NetworkX

---

## NetworkX

### Applications of Data Science - Class 8

### Giora Simchoni

#### `gsimchoni@gmail.com and add #dsapps in subject`

### Stat. and OR Department, TAU
### 2023-02-23

---

<div class="my-footer">
  <span>
    <a href="https://dsapps-2023.github.io/Class_Slides/" target="_blank">Applications of Data Science
    </a>
  </span>
</div>

---

# Why NetworkX?

---

## Pros and Cons of `NetworkX`

Pros:
- Maintained!
- Friendly: Community, Docs, Installation (as opposed to `igraph`)
- Relatively easy interface (as opposed to `igraph`)
- Speaks pandas, numpy and other important python tools
- Nodes can be anything, including python objects

Cons:
- Slow, probably not for large networks (as opposed to `igraph`, `graph-tool`)
- Unimpressive viz (as opposed to Gephi, ggraph, others)
- Not the most comprehensive algorithms selection

---

# Creating a Graph(): Manually

---

## Four types of Graph objects

Undirected:

```python
import networkx as nx

G = nx.Graph()
```

Directed:

```python
G = nx.DiGraph()
```

Multiedge undirected:

```python
G = nx.MultiGraph()
```

Multiedge directed:

```python
G = nx.MultiDiGraph()
```

---

## Add Nodes

One or more nodes:

```python
G = nx.Graph()
G.add_node('John')
G.add_nodes_from(['Paul', 'George', 'Ringo'])
```

Watch a Graph's nodes:

```python
G.nodes
```

```
## NodeView(('John', 'Paul', 'George', 'Ringo'))
```

Print nicer as a list:

```python
list(G.nodes)
```

```
## ['John', 'Paul', 'George', 'Ringo']
```

---

## Add Edges

One or more edges:

```python
G.add_edge('John', 'Paul')
G.add_edges_from([('Paul', 'George'), ('John', 'Ringo')])
```

Surprisingly, this would also work:

```python
G.add_edge('John', 'Brian')
```

Watch a Graph's edges:

```python
print(list(G.edges))
```

```
## [('John', 'Paul'), ('John', 'Ringo'), ('John', 'Brian'), ('Paul', 'George')]
```

---

## Remove nodes or edges

```python
G.remove_node('Brian')
```

Did `NetworkX` bother to remove the edge as well?

```python
print(list(G.edges))
```

```
## [('John', 'Paul'), ('John', 'Ringo'), ('Paul', 'George')]
```

Remove an edge, remove from a list:

```python
G.remove_edge('Paul', 'John') # why did it work?
G.remove_nodes_from(['John', 'George'])
```

Break the Beatles altogether:

```python
G.clear()
```

---

## Good to know

- Adding an edge also adds its nodes if they didn't exist before
- Adding a duplicate node/edge is ignored unless `MultiGraph()`
- Removing an edge does not remove its nodes
- Removing a node removes all edges to/from it
- Removing a non-existent node/edge raises an error unless it is part of a list in which case - ignored

---

## Nodes and Edges Attributes

While adding to Graph:

```python
G.add_node('Ringo', alive=True)
G.add_nodes_from([('George', {'alive': False}), ('John', {'alive': False})])
# can also do: G.add_nodes_from(['George', 'John'], alive=False)
G.add_edge('John', 'Paul', year=1957)
G.add_edges_from([('Ringo', 'John'), ('Ringo', 'Paul'), ('Ringo', 'George')], year=1960)
```

Watch attributes with the `nodes()` and `edges()` methods:

```python
print(G.nodes(data=True))
```

```
## [('Ringo', {'alive': True}), ('George', {'alive': False}), ('John', {'alive': False}), ('Paul', {})]
```

```python
print(G.nodes(data='alive'))
```

```
## [('Ringo', True), ('George', False), ('John', False), ('Paul', None)]
```

---

## Nodes and Edges Attributes

Setting an attribute of an existing node/edge through the `nodes` and `edges` dictionary attributes:

```python
print(G.nodes['Paul'])
```

```
## {}
```

```python
G.nodes['Paul']['alive'] = True #?
```

```python
G.add_edge('John', 'George')
G.edges[('John', 'George')]['year'] = 1958
```

