[Transform] Module Interface for Transform (#3636)

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Co-authored-by: Ubuntu <ubuntu@ip-172-31-31-136.us-west-2.compute.internal>

docs/source/api/python/index.rst

@@ -15,3 +15,4 @@ API Reference
    dgl.sampling
    dgl.contrib.UnifiedTensor
    udf
+   transform

docs/source/api/python/transform.rst

+.. _apitransform-namespace:
+
+dgl.transform
+=============
+
+.. currentmodule:: dgl.transform
+.. automodule:: dgl.transform
+
+BaseTransform
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: BaseTransform
+    :members: __call__, __repr__
+    :show-inheritance:
+
+AddSelfLoop
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: AddSelfLoop
+    :show-inheritance:
+
+RemoveSelfLoop
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: RemoveSelfLoop
+    :show-inheritance:
+
+AddReverse
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: AddReverse
+    :show-inheritance:
+
+ToSimple
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: ToSimple
+    :show-inheritance:
+
+LineGraph
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: LineGraph
+    :show-inheritance:
+
+KHopGraph
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: KHopGraph
+    :show-inheritance:
+
+AddMetaPaths
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: AddMetaPaths
+    :show-inheritance:
+
+KNNGraph
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: KNNGraph
+    :show-inheritance:

python/dgl/transform/__init__.py

+"""Transform for structures and features"""
+from .functional import *
+from .module import *

python/dgl/transform.py → python/dgl/transform/functional.py

@@ -13,7 +13,7 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 #
-"""Module for graph transformation utilities."""
+"""Functional interface for transform"""
 
 from collections.abc import Iterable, Mapping
 from collections import defaultdict
@@ -21,22 +21,22 @@ import numpy as np
 import scipy.sparse as sparse
 import scipy.sparse.linalg
 
-from ._ffi.function import _init_api
-from .base import dgl_warning, DGLError
-from . import convert
-from .heterograph import DGLHeteroGraph, DGLBlock
-from .heterograph_index import create_metagraph_index, create_heterograph_from_relations
-from .frame import Frame
-from . import ndarray as nd
-from . import backend as F
-from . import utils, batch
-from .partition import metis_partition_assignment
-from .partition import partition_graph_with_halo
-from .partition import metis_partition
-from . import subgraph
+from .._ffi.function import _init_api
+from ..base import dgl_warning, DGLError
+from .. import convert
+from ..heterograph import DGLHeteroGraph, DGLBlock
+from ..heterograph_index import create_metagraph_index, create_heterograph_from_relations
+from ..frame import Frame
+from .. import ndarray as nd
+from .. import backend as F
+from .. import utils, batch
+from ..partition import metis_partition_assignment
+from ..partition import partition_graph_with_halo
+from ..partition import metis_partition
+from .. import subgraph
 
 # TO BE DEPRECATED
-from ._deprecate.graph import DGLGraph as DGLGraphStale
+from .._deprecate.graph import DGLGraph as DGLGraphStale
 
 __all__ = [
     'line_graph',
@@ -93,7 +93,7 @@ def knn_graph(x, k, algorithm='bruteforce-blas', dist='euclidean'):
     columns correspond to coordinate/feature dimensions.
 
     The nodes of the returned graph correspond to the points, where the predecessors
-    of each point are its k-nearest neighbors measured by the Euclidean distance.
+    of each point are its k-nearest neighbors measured by the chosen distance.
 
     If :attr:`x` is a 3D tensor, then each submatrix will be transformed
     into a separate graph. DGL then composes the graphs into a large
@@ -715,7 +715,7 @@ def to_bidirected(g, copy_ndata=False, readonly=None):
 
 def add_reverse_edges(g, readonly=None, copy_ndata=True,
                       copy_edata=False, ignore_bipartite=False):
-    r"""Add an reversed edge for each edge in the input graph and return a new graph.
+    r"""Add a reversed edge for each edge in the input graph and return a new graph.
 
     For a graph with edges :math:`(i_1, j_1), \cdots, (i_n, j_n)`, this
     function creates a new graph with edges
@@ -740,7 +740,7 @@ def add_reverse_edges(g, readonly=None, copy_ndata=True,
         (Default: True)
     copy_edata: bool, optional
         If True, the features of the reversed edges will be identical to
-        the original ones."
+        the original ones.
 
         If False, the new graph will not have any edge features.
 
@@ -865,7 +865,7 @@ def line_graph(g, backtracking=True, shared=False):
     """Return the line graph of this graph.
 
     The line graph ``L(G)`` of a given graph ``G`` is defined as another graph where
-    the nodes in ``L(G)`` maps to the edges in ``G``.  For any pair of edges ``(u, v)``
+    the nodes in ``L(G)`` correspond to the edges in ``G``.  For any pair of edges ``(u, v)``
     and ``(v, w)`` in ``G``, the corresponding node of edge ``(u, v)`` in ``L(G)`` will
     have an edge connecting to the corresponding node of edge ``(v, w)``.
 
