Container Analysis API . providers . notes . occurrences

Instance Methods

list(name, pageToken=None, x__xgafv=None, pageSize=None, filter=None)

Lists `Occurrences` referencing the specified `Note`. Use this method to

list_next(previous_request, previous_response)

Retrieves the next page of results.

Method Details

list(name, pageToken=None, x__xgafv=None, pageSize=None, filter=None)
Lists `Occurrences` referencing the specified `Note`. Use this method to
get all occurrences referencing your `Note` across all your customer
projects.

Args:
  name: string, The name field will contain the note name for example:
  "provider/{provider_id}/notes/{note_id}" (required)
  pageToken: string, Token to provide to skip to a particular spot in the list.
  x__xgafv: string, V1 error format.
    Allowed values
      1 - v1 error format
      2 - v2 error format
  pageSize: integer, Number of notes to return in the list.
  filter: string, The filter expression.

Returns:
  An object of the form:

    { # Response including listed occurrences for a note.
    "nextPageToken": "A String", # Token to receive the next page of notes.
    "occurrences": [ # The occurrences attached to the specified note.
      { # `Occurrence` includes information about analysis occurrences for an image.
        "resource": { #  #
            # The resource for which the `Occurrence` applies.
            # Resource is an entity that can have metadata. E.g., a Docker image.
          "contentHash": { # Container message for hash values. # The hash of the resource content. E.g., the Docker digest.
            "type": "A String", # The type of hash that was performed.
            "value": "A String", # The hash value.
          },
          "uri": "A String", # The unique URI of the resource. E.g.,
              # "https://gcr.io/project/image@sha256:foo" for a Docker image.
          "name": "A String", # The name of the resource. E.g., the name of a Docker image - "Debian".
        },
        "updateTime": "A String", # Output only. The time this `Occurrence` was last updated.
        "installation": { # This represents how a particular software package may be installed on # Describes the installation of a package on the linked resource.
            # a system.
          "location": [ # All of the places within the filesystem versions of this package
              # have been found.
            { # An occurrence of a particular package installation found within a
                # system's filesystem.
                # e.g. glibc was found in /var/lib/dpkg/status
              "path": "A String", # The path from which we gathered that this package/version is installed.
              "cpeUri": "A String", # The cpe_uri in [cpe format](https://cpe.mitre.org/specification/)
                  # denoting the package manager version distributing a package.
              "version": { # Version contains structured information about the version of the package. # The version installed at this location.
                  # For a discussion of this in Debian/Ubuntu:
                  # http://serverfault.com/questions/604541/debian-packages-version-convention
                  # For a discussion of this in Redhat/Fedora/Centos:
                  # http://blog.jasonantman.com/2014/07/how-yum-and-rpm-compare-versions/
                "kind": "A String", # Distinguish between sentinel MIN/MAX versions and normal versions.
                    # If kind is not NORMAL, then the other fields are ignored.
                "revision": "A String", # The iteration of the package build from the above version.
                "name": "A String", # The main part of the version name.
                "epoch": 42, # Used to correct mistakes in the version numbering scheme.
              },
            },
          ],
          "name": "A String", # Output only. The name of the installed package.
        },
        "name": "A String", # Output only. The name of the `Occurrence` in the form
            # "projects/{project_id}/occurrences/{OCCURRENCE_ID}"
        "kind": "A String", # Output only. This explicitly denotes which of the `Occurrence` details are
            # specified. This field can be used as a filter in list requests.
        "buildDetails": { # Message encapsulating build provenance details. # Build details for a verifiable build.
          "provenance": { # Provenance of a build. Contains all information needed to verify the full # The actual provenance
              # details about the build from source to completion.
            "finishTime": "A String", # Time at which execution of the build was finished.
            "commands": [ # Commands requested by the build.
              { # Command describes a step performed as part of the build pipeline.
                "waitFor": [ # The ID(s) of the Command(s) that this Command depends on.
                  "A String",
                ],
                "name": "A String", # Name of the command, as presented on the command line, or if the command is
                    # packaged as a Docker container, as presented to `docker pull`.
                "args": [ # Command-line arguments used when executing this Command.
                  "A String",
                ],
                "env": [ # Environment variables set before running this Command.
                  "A String",
                ],
                "id": "A String", # Optional unique identifier for this Command, used in wait_for to reference
                    # this Command as a dependency.
                "dir": "A String", # Working directory (relative to project source root) used when running
                    # this Command.
              },
            ],