As a dictionary you can also delete an attribute this way:

```python
del G.nodes['Paul']['alive']
del G.edges[('John', 'George')]['year']
```

---

## Nodes and Edges Attributes

Finally you can set an attribute from a simple dictionary:

```python
year_met = {
  ('John', 'Paul'): 1957,
  ('Paul', 'Ringo'): 1960,
  ('Paul', 'George'): 1958,
  ('John', 'George'): 1958
}
nx.set_edge_attributes(G, year_met, 'year')
```

Or multiple attributes with a nested dictionary:

```python
node_attrs = {
  'John': {'alive': False, 'born': 1940},
  'Paul': {'alive': True, 'born': 1942},
  'George': {'alive': False, 'born': 1943},
  'Ringo': {'alive': True, 'born': 1940}
}
nx.set_node_attributes(G, node_attrs)
```

---

### Good to have

The edge attribute `weight` is so important it got its own method:

```python
G = nx.Graph()
G.add_weighted_edges_from([(0, 1, 3.0), (1, 2, 7.5)])
```

Quick methods to know:

```python
print(G.number_of_nodes())
```

```
## 3
```

```python
print(G.number_of_edges())
```

```
## 2
```

```python
print(G.is_directed())
```

```
## False
```

---

### Good to have

```python
print(G.has_node(3))
```

```
## False
```

```python
print(G.has_edge(0, 1))
```

```
## True
```

```python
print(G.subgraph([0, 1]).number_of_edges())
```

```
## 1
```

```python
print(list(G.neighbors(1)))
```

```
## [0, 2]
```

---

## What about `DiGraph()`?

Convert an undirected graph to a directed graph:

```python
D = nx.DiGraph(G)
```

Edges direction matters:

```python
D.add_edge(2, 3, weight=10.0)
print(D.has_edge(2, 3))
```

```
## True
```

```python
print(D.has_edge(3, 2))
```

```
## False
```

Other than that...

---

# Visualize a Graph(): matplotlib

---

### `draw()`: The very basic

```python
import matplotlib.pyplot as plt

G = nx.erdos_renyi_graph(n=5, p=0.6) # a.k.a binomial_graph()

nx.draw(G)
plt.show()
```

---

### `draw_networkx()`: Some more options

```python
nx.draw_networkx(G, node_size=800, node_color='red', edge_color='pink',
  width=5, font_size=16)
plt.show()
```

---

You may like the unpacking a dictionary option better:

```python
options = {
    'node_color': 'black',
    'node_size': 100,
    'width': 3,
}

nx.draw_networkx(G, **options)
plt.show()
```

---

### Layouts Galore

```python
plt.subplot(121)
limits = plt.axis('off')
nx.draw_networkx(G, pos=nx.kamada_kawai_layout(G))
plt.subplot(122)
limits = plt.axis('off')
nx.draw_networkx(G, pos=nx.fruchterman_reingold_layout(G))
plt.show()
```

---

### Use a Layout directly

```python
nx.draw_kamada_kawai(G)
plt.show()
```

---

### For nicer graphs go to...

- [Grave](https://networkx.github.io/grave/)?
- [nxviz](https://nxviz.readthedocs.io/en/latest/index.html)?
- [Plotly](https://plot.ly/python/) (interactive graphs for Python or R)
- [Gephi](https://gephi.org/) (a Desktop app, neither R nor Python but both export to Gephi format)
- [d3](https://d3js.org/) (JS delight)
- [ggraph](https://ggraph.data-imaginist.com/) (R)?
---

# Create a Graph(): In Real Life

---

### Numpy

```python
import numpy as np

A = np.array([
[0, 1, 1, 0, 0],
[1, 0, 1, 0, 1],
[1, 1, 0, 0, 1],
[0, 0, 0, 0, 0],
[0, 1, 1, 0, 0]])

G = nx.from_numpy_array(A)
nx.draw_networkx(G, node_size=0, font_size=40)
plt.show()
```

