Namespace xn¶
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Detailed Description¶
View Classes provide node, edge and degree “views” of a graph.
Views for nodes, edges and degree are provided for all base graph classes. A view means a read-only object that is quick to create, automatically updated when the graph changes, and provides basic access like n : V, for n : V, V[n] and sometimes set operations.
The views are read-only iterable containers that are updated as the graph is updated. As with dicts, the graph should not be updated while (iterating through the view. Views can be iterated multiple times.
Edge and Node views also allow data attribute lookup. The resulting attribute dict is writable as G.edges[3, 4]["color"]="red" Degree views allow lookup of degree values for single nodes. Weighted degree is supported with the weight argument.
Template Class NodeView
V = G.nodes (or V = G.nodes()) allows len(V), n : V, set operations e.g. “G.nodes & H.nodes”, and dd = G.nodes[n], where dd is the node data dict. Iteration is over the nodes by default.
NodeDataView
To iterate over (node, data) pairs, use arguments to G.nodes() to create a DataView e.g. DV = G.nodes(data=”color”, default=”red”). The DataView iterates as for n, color : DV and allows (n, “red”] : DV. Using DV = G.nodes(data=true), the DataViews use the full datadict : writeable form also allowing contain testing as (n, {“color”: “red”}] : VD. DataViews allow set operations when data attributes are hashable.
DegreeView
V = G.degree allows iteration over (node, degree) pairs as well as lookup: deg=V[n]. There are many flavors of DegreeView for In/Out/Directed/Multi. For Directed Graphs, G.degree counts both : and out going edges. G.out_degree && G.in_degree count only specific directions. Weighted degree using edge data attributes is provide via V = G.degree(weight=”attr_name”) where any string with the attribute name can be used. weight=None is the default. No set operations are implemented for degrees, use NodeView.
The argument nbunch restricts iteration to nodes : nbunch. The DegreeView can still lookup any node even if (nbunch is specified.
V = G.edges or V = G.edges() allows iteration over edges as well as e : V, set operations and edge data lookup dd = G.edges[2, 3]. Iteration is over 2-tuples (u, v) for Graph/DiGraph. For multigraphs edges 3-tuples (u, v, key) are the default but 2-tuples can be obtained via V = G.edges(keys=false).
Set operations for directed graphs treat the edges as a set of 2-tuples. For undirected graphs, 2-tuples are not a unique representation of edges. So long as the set being compared to contains unique representations of its edges, the set operations will act as expected. If the other set contains both (0, 1) and (1, 0) however, the result of set operations may contain both representations of the same edge.
EdgeDataView
Edge data can be reported using an EdgeDataView typically created by calling an EdgeView: DV = G.edges(data=”weight”, default=1). The EdgeDataView allows iteration over edge tuples, membership checking but no set operations.
Iteration depends on data and default and for multigraph keys If data == false (the default) then iterate over 2-tuples (u, v). If data is true iterate over 3-tuples (u, v, datadict). Otherwise iterate over (u, v, datadict.get(data, default)). For Multigraphs, if (keys is true, replace u, v with u, v, key to create 3-tuples and 4-tuples.
The argument nbunch restricts edges to those incident to nodes : nbunch. Exceptions Base exceptions and errors for XNetwork.