            "sourceProvenance": { # Source describes the location of the source used for the build. # Details of the Source input to the build.
              "artifactStorageSource": { # StorageSource describes the location of the source in an archive file in # If provided, the input binary artifacts for the build came from this
                  # location.
                  # Google Cloud Storage.
                "generation": "A String", # Google Cloud Storage generation for the object.
                "object": "A String", # Google Cloud Storage object containing source.
                "bucket": "A String", # Google Cloud Storage bucket containing source (see [Bucket Name
                    # Requirements]
                    # (https://cloud.google.com/storage/docs/bucket-naming#requirements)).
              },
              "repoSource": { # RepoSource describes the location of the source in a Google Cloud Source # If provided, get source from this location in a Cloud Repo.
                  # Repository.
                "projectId": "A String", # ID of the project that owns the repo.
                "branchName": "A String", # Name of the branch to build.
                "repoName": "A String", # Name of the repo.
                "tagName": "A String", # Name of the tag to build.
                "commitSha": "A String", # Explicit commit SHA to build.
              },
              "fileHashes": { # Hash(es) of the build source, which can be used to verify that the original
                  # source integrity was maintained in the build.
                  #
                  # The keys to this map are file paths used as build source and the values
                  # contain the hash values for those files.
                  #
                  # If the build source came in a single package such as a gzipped tarfile
                  # (.tar.gz), the FileHash will be for the single path to that file.
                "a_key": { # Container message for hashes of byte content of files, used in Source
                    # messages to verify integrity of source input to the build.
                  "fileHash": [ # Collection of file hashes.
                    { # Container message for hash values.
                      "type": "A String", # The type of hash that was performed.
                      "value": "A String", # The hash value.
                    },
                  ],
                },
              },
              "additionalContexts": [ # If provided, some of the source code used for the build may be found in
                  # these locations, in the case where the source repository had multiple
                  # remotes or submodules. This list will not include the context specified in
                  # the context field.
                { # A SourceContext is a reference to a tree of files. A SourceContext together
                    # with a path point to a unique revision of a single file or directory.
                  "git": { # A GitSourceContext denotes a particular revision in a third party Git # A SourceContext referring to any third party Git repo (e.g., GitHub).
                      # repository (e.g., GitHub).
                    "url": "A String", # Git repository URL.
                    "revisionId": "A String", # Required.
                        # Git commit hash.
                  },
                  "cloudRepo": { # A CloudRepoSourceContext denotes a particular revision in a Google Cloud # A SourceContext referring to a revision in a Google Cloud Source Repo.
                      # Source Repo.
                    "aliasContext": { # An alias to a repo revision. # An alias, which may be a branch or tag.
                      "kind": "A String", # The alias kind.
                      "name": "A String", # The alias name.
                    },
                    "revisionId": "A String", # A revision ID.
                    "repoId": { # A unique identifier for a Cloud Repo. # The ID of the repo.
                      "projectRepoId": { # Selects a repo using a Google Cloud Platform project ID (e.g., # A combination of a project ID and a repo name.
                          # winged-cargo-31) and a repo name within that project.
                        "projectId": "A String", # The ID of the project.
                        "repoName": "A String", # The name of the repo. Leave empty for the default repo.
                      },
                      "uid": "A String", # A server-assigned, globally unique identifier.
                    },
                  },
                  "labels": { # Labels with user defined metadata.
                    "a_key": "A String",
                  },
                  "gerrit": { # A SourceContext referring to a Gerrit project. # A SourceContext referring to a Gerrit project.
                    "aliasContext": { # An alias to a repo revision. # An alias, which may be a branch or tag.
                      "kind": "A String", # The alias kind.
                      "name": "A String", # The alias name.
                    },
                    "revisionId": "A String", # A revision (commit) ID.
                    "gerritProject": "A String", # The full project name within the host. Projects may be nested, so
                        # "project/subproject" is a valid project name. The "repo name" is
                        # the hostURI/project.
                    "hostUri": "A String", # The URI of a running Gerrit instance.
                  },
                },
              ],
              "context": { # A SourceContext is a reference to a tree of files. A SourceContext together # If provided, the source code used for the build came from this location.
                  # with a path point to a unique revision of a single file or directory.