@@ -1050,7 +1050,7 @@ def khop_graph(g, k, copy_ndata=True):
     col = np.repeat(adj_k.col, multiplicity)
     # TODO(zihao): we should support creating multi-graph from scipy sparse matrix
     # in the future.
-    new_g = convert.graph((row, col), num_nodes=n)
+    new_g = convert.graph((row, col), num_nodes=n, idtype=g.idtype, device=g.device)
 
     # handle ndata
     if copy_ndata:
@@ -3195,4 +3195,4 @@ def rcmk_perm(g):
     perm = sparse.csgraph.reverse_cuthill_mckee(csr_adj)
     return perm.copy()
 
-_init_api("dgl.transform")
+_init_api("dgl.transform", __name__)

python/dgl/transform/module.py

+##
+#   Copyright 2019-2021 Contributors
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+#
+"""Modules for transform"""
+
+from .. import convert
+from .. import backend as F
+from . import functional
+
+__all__ = [
+    'BaseTransform',
+    'AddSelfLoop',
+    'RemoveSelfLoop',
+    'AddReverse',
+    'ToSimple',
+    'LineGraph',
+    'KHopGraph',
+    'AddMetaPaths',
+    'Compose'
+]
+
+def update_graph_structure(g, data_dict, copy_edata=True):
+    r"""
+
+    Description
+    -----------
+    Update the structure of a graph.
+
+    Parameters
+    ----------
+    g : DGLGraph
+        The graph to update.
+    data_dict : graph data
+        The dictionary data for constructing a heterogeneous graph.
+    copy_edata : bool
+        If True, it will copy the edge features to the updated graph.
+
+    Returns
+    -------
+    DGLGraph
+        The updated graph.
+    """
+    device = g.device
+    idtype = g.idtype
+    num_nodes_dict = dict()
+
+    for ntype in g.ntypes:
+        num_nodes_dict[ntype] = g.num_nodes(ntype)
+
+    new_g = convert.heterograph(data_dict, num_nodes_dict=num_nodes_dict,
+                                idtype=idtype, device=device)
+
+    # Copy features
+    for ntype in g.ntypes:
+        for key, feat in g.nodes[ntype].data.items():
+            new_g.nodes[ntype].data[key] = feat
+
+    if copy_edata:
+        for c_etype in g.canonical_etypes:
+            for key, feat in g.edges[c_etype].data.items():
+                new_g.edges[c_etype].data[key] = feat
+
+    return new_g
+
+class BaseTransform:
+    r"""
+
+    Description
+    -----------
+    An abstract class for writing transforms.
+    """
+    def __call__(self, g):
+        raise NotImplementedError
+
+    def __repr__(self):
+        return self.__class__.__name__ + '()'
+
+class AddSelfLoop(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Add self-loops for each node in the graph and return a new graph.
+
+    For heterogeneous graphs, self-loops are added only for edge types with same
+    source and destination node types.
+
+    Parameters
+    ----------
+    allow_duplicate : bool, optional
+        If False, it will first remove self-loops to prevent duplicate self-loops.
+    new_etypes : bool, optional
+        If True, it will add an edge type 'self' per node type, which holds self-loops.
+
+    Example
+    -------
+
+    >>> import dgl
+    >>> from dgl import AddSelfLoop
+
+    Case1: Add self-loops for a homogeneous graph
+
+    >>> transform = AddSelfLoop()
+    >>> g = dgl.graph(([1, 1], [1, 2]))
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([1, 0, 1, 2]), tensor([2, 0, 1, 2]))
+
+    Case2: Add self-loops for a heterogeneous graph
+
+    >>> g = dgl.heterograph({
+    ...     ('user', 'plays', 'game'): ([0], [1]),
+    ...     ('user', 'follows', 'user'): ([1], [2])
+    ... })
+    >>> new_g = transform(g)
+    >>> print(new_g.edges(etype='plays'))
+    (tensor([0]), tensor([1]))
+    >>> print(new_g.edges(etype='follows'))
+    (tensor([1, 0, 1, 2]), tensor([2, 0, 1, 2]))
+
+    Case3: Add self-etypes for a heterogeneous graph
+
+    >>> transform = AddSelfLoop(new_etypes=True)
+    >>> new_g = transform(g)
+    >>> print(new_g.edges(etype='follows'))
+    (tensor([1, 0, 1, 2]), tensor([2, 0, 1, 2]))
+    >>> print(new_g.edges(etype=('game', 'self', 'game')))
+    (tensor([0, 1]), tensor([0, 1]))
+    """
+    def __init__(self, allow_duplicate=False, new_etypes=False):
+        self.allow_duplicate = allow_duplicate
+        self.new_etypes = new_etypes
+
+    def transform_etype(self, c_etype, g):
+        r"""
+
+        Description
+        -----------
+        Transform the graph corresponding to a canonical edge type.
+
+        Parameters
+        ----------
+        c_etype : tuple of str
+            A canonical edge type.
+        g : DGLGraph
+            The graph.
+
+        Returns
+        -------
+        DGLGraph
+            The transformed graph.
+        """
+        utype, _, vtype = c_etype
+        if utype != vtype:
+            return g
+
+        if not self.allow_duplicate:
+            g = functional.remove_self_loop(g, etype=c_etype)
+        return functional.add_self_loop(g, etype=c_etype)
+
+    def __call__(self, g):
+        for c_etype in g.canonical_etypes:
+            g = self.transform_etype(c_etype, g)
+
+        if self.new_etypes:
+            device = g.device
+            idtype = g.idtype
+            data_dict = dict()
+
+            # Add self etypes
+            for ntype in g.ntypes:
+                nids = F.arange(0, g.num_nodes(ntype), idtype, device)
+                data_dict[(ntype, 'self', ntype)] = (nids, nids)
+
+            # Copy edges
+            for c_etype in g.canonical_etypes:
+                data_dict[c_etype] = g.edges(etype=c_etype)
+
+            g = update_graph_structure(g, data_dict)
+        return g
+
+class RemoveSelfLoop(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Remove self-loops for each node in the graph and return a new graph.
+
+    For heterogeneous graphs, this operation only applies to edge types with same
+    source and destination node types.
+
+    Example
+    -------
+
+    >>> import dgl
+    >>> from dgl import RemoveSelfLoop
+
+    Case1: Remove self-loops for a homogeneous graph
+
+    >>> transform = RemoveSelfLoop()
+    >>> g = dgl.graph(([1, 1], [1, 2]))
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([1]), tensor([2]))
+
+    Case2: Remove self-loops for a heterogeneous graph
+
+    >>> g = dgl.heterograph({
+    ...     ('user', 'plays', 'game'): ([0, 1], [1, 1]),
+    ...     ('user', 'follows', 'user'): ([1, 2], [2, 2])
+    ... })
+    >>> new_g = transform(g)
+    >>> print(new_g.edges(etype='plays'))
+    (tensor([0, 1]), tensor([1, 1]))
+    >>> print(new_g.edges(etype='follows'))
+    (tensor([1]), tensor([2]))
+    """
+    def transform_etype(self, c_etype, g):
+        r"""
+
+        Description
+        -----------
+        Transform the graph corresponding to a canonical edge type.
+
+        Parameters
+        ----------
+        c_etype : tuple of str
+            A canonical edge type.
+        g : DGLGraph
+            The graph.
+
+        Returns
+        -------
+        DGLGraph
+            The transformed graph.
+        """
+        utype, _, vtype = c_etype
+        if utype == vtype:
+            g = functional.remove_self_loop(g, etype=c_etype)
+        return g
+
+    def __call__(self, g):
+        for c_etype in g.canonical_etypes:
+            g = self.transform_etype(c_etype, g)
+        return g
+
+class AddReverse(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Add a reverse edge :math:`(i,j)` for each edge :math:`(j,i)` in the input graph and
+    return a new graph.
+
+    For a heterogeneous graph, it adds a "reverse" edge type for each edge type
+    to hold the reverse edges. For example, for a canonical edge type ('A', 'r', 'B'),
+    it adds a canonical edge type ('B', 'rev_r', 'A').
+
+    Parameters
+    ----------
+    copy_edata : bool, optional
+        If True, the features of the reverse edges will be identical to the original ones.
+    sym_new_etype : bool, optional
+        If False, it will not add a reverse edge type if the source and destination node type
+        in a canonical edge type are identical. Instead, it will directly add edges to the
+        original edge type.
+
+    Example
+    -------
+
+    The following example uses PyTorch backend.
+
+    >>> import dgl
+    >>> import torch
+    >>> from dgl import AddReverse
+
+    Case1: Add reverse edges for a homogeneous graph
+
+    >>> transform = AddReverse()
+    >>> g = dgl.graph(([0], [1]))
+    >>> g.edata['w'] = torch.ones(1, 2)
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([0, 1]), tensor([1, 0]))
+    >>> print(new_g.edata['w'])
+    tensor([[1., 1.],
+            [0., 0.]])
+
+    Case2: Add reverse edges for a homogeneous graph and copy edata
+
+    >>> transform = AddReverse(copy_edata=True)
+    >>> new_g = transform(g)
+    >>> print(new_g.edata['w'])
+    tensor([[1., 1.],
+            [1., 1.]])
+
+    Case3: Add reverse edges for a heterogeneous graph
+
+    >>> g = dgl.heterograph({
+    ...     ('user', 'plays', 'game'): ([0, 1], [1, 1]),
+    ...     ('user', 'follows', 'user'): ([1, 2], [2, 2])
+    ... })
+    >>> new_g = transform(g)
+    >>> print(new_g.canonical_etypes)
+    [('game', 'rev_plays', 'user'), ('user', 'follows', 'user'), ('user', 'plays', 'game')]
+    >>> print(new_g.edges(etype='rev_plays'))
+    (tensor([1, 1]), tensor([0, 1]))
+    >>> print(new_g.edges(etype='follows'))
+    (tensor([1, 2, 2, 2]), tensor([2, 2, 1, 2]))
+    """
+    def __init__(self, copy_edata=False, sym_new_etype=False):
+        self.copy_edata = copy_edata
+        self.sym_new_etype = sym_new_etype
+
+    def transform_symmetric_etype(self, c_etype, g, data_dict):
+        r"""
+
+        Description
+        -----------
+        Transform the graph corresponding to a symmetric canonical edge type.
+
+        Parameters
+        ----------
+        c_etype : tuple of str
+            A canonical edge type.
+        g : DGLGraph
+            The graph.
+        data_dict : dict
+            The edge data to update.
+        """
+        if self.sym_new_etype:
+            self.transform_asymmetric_etype(c_etype, g, data_dict)
+        else:
+            src, dst = g.edges(etype=c_etype)
+            src, dst = F.cat([src, dst], dim=0), F.cat([dst, src], dim=0)
+            data_dict[c_etype] = (src, dst)
+
+    def transform_asymmetric_etype(self, c_etype, g, data_dict):
+        r"""
+
+        Description
+        -----------
+        Transform the graph corresponding to an asymmetric canonical edge type.
