# The BlobKey Class - Google App Engine - Google Code

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# The BlobKey Class

An instance of the `BlobKey` class represents the key of a Blobstore value. This value can be stored as a property value in a datastore entity, used to get the `BlobInfo` object that corresponds with a Blobstore value, and returned in a response header to serve the Blobstore value to a user.

The `BlobKey` class is provided by the `google.appengine.ext.blobstore` module.

Your Blobstore upload handler can access the string form of the Blobstore key in the request data, as the `X-AppEngine-BlobKey` header in the MIME part that corresponds with the uploaded file. You can convert this string to a `BlobKey` value by passing the string to the `BlobKey()` constructor.

You can get the string form of a `BlobKey` value by passing it to the `str()` builtin. The string form avoids special characters used in URLs and HTML, so it can be used in user-facing data without being escaped. A Blobstore value can only be retrieved by a user if the application sets the `X-AppEngine-BlobKey` header in a response, so the application can control access to values even if Blobstore keys are public. If access to your Blobstore values must be restricted, be careful when serving values based on keys calculated from user-submitted data.

You can use the `BlobReferenceProperty` class with the Python data modeling interface to create references to BlobInfo entities from other entities. The actual value stored for the property is a BlobKey value.

You can fetch the `BlobInfo` entity with a `BlobKey` by passing the key to the `BlobInfo.get()` class method.