                "git": { # A GitSourceContext denotes a particular revision in a third party Git # A SourceContext referring to any third party Git repo (e.g., GitHub).
                    # repository (e.g., GitHub).
                  "url": "A String", # Git repository URL.
                  "revisionId": "A String", # Required.
                      # Git commit hash.
                },
                "cloudRepo": { # A CloudRepoSourceContext denotes a particular revision in a Google Cloud # A SourceContext referring to a revision in a Google Cloud Source Repo.
                    # Source Repo.
                  "aliasContext": { # An alias to a repo revision. # An alias, which may be a branch or tag.
                    "kind": "A String", # The alias kind.
                    "name": "A String", # The alias name.
                  },
                  "revisionId": "A String", # A revision ID.
                  "repoId": { # A unique identifier for a Cloud Repo. # The ID of the repo.
                    "projectRepoId": { # Selects a repo using a Google Cloud Platform project ID (e.g., # A combination of a project ID and a repo name.
                        # winged-cargo-31) and a repo name within that project.
                      "projectId": "A String", # The ID of the project.
                      "repoName": "A String", # The name of the repo. Leave empty for the default repo.
                    },
                    "uid": "A String", # A server-assigned, globally unique identifier.
                  },
                },
                "labels": { # Labels with user defined metadata.
                  "a_key": "A String",
                },
                "gerrit": { # A SourceContext referring to a Gerrit project. # A SourceContext referring to a Gerrit project.
                  "aliasContext": { # An alias to a repo revision. # An alias, which may be a branch or tag.
                    "kind": "A String", # The alias kind.
                    "name": "A String", # The alias name.
                  },
                  "revisionId": "A String", # A revision (commit) ID.
                  "gerritProject": "A String", # The full project name within the host. Projects may be nested, so
                      # "project/subproject" is a valid project name. The "repo name" is
                      # the hostURI/project.
                  "hostUri": "A String", # The URI of a running Gerrit instance.
                },
              },
              "storageSource": { # StorageSource describes the location of the source in an archive file in # If provided, get the source from this location in in Google Cloud
                  # Storage.
                  # Google Cloud Storage.
                "generation": "A String", # Google Cloud Storage generation for the object.
                "object": "A String", # Google Cloud Storage object containing source.
                "bucket": "A String", # Google Cloud Storage bucket containing source (see [Bucket Name
                    # Requirements]
                    # (https://cloud.google.com/storage/docs/bucket-naming#requirements)).
              },
            },
            "buildOptions": { # Special options applied to this build. This is a catch-all field where
                # build providers can enter any desired additional details.
              "a_key": "A String",
            },
            "creator": "A String", # E-mail address of the user who initiated this build. Note that this was the
                # user's e-mail address at the time the build was initiated; this address may
                # not represent the same end-user for all time.
            "logsBucket": "A String", # Google Cloud Storage bucket where logs were written.
            "builderVersion": "A String", # Version string of the builder at the time this build was executed.
            "createTime": "A String", # Time at which the build was created.
            "builtArtifacts": [ # Output of the build.
              { # Artifact describes a build product.
                "checksum": "A String", # Hash or checksum value of a binary, or Docker Registry 2.0 digest of a
                    # container.
                "id": "A String", # Artifact ID, if any; for container images, this will be a URL by digest
                    # like gcr.io/projectID/imagename@sha256:123456
                "name": "A String", # Name of the artifact. This may be the path to a binary or jar file, or in
                    # the case of a container build, the name used to push the container image to
                    # Google Container Registry, as presented to `docker push`.
                    #
                    # This field is deprecated in favor of the plural `names` field; it continues
                    # to exist here to allow existing BuildProvenance serialized to json in
                    # google.devtools.containeranalysis.v1alpha1.BuildDetails.provenance_bytes to
                    # deserialize back into proto.
                "names": [ # Related artifact names. This may be the path to a binary or jar file, or in
                    # the case of a container build, the name used to push the container image to
                    # Google Container Registry, as presented to `docker push`. Note that a
                    # single Artifact ID can have multiple names, for example if two tags are
                    # applied to one image.
                  "A String",
                ],
              },
            ],
            "triggerId": "A String", # Trigger identifier if the build was triggered automatically; empty if not.
            "startTime": "A String", # Time at which execution of the build was started.