+
+        Parameters
+        ----------
+        c_etype : tuple of str
+            A canonical edge type.
+        g : DGLGraph
+            The graph.
+        data_dict : dict
+            The edge data to update.
+        """
+        utype, etype, vtype = c_etype
+        src, dst = g.edges(etype=c_etype)
+        data_dict.update({
+            c_etype: (src, dst),
+            (vtype, 'rev_{}'.format(etype), utype): (dst, src)
+        })
+
+    def transform_etype(self, c_etype, g, data_dict):
+        r"""
+
+        Description
+        -----------
+        Transform the graph corresponding to a canonical edge type.
+
+        Parameters
+        ----------
+        c_etype : tuple of str
+            A canonical edge type.
+        g : DGLGraph
+            The graph.
+        data_dict : dict
+            The edge data to update.
+        """
+        utype, _, vtype = c_etype
+        if utype == vtype:
+            self.transform_symmetric_etype(c_etype, g, data_dict)
+        else:
+            self.transform_asymmetric_etype(c_etype, g, data_dict)
+
+    def __call__(self, g):
+        data_dict = dict()
+        for c_etype in g.canonical_etypes:
+            self.transform_etype(c_etype, g, data_dict)
+        new_g = update_graph_structure(g, data_dict, copy_edata=False)
+
+        # Copy and expand edata
+        for c_etype in g.canonical_etypes:
+            utype, etype, vtype = c_etype
+            if utype != vtype or self.sym_new_etype:
+                rev_c_etype = (vtype, 'rev_{}'.format(etype), utype)
+                for key, feat in g.edges[c_etype].data.items():
+                    new_g.edges[c_etype].data[key] = feat
+                    if self.copy_edata:
+                        new_g.edges[rev_c_etype].data[key] = feat
+            else:
+                for key, feat in g.edges[c_etype].data.items():
+                    new_feat = feat if self.copy_edata else F.zeros(
+                        F.shape(feat), F.dtype(feat), F.context(feat))
+                    new_g.edges[c_etype].data[key] = F.cat([feat, new_feat], dim=0)
+
+        return new_g
+
+class ToSimple(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Convert a graph to a simple graph without parallel edges and return a new graph.
+
+    Parameters
+    ----------
+    return_counts : str, optional
+        The edge feature name to hold the edge count in the original graph.
+    aggregator : str, optional
+        The way to coalesce features of duplicate edges.
+
+        * ``'arbitrary'``: select arbitrarily from one of the duplicate edges
+        * ``'sum'``: take the sum over the duplicate edges
+        * ``'mean'``: take the mean over the duplicate edges
+
+    Example
+    -------
+
+    The following example uses PyTorch backend.
+
+    >>> import dgl
+    >>> import torch
+    >>> from dgl import ToSimple
+
+    Case1: Convert a homogeneous graph to a simple graph
+
+    >>> transform = ToSimple()
+    >>> g = dgl.graph(([0, 1, 1], [1, 2, 2]))
+    >>> g.edata['w'] = torch.tensor([[0.1], [0.2], [0.3]])
+    >>> sg = transform(g)
+    >>> print(sg.edges())
+    (tensor([0, 1]), tensor([1, 2]))
+    >>> print(sg.edata['count'])
+    tensor([1, 2])
+    >>> print(sg.edata['w'])
+    tensor([[0.1000], [0.2000]])
+
+    Case2: Convert a heterogeneous graph to a simple graph
+
+    >>> g = dgl.heterograph({
+    ...     ('user', 'follows', 'user'): ([0, 1, 1], [1, 2, 2]),
+    ...     ('user', 'plays', 'game'): ([0, 1, 0], [1, 1, 1])
+    ... })
+    >>> sg = transform(g)
+    >>> print(sg.edges(etype='follows'))
+    (tensor([0, 1]), tensor([1, 2]))
+    >>> print(sg.edges(etype='plays'))
+    (tensor([0, 1]), tensor([1, 1]))
+    """
+    def __init__(self, return_counts='count', aggregator='arbitrary'):
+        self.return_counts = return_counts
+        self.aggregator = aggregator
+
+    def __call__(self, g):
+        return functional.to_simple(g,
+                                    return_counts=self.return_counts,
+                                    copy_edata=True,
+                                    aggregator=self.aggregator)
+
+class LineGraph(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Return the line graph of the input graph.
+
+    The line graph :math:`L(G)` of a given graph :math:`G` is a graph where
+    the nodes in :math:`L(G)` correspond to the edges in :math:`G`. For a pair
+    of edges :math:`(u, v)` and :math:`(v, w)` in :math:`G`, there will be an
+    edge from the node corresponding to :math:`(u, v)` to the node corresponding to
+    :math:`(v, w)` in :math:`L(G)`.
+
+    This module only works for homogeneous graphs.
+
+    Parameters
+    ----------
+    backtracking : bool, optional
+        If False, there will be an edge from the line graph node corresponding to
+        :math:`(u, v)` to the line graph node corresponding to :math:`(v, u)`.
+
+    Example
+    -------
+
+    The following example uses PyTorch backend.
+
+    >>> import dgl
+    >>> import torch
+    >>> from dgl import LineGraph
+
+    Case1: Backtracking is True
+
+    >>> transform = LineGraph()
+    >>> g = dgl.graph(([0, 1, 1], [1, 0, 2]))
