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# Source code for networkx.algorithms.structuralholes

# -*- encoding: utf-8 -*-
#
#    Aric Hagberg <hagberg@lanl.gov>
#    Dan Schult <dschult@colgate.edu>
#    Pieter Swart <swart@lanl.gov>
"""Functions for computing measures of structural holes."""
from __future__ import division

import networkx as nx

__all__ = ['constraint', 'local_constraint', 'effective_size']

def mutual_weight(G, u, v, weight=None):
"""Returns the sum of the weights of the edge from u to v and
the edge from v to u in G.

weight is the edge data key that represents the edge weight. If
the specified key is None or is not in the edge data for an edge,
that edge is assumed to have weight 1.

Pre-conditions: u and v must both be in G.

"""
try:
a_uv = G[u][v].get(weight, 1)
except KeyError:
a_uv = 0
try:
a_vu = G[v][u].get(weight, 1)
except KeyError:
a_vu = 0
return a_uv + a_vu

def normalized_mutual_weight(G, u, v, norm=sum, weight=None):
"""Returns normalized mutual weight of the edges from u to v
with respect to the mutual weights of the neighbors of u in G.

norm specifies how the normalization factor is computed. It must
be a function that takes a single argument and returns a number.
The argument will be an iterable of mutual weights
of pairs (u, w), where w ranges over each (in- and
out-)neighbor of u. Commons values for normalization are
sum and max.

weight can be None or a string, if None, all edge weights
are considered equal. Otherwise holds the name of the edge
attribute used as weight.

"""
scale = norm(mutual_weight(G, u, w, weight)
for w in set(nx.all_neighbors(G, u)))
return 0 if scale == 0 else mutual_weight(G, u, v, weight) / scale

[docs]def effective_size(G, nodes=None, weight=None):
r"""Returns the effective size of all nodes in the graph G.

The *effective size* of a node's ego network is based on the concept
of redundancy. A person's ego network has redundancy to the extent
that her contacts are connected to each other as well. The
nonredundant part of a person's relationships it's the effective
size of her ego network _.  Formally, the effective size of a
node $u$, denoted $e(u)$, is defined by

.. math::

e(u) = \sum_{v \in N(u) \setminus \{u\}}
\left(1 - \sum_{w \in N(v)} p_{uw} m_{vw}\right)

where $N(u)$ is the set of neighbors of $u$ and $p_{uw}$ is the
normalized mutual weight of the (directed or undirected) edges
joining $u$ and $v$, for each vertex $u$ and $v$ _. And $m_{vw}$
is the mutual weight of $v$ and $w$ divided by $v$ highest mutual
weight with any of its neighbors. The *mutual weight* of $u$ and $v$
is the sum of the weights of edges joining them (edge weights are
assumed to be one if the graph is unweighted).

For the case of unweighted and undirected graphs, Borgatti proposed
a simplified formula to compute effective size _

.. math::

e(u) = n - \frac{2t}{n}

where t is the number of ties in the ego network (not including
ties to ego) and n is the number of nodes (excluding ego).

Parameters
----------
G : NetworkX graph
The graph containing v. Directed graphs are treated like
undirected graphs when computing neighbors of v.

nodes : container, optional
Container of nodes in the graph G to compute the effective size.
If None, the effective size of every node is computed.

weight : None or string, optional
If None, all edge weights are considered equal.
Otherwise holds the name of the edge attribute used as weight.

Returns
-------
dict
Dictionary with nodes as keys and the constraint on the node as values.

Notes
-----
Burt also defined the related concept of *efficiency* of a node's ego
network, which is its effective size divided by the degree of that
node _. So you can easily compute efficiency:

>>> G = nx.DiGraph()
>>> G.add_edges_from([(0, 1), (0, 2), (1, 0), (2, 1)])
>>> esize = nx.effective_size(G)
>>> efficiency = {n: v / G.degree(n) for n, v in esize.items()}

--------
constraint

References
----------
..  Burt, Ronald S.
*Structural Holes: The Social Structure of Competition.*
Cambridge: Harvard University Press, 1995.