            "projectId": "A String", # ID of the project.
            "id": "A String", # Unique identifier of the build.
          },
          "provenanceBytes": "A String", # Serialized JSON representation of the provenance, used in generating the
              # `BuildSignature` in the corresponding Result. After verifying the
              # signature, `provenance_bytes` can be unmarshalled and compared to the
              # provenance to confirm that it is unchanged. A base64-encoded string
              # representation of the provenance bytes is used for the signature in order
              # to interoperate with openssl which expects this format for signature
              # verification.
              #
              # The serialized form is captured both to avoid ambiguity in how the
              # provenance is marshalled to json as well to prevent incompatibilities with
              # future changes.
        },
        "discovered": { # Provides information about the scan status of a discovered resource. # Describes the initial scan status for this resource.
          "analysisStatus": "A String", # The status of discovery for the resource.
          "operation": { # This resource represents a long-running operation that is the result of a # Output only. An operation that indicates the status of the current scan.
              # This field is deprecated, do not use.
              # network API call.
            "error": { # The `Status` type defines a logical error model that is suitable for # The error result of the operation in case of failure or cancellation.
                # different programming environments, including REST APIs and RPC APIs. It is
                # used by [gRPC](https://github.com/grpc). Each `Status` message contains
                # three pieces of data: error code, error message, and error details.
                #
                # You can find out more about this error model and how to work with it in the
                # [API Design Guide](https://cloud.google.com/apis/design/errors).
              "message": "A String", # A developer-facing error message, which should be in English. Any
                  # user-facing error message should be localized and sent in the
                  # google.rpc.Status.details field, or localized by the client.
              "code": 42, # The status code, which should be an enum value of google.rpc.Code.
              "details": [ # A list of messages that carry the error details.  There is a common set of
                  # message types for APIs to use.
                {
                  "a_key": "", # Properties of the object. Contains field @type with type URL.
                },
              ],
            },
            "done": True or False, # If the value is `false`, it means the operation is still in progress.
                # If `true`, the operation is completed, and either `error` or `response` is
                # available.
            "response": { # The normal response of the operation in case of success.  If the original
                # method returns no data on success, such as `Delete`, the response is
                # `google.protobuf.Empty`.  If the original method is standard
                # `Get`/`Create`/`Update`, the response should be the resource.  For other
                # methods, the response should have the type `XxxResponse`, where `Xxx`
                # is the original method name.  For example, if the original method name
                # is `TakeSnapshot()`, the inferred response type is
                # `TakeSnapshotResponse`.
              "a_key": "", # Properties of the object. Contains field @type with type URL.
            },
            "name": "A String", # The server-assigned name, which is only unique within the same service that
                # originally returns it. If you use the default HTTP mapping, the
                # `name` should be a resource name ending with `operations/{unique_id}`.
            "metadata": { # Service-specific metadata associated with the operation.  It typically
                # contains progress information and common metadata such as create time.
                # Some services might not provide such metadata.  Any method that returns a
                # long-running operation should document the metadata type, if any.
              "a_key": "", # Properties of the object. Contains field @type with type URL.
            },
          },
          "analysisStatusError": { # The `Status` type defines a logical error model that is suitable for # When an error is encountered this will contain a LocalizedMessage under
              # details to show to the user. The LocalizedMessage output only and
              # populated by the API.
              # different programming environments, including REST APIs and RPC APIs. It is
              # used by [gRPC](https://github.com/grpc). Each `Status` message contains
              # three pieces of data: error code, error message, and error details.
              #
              # You can find out more about this error model and how to work with it in the
              # [API Design Guide](https://cloud.google.com/apis/design/errors).
            "message": "A String", # A developer-facing error message, which should be in English. Any
                # user-facing error message should be localized and sent in the
                # google.rpc.Status.details field, or localized by the client.
            "code": 42, # The status code, which should be an enum value of google.rpc.Code.
            "details": [ # A list of messages that carry the error details.  There is a common set of
                # message types for APIs to use.
              {