+    >>> g.ndata['h'] = torch.tensor([[0.], [1.], [2.]])
+    >>> g.edata['w'] = torch.tensor([[0.], [0.1], [0.2]])
+    >>> new_g = transform(g)
+    >>> print(new_g)
+    Graph(num_nodes=3, num_edges=3,
+          ndata_schemes={'w': Scheme(shape=(1,), dtype=torch.float32)}
+          edata_schemes={})
+    >>> print(new_g.edges())
+    (tensor([0, 0, 1]), tensor([1, 2, 0]))
+
+    Case2: Backtracking is False
+
+    >>> transform = LineGraph(backtracking=False)
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([0]), tensor([2]))
+    """
+    def __init__(self, backtracking=True):
+        self.backtracking = backtracking
+
+    def __call__(self, g):
+        return functional.line_graph(g, backtracking=self.backtracking, shared=True)
+
+class KHopGraph(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Return the graph whose edges connect the :math:`k`-hop neighbors of the original graph.
+
+    This module only works for homogeneous graphs.
+
+    Parameters
+    ----------
+    k : int
+        The number of hops.
+
+    Example
+    -------
+
+    >>> import dgl
+    >>> from dgl import KHopGraph
+
+    >>> transform = KHopGraph(2)
+    >>> g = dgl.graph(([0, 1], [1, 2]))
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([0]), tensor([2]))
+    """
+    def __init__(self, k):
+        self.k = k
+
+    def __call__(self, g):
+        return functional.khop_graph(g, self.k)
+
+class AddMetaPaths(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Add new edges to an input graph based on given metapaths, as described in
+    `Heterogeneous Graph Attention Network <https://arxiv.org/abs/1903.07293>`__. Formally,
+    a metapath is a path of the form
+
+    .. math::
+
+        \mathcal{V}_1 \xrightarrow{R_1} \mathcal{V}_2 \xrightarrow{R_2} \ldots
+        \xrightarrow{R_{\ell-1}} \mathcal{V}_{\ell}
+
+    in which :math:`\mathcal{V}_i` represents a node type and :math:`\xrightarrow{R_j}`
+    represents a relation type connecting its two adjacent node types. The adjacency matrix
+    corresponding to the metapath is obtained by sequential multiplication of adjacency matrices
+    along the metapath.
+
+    Parameters
+    ----------
+    metapaths : dict[str, list]
+        The metapaths to add, mapping a metapath name to a metapath. For example,
+        :attr:`{'co-author': [('person', 'author', 'paper'), ('paper', 'authored by', 'person')]}`
+    keep_orig_edges : bool, optional
+        If True, it will keep the edges of the original graph. Otherwise, it will drop them.
+
+    Example
+    -------
+
+    >>> import dgl
+    >>> from dgl import AddMetaPaths
+
+    >>> transform = AddMetaPaths({
+    ...     'accepted': [('person', 'author', 'paper'), ('paper', 'accepted', 'venue')],
+    ...     'rejected': [('person', 'author', 'paper'), ('paper', 'rejected', 'venue')]
+    ... })
+    >>> g = dgl.heterograph({
+    ...     ('person', 'author', 'paper'): ([0, 0, 1], [1, 2, 2]),
+    ...     ('paper', 'accepted', 'venue'): ([1], [0]),
+    ...     ('paper', 'rejected', 'venue'): ([2], [1])
+    ... })
+    >>> new_g = transform(g)
+    >>> print(new_g.edges(etype=('person', 'accepted', 'venue')))
+    (tensor([0]), tensor([0]))
+    >>> print(new_g.edges(etype=('person', 'rejected', 'venue')))
+    (tensor([0, 1]), tensor([1, 1]))
+    """
+    def __init__(self, metapaths, keep_orig_edges=True):
+        self.metapaths = metapaths
+        self.keep_orig_edges = keep_orig_edges
+
+    def __call__(self, g):
+        data_dict = dict()
+
+        for meta_etype, metapath in self.metapaths.items():
+            meta_g = functional.metapath_reachable_graph(g, metapath)
+            u_type = metapath[0][0]
+            v_type = metapath[-1][-1]
+            data_dict[(u_type, meta_etype, v_type)] = meta_g.edges()
+
+        if self.keep_orig_edges:
+            for c_etype in g.canonical_etypes:
+                data_dict[c_etype] = g.edges(etype=c_etype)
+            new_g = update_graph_structure(g, data_dict, copy_edata=True)
+        else:
+            new_g = update_graph_structure(g, data_dict, copy_edata=False)
+
+        return new_g
+
+class Compose(BaseTransform):
+    r"""
+
+    Description
+    -----------
+    Create a transform composed of multiple transforms in sequence.
+
+    Parameters
+    ----------
+    transforms : list of Callable
+        A list of transform objects to apply in order. A transform object should inherit
+        :class:`~dgl.BaseTransform` and implement :func:`~dgl.BaseTransform.__call__`.
+
+    Example
+    -------
+
+    >>> import dgl
+    >>> from dgl import transform as T
+
+    >>> g = dgl.graph(([0, 0], [1, 1]))
+    >>> transform = T.Compose([T.ToSimple(), T.AddReverse()])
+    >>> new_g = transform(g)
+    >>> print(new_g.edges())
+    (tensor([0, 1]), tensor([1, 0]))
+    """
+    def __init__(self, transforms):
+        self.transforms = transforms
+
+    def __call__(self, g):
+        for transform in self.transforms:
+            g = transform(g)
+        return g
+
+    def __repr__(self):
+        args = ['  ' + str(transform) for transform in self.transforms]
+        return self.__class__.__name__ + '([\n' + ',\n'.join(args) + '\n])'