..  Borgatti, S.
"Structural Holes: Unpacking Burt's Redundancy Measures"
CONNECTIONS 20(1):35-38.
http://www.analytictech.com/connections/v20(1)/holes.htm

"""
def redundancy(G, u, v, weight=None):
nmw = normalized_mutual_weight
r = sum(nmw(G, u, w, weight=weight) * nmw(G, v, w, norm=max, weight=weight)
for w in set(nx.all_neighbors(G, u)))
return 1 - r
effective_size = {}
if nodes is None:
nodes = G
# Use Borgatti's simplified formula for unweighted and undirected graphs
if not G.is_directed() and weight is None:
for v in nodes:
# Effective size is not defined for isolated nodes
if len(G[v]) == 0:
effective_size[v] = float('nan')
continue
E = nx.ego_graph(G, v, center=False, undirected=True)
effective_size[v] = len(E) - (2 * E.size()) / len(E)
else:
for v in nodes:
# Effective size is not defined for isolated nodes
if len(G[v]) == 0:
effective_size[v] = float('nan')
continue
effective_size[v] = sum(redundancy(G, v, u, weight)
for u in set(nx.all_neighbors(G, v)))
return effective_size

[docs]def constraint(G, nodes=None, weight=None):
r"""Returns the constraint on all nodes in the graph G.

The *constraint* is a measure of the extent to which a node *v* is
invested in those nodes that are themselves invested in the
neighbors of *v*. Formally, the *constraint on v*, denoted c(v),
is defined by

.. math::

c(v) = \sum_{w \in N(v) \setminus \{v\}} \ell(v, w)

where N(v) is the subset of the neighbors of v that are either
predecessors or successors of v and \ell(v, w) is the local
constraint on v with respect to w _. For the definition of local
constraint, see :func:local_constraint.

Parameters
----------
G : NetworkX graph
The graph containing v. This can be either directed or undirected.

nodes : container, optional
Container of nodes in the graph G to compute the constraint. If
None, the constraint of every node is computed.

weight : None or string, optional
If None, all edge weights are considered equal.
Otherwise holds the name of the edge attribute used as weight.

Returns
-------
dict
Dictionary with nodes as keys and the constraint on the node as values.

--------
local_constraint

References
----------
..  Burt, Ronald S.
"Structural holes and good ideas".
American Journal of Sociology (110): 349–399.

"""
if nodes is None:
nodes = G
constraint = {}
for v in nodes:
# Constraint is not defined for isolated nodes
if len(G[v]) == 0:
constraint[v] = float('nan')
continue
constraint[v] = sum(local_constraint(G, v, n, weight)
for n in set(nx.all_neighbors(G, v)))
return constraint

[docs]def local_constraint(G, u, v, weight=None):
r"""Returns the local constraint on the node u with respect to
the node v in the graph G.

Formally, the *local constraint on u with respect to v*, denoted
$\ell(v)$, is defined by

.. math::

\ell(u, v) = \left(p_{uv} + \sum_{w \in N(v)} p_{uw} p{wv}\right)^2,

where $N(v)$ is the set of neighbors of $v$ and $p_{uv}$ is the
normalized mutual weight of the (directed or undirected) edges
joining $u$ and $v$, for each vertex $u$ and $v$ _. The *mutual
weight* of $u$ and $v$ is the sum of the weights of edges joining
them (edge weights are assumed to be one if the graph is
unweighted).

Parameters
----------
G : NetworkX graph
The graph containing u and v. This can be either
directed or undirected.

u : node
A node in the graph G.

v : node
A node in the graph G.

weight : None or string, optional
If None, all edge weights are considered equal.
Otherwise holds the name of the edge attribute used as weight.

Returns
-------
float
The constraint of the node v in the graph G.

--------
constraint

References
----------
..  Burt, Ronald S.
"Structural holes and good ideas".
American Journal of Sociology (110): 349–399.

"""
nmw = normalized_mutual_weight
direct = nmw(G, u, v, weight=weight)
indirect = sum(nmw(G, u, w, weight=weight) * nmw(G, w, v, weight=weight)
for w in set(nx.all_neighbors(G, u)))
return (direct + indirect) ** 2