---

### Edgelist - a list of edges

```python
beatles_edgelist = [('John', 'Paul', {'weight': 1.0}), ('John', 'George', {'weight': 0.5}),
  ('John', 'Ringo', {'weight': 0.5})]

G = nx.from_edgelist(beatles_edgelist)

weights = [10 * attrs['weight'] for u, v, attrs in G.edges(data=True)]
nx.draw_networkx(G, width=weights)
plt.show()
```

---

### Edgelist - a file

This is how the `beatles.edgelist` file looks like:

```python
John Paul 1
John George 0.5
John Ringo 0.5
```

Read it with `read_edgelist()` or `read_weighted_edgelist()`:

```python
with open('../data/beatles.edgelist', 'rb') as edgelist_file:
  G = nx.read_edgelist(edgelist_file, data=(('weight', float),))
print(G.edges(data=True))
```

```
## [('John', 'Paul', {'weight': 1.0}), ('John', 'George', {'weight': 0.5}), ('John', 'Ringo', {'weight': 0.5})]
```

```python
with open('../data/beatles.edgelist', 'rb') as edgelist_file:
  G = nx.read_weighted_edgelist(edgelist_file)
print(G.edges(data=True))
```

```
## [('John', 'Paul', {'weight': 1.0}), ('John', 'George', {'weight': 0.5}), ('John', 'Ringo', {'weight': 0.5})]
```

---

### CSV

This is how the `beatles.csv` file looks like:

```python
John,Paul,1957,1.0
John,George,1958,0.5
John,Ringo,1960,0.5
```

You can still use `read_edgelist()` to read it:

```python
with open('../data/beatles.csv', 'rb') as edgelist_file:
  G = nx.read_edgelist(edgelist_file, delimiter=',', data=(('year', int),('weight', float),))
print(G.edges(data=True))
```

```
## [('John', 'Paul', {'year': 1957, 'weight': 1.0}), ('John', 'George', {'year': 1958, 'weight': 0.5}), ('John', 'Ringo', {'year': 1960, 'weight': 0.5})]
```

Or, you can just use Pandas `read_csv()` and...

---

### Pandas

```python
import pandas as pd

beatles_df = pd.read_csv('../data/beatles.csv', header=None)
beatles_df.columns = ['source', 'target', 'year', 'weight']

G = nx.from_pandas_edgelist(beatles_df, 'source', 'target', ['year', 'weight'])

nx.draw_networkx(G)
plt.show()
```

---

### What about writing?

All methods we've seen have a `write_` or `to_` complementary function:

```python
print(nx.to_edgelist(G))
```

```
## [('John', 'Paul', {'year': 1957, 'weight': 1.0}), ('John', 'George', {'year': 1958, 'weight': 0.5}), ('John', 'Ringo', {'year': 1960, 'weight': 0.5})]
```

### What about `DiGraph()`?

All methods we've seen have a `create_using` parameter:

```python
D = nx.from_pandas_edgelist(beatles_df, 'source', 'target',
  ['year', 'weight'], create_using=nx.DiGraph)
```

.warning[
⚠️ When creating `DiGraph()` NetworkX assumes `\(A_{ij}\)` corresponds to the edge from i to j, contrary to our convention. Therefore, you should use `A.transpose()`
]
---

### Bipartite

For creating a bipartite network, use the `bipartite` node attribute:

```python
G = nx.Graph()
G.add_nodes_from([1, 2, 3, 4], bipartite=0)
G.add_nodes_from(['a', 'b', 'c'], bipartite=1)
G.add_edges_from([(1, 'a'), (1, 'b'), (2, 'b'), (2, 'c'), (3, 'c'), (4, 'a')])

nx.draw_networkx(G, pos=nx.bipartite_layout(G, [1, 2, 3, 4]), node_size=0, font_size=20)
plt.show()
```

It is your responsibility though to check that indeed the two sets of nodes are not connected.