tests/compute/test_transform.py

@@ -1800,5 +1800,361 @@ def test_reorder_graph(idtype):
     rfg = dgl.reorder_graph(fg)
     assert 'csr' in sum(rfg.formats().values(), [])
 
+@parametrize_dtype
+def test_module_add_self_loop(idtype):
+    g = dgl.graph(([1, 1], [1, 2]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.randn((g.num_nodes(), 2))
+    g.edata['w'] = F.randn((g.num_edges(), 3))
+
+    # Case1: add self-loops with the default setting
+    transform = dgl.AddSelfLoop()
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_nodes()
+    assert new_g.num_edges() == 4
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 0), (1, 1), (1, 2), (2, 2)}
+    assert 'h' in new_g.ndata
+    assert 'w' in new_g.edata
+
+    # Case2: Remove self-loops first to avoid duplicate ones
+    transform = dgl.AddSelfLoop(allow_duplicate=True)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_nodes()
+    assert new_g.num_edges() == 5
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 0), (1, 1), (1, 2), (2, 2)}
+    assert 'h' in new_g.ndata
+    assert 'w' in new_g.edata
+
+    # Create a heterogeneous graph
+    g = dgl.heterograph({
+        ('user', 'plays', 'game'): ([0], [1]),
+        ('user', 'follows', 'user'): ([1], [3])
+    }, idtype=idtype, device=F.ctx())
+    g.nodes['user'].data['h1'] = F.randn((4, 2))
+    g.edges['plays'].data['w1'] = F.randn((1, 3))
+    g.nodes['game'].data['h2'] = F.randn((2, 4))
+    g.edges['follows'].data['w2'] = F.randn((1, 5))
+
+    # Case3: add self-loops for a heterogeneous graph
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.ntypes == g.ntypes
+    assert new_g.canonical_etypes == g.canonical_etypes
+    for nty in new_g.ntypes:
+        assert new_g.num_nodes(nty) == g.num_nodes(nty)
+    assert new_g.num_edges('plays') == 1
+    assert new_g.num_edges('follows') == 5
+    assert 'h1' in new_g.nodes['user'].data
+    assert 'h2' in new_g.nodes['game'].data
+    assert 'w1' in new_g.edges['plays'].data
+    assert 'w2' in new_g.edges['follows'].data
+
+    # Case4: add self-etypes for a heterogeneous graph
+    transform = dgl.AddSelfLoop(new_etypes=True)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.ntypes == g.ntypes
+    assert set(new_g.canonical_etypes) == {
+        ('user', 'plays', 'game'), ('user', 'follows', 'user'),
+        ('user', 'self', 'user'), ('game', 'self', 'game')
+    }
+    for nty in new_g.ntypes:
+        assert new_g.num_nodes(nty) == g.num_nodes(nty)
+    assert new_g.num_edges('plays') == 1
+    assert new_g.num_edges('follows') == 5
+    assert new_g.num_edges(('user', 'self', 'user')) == 4
+    assert new_g.num_edges(('game', 'self', 'game')) == 2
+    assert 'h1' in new_g.nodes['user'].data
+    assert 'h2' in new_g.nodes['game'].data
+    assert 'w1' in new_g.edges['plays'].data
+    assert 'w2' in new_g.edges['follows'].data
+
+@parametrize_dtype
+def test_module_remove_self_loop(idtype):
+    transform = dgl.RemoveSelfLoop()
+
+    # Case1: homogeneous graph
+    g = dgl.graph(([1, 1], [1, 2]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.randn((g.num_nodes(), 2))
+    g.edata['w'] = F.randn((g.num_edges(), 3))
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_nodes()
+    assert new_g.num_edges() == 1
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(1, 2)}
+    assert 'h' in new_g.ndata
+    assert 'w' in new_g.edata
+
+    # Case2: heterogeneous graph
+    g = dgl.heterograph({
+        ('user', 'plays', 'game'): ([0, 1], [1, 1]),
+        ('user', 'follows', 'user'): ([1, 2], [2, 2])
+    }, idtype=idtype, device=F.ctx())
+    g.nodes['user'].data['h1'] = F.randn((3, 2))
+    g.edges['plays'].data['w1'] = F.randn((2, 3))
+    g.nodes['game'].data['h2'] = F.randn((2, 4))
+    g.edges['follows'].data['w2'] = F.randn((2, 5))
+
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.ntypes == g.ntypes
+    assert new_g.canonical_etypes == g.canonical_etypes
+    for nty in new_g.ntypes:
+        assert new_g.num_nodes(nty) == g.num_nodes(nty)
+    assert new_g.num_edges('plays') == 2
+    assert new_g.num_edges('follows') == 1
+    assert 'h1' in new_g.nodes['user'].data