                "a_key": "", # Properties of the object. Contains field @type with type URL.
              },
            ],
          },
          "continuousAnalysis": "A String", # Whether the resource is continuously analyzed.
        },
        "attestation": { # Occurrence that represents a single "attestation".  The authenticity of an # Describes an attestation of an artifact.
            # Attestation can be verified using the attached signature. If the verifier
            # trusts the public key of the signer, then verifying the signature is
            # sufficient to establish trust.  In this circumstance, the
            # AttestationAuthority to which this Attestation is attached is primarily
            # useful for look-up (how to find this Attestation if you already know the
            # Authority and artifact to be verified) and intent (which authority was this
            # attestation intended to sign for).
          "pgpSignedAttestation": { # An attestation wrapper with a PGP-compatible signature.
              # This message only supports `ATTACHED` signatures, where the payload that is
              # signed is included alongside the signature itself in the same file.
            "pgpKeyId": "A String", # The cryptographic fingerprint of the key used to generate the signature,
                # as output by, e.g. `gpg --list-keys`. This should be the version 4, full
                # 160-bit fingerprint, expressed as a 40 character hexadecimal string. See
                # https://tools.ietf.org/html/rfc4880#section-12.2 for details.
                # Implementations may choose to acknowledge "LONG", "SHORT", or other
                # abbreviated key IDs, but only the full fingerprint is guaranteed to work.
                # In gpg, the full fingerprint can be retrieved from the `fpr` field
                # returned when calling --list-keys with --with-colons.  For example:
                # ```
                # gpg --with-colons --with-fingerprint --force-v4-certs \
                #     --list-keys attester@example.com
                # tru::1:1513631572:0:3:1:5
                # pub:......
                # fpr:::::::::24FF6481B76AC91E66A00AC657A93A81EF3AE6FB:
                # ```
                # Above, the fingerprint is `24FF6481B76AC91E66A00AC657A93A81EF3AE6FB`.
            "contentType": "A String", # Type (for example schema) of the attestation payload that was signed.
                # The verifier must ensure that the provided type is one that the verifier
                # supports, and that the attestation payload is a valid instantiation of that
                # type (for example by validating a JSON schema).
            "signature": "A String", # The raw content of the signature, as output by GNU Privacy Guard (GPG) or
                # equivalent.  Since this message only supports attached signatures, the
                # payload that was signed must be attached. While the signature format
                # supported is dependent on the verification implementation, currently only
                # ASCII-armored (`--armor` to gpg), non-clearsigned (`--sign` rather than
                # `--clearsign` to gpg) are supported. Concretely, `gpg --sign --armor
                # --output=signature.gpg payload.json` will create the signature content
                # expected in this field in `signature.gpg` for the `payload.json`
                # attestation payload.
          },
        },
        "noteName": "A String", # An analysis note associated with this image, in the form
            # "providers/{provider_id}/notes/{NOTE_ID}"
            # This field can be used as a filter in list requests.
        "deployment": { # The period during which some deployable was active in a runtime. # Describes the deployment of an artifact on a runtime.
          "resourceUri": [ # Output only. Resource URI for the artifact being deployed taken from the
              # deployable field with the same name.
            "A String",
          ],
          "userEmail": "A String", # Identity of the user that triggered this deployment.
          "address": "A String", # Address of the runtime element hosting this deployment.
          "platform": "A String", # Platform hosting this deployment.
          "deployTime": "A String", # Beginning of the lifetime of this deployment.
          "undeployTime": "A String", # End of the lifetime of this deployment.
          "config": "A String", # Configuration used to create this deployment.
        },
        "remediation": "A String", # A description of actions that can be taken to remedy the `Note`
        "vulnerabilityDetails": { # Used by Occurrence to point to where the vulnerability exists and how # Details of a security vulnerability note.
            # to fix it.
          "packageIssue": [ # The set of affected locations and their fixes (if available) within
              # the associated resource.
            { # This message wraps a location affected by a vulnerability and its
                # associated fix (if one is available).
              "severityName": "A String",
              "affectedLocation": { # The location of the vulnerability # The location of the vulnerability.
                "cpeUri": "A String", # The cpe_uri in [cpe format] (https://cpe.mitre.org/specification/)
                    # format. Examples include distro or storage location for vulnerable jar.
                    # This field can be used as a filter in list requests.
                "version": { # Version contains structured information about the version of the package. # The version of the package being described. This field can be used as a
                    # filter in list requests.
                    # For a discussion of this in Debian/Ubuntu:
                    # http://serverfault.com/questions/604541/debian-packages-version-convention
                    # For a discussion of this in Redhat/Fedora/Centos:
                    # http://blog.jasonantman.com/2014/07/how-yum-and-rpm-compare-versions/
                  "kind": "A String", # Distinguish between sentinel MIN/MAX versions and normal versions.
                      # If kind is not NORMAL, then the other fields are ignored.
                  "revision": "A String", # The iteration of the package build from the above version.
                  "name": "A String", # The main part of the version name.
                  "epoch": 42, # Used to correct mistakes in the version numbering scheme.
                },
                "package": "A String", # The package being described.
              },
              "fixedLocation": { # The location of the vulnerability # The location of the available fix for vulnerability.
                "cpeUri": "A String", # The cpe_uri in [cpe format] (https://cpe.mitre.org/specification/)
                    # format. Examples include distro or storage location for vulnerable jar.
                    # This field can be used as a filter in list requests.
                "version": { # Version contains structured information about the version of the package. # The version of the package being described. This field can be used as a
                    # filter in list requests.
                    # For a discussion of this in Debian/Ubuntu:
                    # http://serverfault.com/questions/604541/debian-packages-version-convention
                    # For a discussion of this in Redhat/Fedora/Centos:
                    # http://blog.jasonantman.com/2014/07/how-yum-and-rpm-compare-versions/
                  "kind": "A String", # Distinguish between sentinel MIN/MAX versions and normal versions.
                      # If kind is not NORMAL, then the other fields are ignored.
                  "revision": "A String", # The iteration of the package build from the above version.
                  "name": "A String", # The main part of the version name.
                  "epoch": 42, # Used to correct mistakes in the version numbering scheme.
                },
                "package": "A String", # The package being described.
              },
            },
          ],
          "type": "A String", # The type of package; whether native or non native(ruby gems,
              # node.js packages etc)
          "cvssScore": 3.14, # Output only. The CVSS score of this vulnerability. CVSS score is on a
              # scale of 0-10 where 0 indicates low severity and 10 indicates high
              # severity.
          "severity": "A String", # Output only. The note provider assigned Severity of the vulnerability.
          "effectiveSeverity": "A String", # The distro assigned severity for this vulnerability when that is
              # available and note provider assigned severity when distro has not yet
              # assigned a severity for this vulnerability.
        },
        "createTime": "A String", # Output only. The time this `Occurrence` was created.
        "derivedImage": { # Derived describes the derived image portion (Occurrence) of the # Describes how this resource derives from the basis
            # in the associated note.
            # DockerImage relationship.  This image would be produced from a Dockerfile
            # with FROM .
          "distance": 42, # Output only. The number of layers by which this image differs from the
              # associated image basis.
          "baseResourceUrl": "A String", # Output only. This contains the base image URL for the derived image
              # occurrence.
          "layerInfo": [ # This contains layer-specific metadata, if populated it has length
              # "distance" and is ordered with [distance] being the layer immediately
              # following the base image and [1] being the final layer.
            { # Layer holds metadata specific to a layer of a Docker image.
              "arguments": "A String", # The recovered arguments to the Dockerfile directive.
              "directive": "A String", # The recovered Dockerfile directive used to construct this layer.
            },
          ],
          "fingerprint": { # A set of properties that uniquely identify a given Docker image. # The fingerprint of the derived image.
            "v1Name": "A String", # The layer-id of the final layer in the Docker image's v1
                # representation.
                # This field can be used as a filter in list requests.
            "v2Blob": [ # The ordered list of v2 blobs that represent a given image.
              "A String",
            ],
            "v2Name": "A String", # Output only. The name of the image's v2 blobs computed via:
                #   [bottom] := v2_blobbottom := sha256(v2_blob[N] + " " + v2_name[N+1])
                # Only the name of the final blob is kept.
                # This field can be used as a filter in list requests.
          },
        },
        "resourceUrl": "A String", # The unique URL of the image or the container for which the `Occurrence`
            # applies. For example, https://gcr.io/project/image@sha256:foo This field
            # can be used as a filter in list requests.
      },
    ],
  }
list_next(previous_request, previous_response)
Retrieves the next page of results.

Args:
  previous_request: The request for the previous page. (required)
  previous_response: The response from the request for the previous page. (required)

Returns:
  A request object that you can call 'execute()' on to request the next
  page. Returns None if there are no more items in the collection.