` call. That root’s `ResolvedObjectCache` will be used to ensure that if the link is pointing to an object that has already been resolved, then that link will point to the same, single instance in the cache. This ensures working with STAC objects in memory doesn’t create a situation where multiple copies of the same STAC objects are created from different links, manipulated, and written over each other.

## Working with STAC JSON

The `pystac.serialization` package has some functionality around working directly with STAC JSON objects, without utilizing PySTAC object types. This is used internally by PySTAC, but might also be useful to users working directly with JSON (e.g. on validation).

### Identifying STAC objects from JSON

Users can identify STAC information, including the object type, version and extensions, from JSON. The main method for this is `identify_stac_object()`, which returns an object that contains the object type, the range of versions this object is valid for (according to PySTAC’s best guess), the common extensions implemented by this object, and any custom extensions (represented by URIs to JSON Schemas).

    from pystac.serialization import identify_stac_object

    json_dict = ...

    info = identify_stac_object(json_dict)

    # The object type
    info.object_type

    # The version range
    info.version_range

    # The common extensions
    info.common_extensions

    # The custom Extensions
    info.custom_extensions

### Merging common properties

For pre-1.0.0 STAC, The `merge_common_properties()` will take a JSON dict that represents an item, and if it is associated with a collection, merge in the collection’s properties. You can pass in a dict that contains previously read collections that caches collections by the HREF of the collection link and/or the collection ID, which can help avoid multiple reads of collection links.

Note that this feature was dropped in STAC 1.0.0-beta.1

## Geo interface

`Item` implements `__geo_interface__`, a de-facto standard for describing geospatial objects in Python: https://gist.github.com/sgillies/2217756. Many packages can automatically use objects that implement this protocol, e.g. shapely:

    >>> from pystac import Item
    >>> from shapely.geometry import mapping, shape
    >>> item = Item.from_file("data-files/item/sample-item.json")
    >>> print(shape(item))
    POLYGON ((-122.308150179 37.488035566, -122.597502109 37.538869539,
    -122.576687533 37.613537207, -122.2880486 37.562818007, -122.308150179
    37.488035566))