+    assert 'h2' in new_g.nodes['game'].data
+    assert 'w1' in new_g.edges['plays'].data
+    assert 'w2' in new_g.edges['follows'].data
+
+@parametrize_dtype
+def test_module_add_reverse(idtype):
+    transform = dgl.AddReverse()
+
+    # Case1: Add reverse edges for a homogeneous graph
+    g = dgl.graph(([0], [1]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.randn((g.num_nodes(), 3))
+    g.edata['w'] = F.randn((g.num_edges(), 2))
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert g.num_nodes() == new_g.num_nodes()
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 0)}
+    assert F.allclose(g.ndata['h'], new_g.ndata['h'])
+    assert F.allclose(g.edata['w'], F.narrow_row(new_g.edata['w'], 0, 1))
+    assert F.allclose(F.narrow_row(new_g.edata['w'], 1, 2), F.zeros((1, 2), F.float32, F.ctx()))
+
+    # Case2: Add reverse edges for a homogeneous graph and copy edata
+    transform = dgl.AddReverse(copy_edata=True)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert g.num_nodes() == new_g.num_nodes()
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 0)}
+    assert F.allclose(g.ndata['h'], new_g.ndata['h'])
+    assert F.allclose(g.edata['w'], F.narrow_row(new_g.edata['w'], 0, 1))
+    assert F.allclose(g.edata['w'], F.narrow_row(new_g.edata['w'], 1, 2))
+
+    # Case3: Add reverse edges for a heterogeneous graph
+    g = dgl.heterograph({
+        ('user', 'plays', 'game'): ([0, 1], [1, 1]),
+        ('user', 'follows', 'user'): ([1, 2], [2, 2])
+    }, device=F.ctx())
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert g.ntypes == new_g.ntypes
+    assert set(new_g.canonical_etypes) == {
+        ('user', 'plays', 'game'), ('user', 'follows', 'user'), ('game', 'rev_plays', 'user')}
+    for nty in g.ntypes:
+        assert g.num_nodes(nty) == new_g.num_nodes(nty)
+
+    src, dst = new_g.edges(etype='plays')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 1)}
+
+    src, dst = new_g.edges(etype='follows')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(1, 2), (2, 2), (2, 1)}
+
+    src, dst = new_g.edges(etype='rev_plays')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(1, 1), (1, 0)}
+
+    # Case4: Enforce reverse edge types for symmetric canonical edge types
+    transform = dgl.AddReverse(sym_new_etype=True)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert g.ntypes == new_g.ntypes
+    assert set(new_g.canonical_etypes) == {
+        ('user', 'plays', 'game'), ('user', 'follows', 'user'),
+        ('game', 'rev_plays', 'user'), ('user', 'rev_follows', 'user')}
+    for nty in g.ntypes:
+        assert g.num_nodes(nty) == new_g.num_nodes(nty)
+
+    src, dst = new_g.edges(etype='plays')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 1)}
+
+    src, dst = new_g.edges(etype='follows')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(1, 2), (2, 2)}
+
+    src, dst = new_g.edges(etype='rev_plays')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(1, 1), (1, 0)}
+
+    src, dst = new_g.edges(etype='rev_follows')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(2, 1), (2, 2)}
+
+@unittest.skipIf(F._default_context_str == 'gpu', reason="GPU not supported for to_simple")
+@parametrize_dtype
+def test_module_to_simple(idtype):
+    transform = dgl.ToSimple()
+    g = dgl.graph(([0, 1, 1], [1, 2, 2]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.randn((g.num_nodes(), 2))
+    g.edata['w'] = F.tensor([[0.1], [0.2], [0.3]])
+    sg = transform(g)
+    assert sg.device == g.device
+    assert sg.idtype == g.idtype
+    assert sg.num_nodes() == g.num_nodes()
+    assert sg.num_edges() == 2
+    src, dst = sg.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 2)}
+    assert F.allclose(sg.edata['count'], F.tensor([1, 2]))
+    assert F.allclose(sg.ndata['h'], g.ndata['h'])
+
+    g = dgl.heterograph({
+        ('user', 'follows', 'user'): ([0, 1, 1], [1, 2, 2]),
+        ('user', 'plays', 'game'): ([0, 1, 0], [1, 1, 1])
+    })
+    sg = transform(g)
+    assert sg.device == g.device
+    assert sg.idtype == g.idtype
+    assert sg.ntypes == g.ntypes
+    assert sg.canonical_etypes == g.canonical_etypes
+    for nty in sg.ntypes:
+        assert sg.num_nodes(nty) == g.num_nodes(nty)
+    for ety in sg.canonical_etypes:
+        assert sg.num_edges(ety) == 2
+
+    src, dst = sg.edges(etype='follows')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 2)}
+
+    src, dst = sg.edges(etype='plays')
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 1)}
+
+@parametrize_dtype
+def test_module_line_graph(idtype):
+    transform = dgl.LineGraph()
+    g = dgl.graph(([0, 1, 1], [1, 0, 2]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.tensor([[0.], [1.], [2.]])
+    g.edata['w'] = F.tensor([[0.], [0.1], [0.2]])
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_edges()
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (0, 2), (1, 0)}
+
+    transform = dgl.LineGraph(backtracking=False)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_edges()
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 2)}
+
+@parametrize_dtype
+def test_module_khop_graph(idtype):
+    transform = dgl.KHopGraph(2)
+    g = dgl.graph(([0, 1], [1, 2]), idtype=idtype, device=F.ctx())
+    g.ndata['h'] = F.randn((g.num_nodes(), 2))
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_nodes() == g.num_nodes()
+    assert F.allclose(g.ndata['h'], new_g.ndata['h'])
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 2)}
+
+@parametrize_dtype
+def test_module_add_metapaths(idtype):
+    g = dgl.heterograph({
+        ('person', 'author', 'paper'): ([0, 0, 1], [1, 2, 2]),
+        ('paper', 'accepted', 'venue'): ([1], [0]),
+        ('paper', 'rejected', 'venue'): ([2], [1])
+    }, idtype=idtype, device=F.ctx())
+    g.nodes['venue'].data['h'] = F.randn((g.num_nodes('venue'), 2))
+    g.edges['author'].data['h'] = F.randn((g.num_edges('author'), 3))
+
+    # Case1: keep_orig_edges is True
+    metapaths = {
+        'accepted': [('person', 'author', 'paper'), ('paper', 'accepted', 'venue')],
+        'rejected': [('person', 'author', 'paper'), ('paper', 'rejected', 'venue')]
+    }
+    transform = dgl.AddMetaPaths(metapaths)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.ntypes == g.ntypes
+    assert set(new_g.canonical_etypes) == {
+        ('person', 'author', 'paper'), ('paper', 'accepted', 'venue'),
+        ('paper', 'rejected', 'venue'), ('person', 'accepted', 'venue'),
+        ('person', 'rejected', 'venue')
+    }
+    for nty in new_g.ntypes:
+        assert new_g.num_nodes(nty) == g.num_nodes(nty)
+    for ety in g.canonical_etypes:
+        assert new_g.num_edges(ety) == g.num_edges(ety)
+    assert F.allclose(g.nodes['venue'].data['h'], new_g.nodes['venue'].data['h'])
+    assert F.allclose(g.edges['author'].data['h'], new_g.edges['author'].data['h'])
+
+    src, dst = new_g.edges(etype=('person', 'accepted', 'venue'))
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 0)}
+
+    src, dst = new_g.edges(etype=('person', 'rejected', 'venue'))
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 1)}
+
+    # Case2: keep_orig_edges is False
+    transform = dgl.AddMetaPaths(metapaths, keep_orig_edges=False)
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.ntypes == g.ntypes
+    assert len(new_g.canonical_etypes) == 2
+    for nty in new_g.ntypes:
+        assert new_g.num_nodes(nty) == g.num_nodes(nty)
+    assert F.allclose(g.nodes['venue'].data['h'], new_g.nodes['venue'].data['h'])
+
+    src, dst = new_g.edges(etype=('person', 'accepted', 'venue'))
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 0)}
+
+    src, dst = new_g.edges(etype=('person', 'rejected', 'venue'))
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 1)}
+
+@parametrize_dtype
+def test_module_compose(idtype):
+    g = dgl.graph(([0, 1], [1, 2]), idtype=idtype, device=F.ctx())
+    transform = dgl.Compose([dgl.AddReverse(), dgl.AddSelfLoop()])
+    new_g = transform(g)
+    assert new_g.device == g.device
+    assert new_g.idtype == g.idtype
+    assert new_g.num_edges() == 7
+
+    src, dst = new_g.edges()
+    eset = set(zip(list(F.asnumpy(src)), list(F.asnumpy(dst))))
+    assert eset == {(0, 1), (1, 2), (1, 0), (2, 1), (0, 0), (1, 1), (2, 2)}
+
 if __name__ == '__main__':
     test_partition_with_halo()