---

"In real life" you would probably have something like the Marvel incidence matrix in a CSV, what `NetworkX` calls a *biadjacency_matrix*, or a list of edges you could make a Scipy *sparse matrix* with, from which you can create a bipartite graph:

```python
from scipy import sparse

marvel = pd.read_csv("../data/marvel_incidence_matrix.csv")
B = marvel.iloc[:, 1:].values

sB = sparse.csr_matrix(B)

G = nx.bipartite.from_biadjacency_matrix(sB)
print(G.nodes(data=True))
```

```
## [(0, {'bipartite': 0}), (1, {'bipartite': 0}), (2, {'bipartite': 0}), (3, {'bipartite': 0}), (4, {'bipartite': 0}), (5, {'bipartite': 0}), (6, {'bipartite': 0}), (7, {'bipartite': 0}), (8, {'bipartite': 0}), (9, {'bipartite': 0}), (10, {'bipartite': 0}), (11, {'bipartite': 0}), (12, {'bipartite': 0}), (13, {'bipartite': 0}), (14, {'bipartite': 0}), (15, {'bipartite': 0}), (16, {'bipartite': 0}), (17, {'bipartite': 0}), (18, {'bipartite': 0}), (19, {'bipartite': 0}), (20, {'bipartite': 0}), (21, {'bipartite': 1}), (22, {'bipartite': 1}), (23, {'bipartite': 1}), (24, {'bipartite': 1}), (25, {'bipartite': 1}), (26, {'bipartite': 1}), (27, {'bipartite': 1}), (28, {'bipartite': 1}), (29, {'bipartite': 1}), (30, {'bipartite': 1}), (31, {'bipartite': 1}), (32, {'bipartite': 1}), (33, {'bipartite': 1}), (34, {'bipartite': 1}), (35, {'bipartite': 1}), (36, {'bipartite': 1})]
```

---

```python
films = marvel['Film'].to_list()
characters = marvel.columns[1:].to_list()

id_to_label = {id: char for id, char in enumerate(films + characters)}
G = nx.relabel_nodes(G, id_to_label)

nx.draw_networkx(G, pos=nx.bipartite_layout(G, films), node_size=0, font_size=10)
plt.show()
```

---

You've seen how to project an incidence matrix with numpy:

```python
P = B.transpose() @ B
np.fill_diagonal(P, 0)
Gc = nx.from_numpy_array(P)

id_to_char = {id: char for id, char in enumerate(characters)}
Gc = nx.relabel_nodes(Gc, id_to_char)
nx.draw_networkx(Gc, node_size=0, font_size=10)
plt.show()
```

---

But NetworkX can do all that for you:

```python
Gc = nx.bipartite.projected_graph(G, characters)

nx.draw_networkx(Gc, node_size=0, font_size=10)
plt.show()
```

---

### Similarity

Let's recreate the sci-fi similarity network. We'll do some filtering:
- The original CSV includes 156 books, we'll take only those marked as "popular"
- Take only books with at least 2 features marked > 0
- Won't be looking at the author's name, gender, or date of release

```python
scifi = pd.read_csv('../data/sci_fi_books.csv', index_col='book')
scifi['n_features'] = (scifi.iloc[:, 6:17] > 0).sum(axis=1)
scifi = scifi[scifi['popular'] == 1]

scifi_fil = scifi[scifi['n_features'] >= 2]
cols_to_drop = ['date', 'author', 'frequency', 'author_gender',
  'quarter_century', 'century', 'popular', 'n_features']
scifi_fil = scifi_fil.drop(cols_to_drop, axis=1)

# to use Pandas corr function need to transpose data frame
scifi_corr = scifi_fil.T.corr(method='spearman')
```

---

We reached a 25x25 symmetric correlation matrix for 25 books:

```python
scifi_corr.values[:5, :5]
```

```
## array([[ 1.        ,  0.62769528,  0.2975283 ,  0.69310571, -0.2111842 ],
##        [ 0.62769528,  1.        ,  0.68760573,  0.3303204 ,  0.23530427],
##        [ 0.2975283 ,  0.68760573,  1.        ,  0.0037037 ,  0.48989795],
##        [ 0.69310571,  0.3303204 ,  0.0037037 ,  1.        , -0.04082483],
##        [-0.2111842 ,  0.23530427,  0.48989795, -0.04082483,  1.        ]])
```

Turn all upper triangle to `Nan`, so we'll get only 25 * 24 / 2 correlations:

```python
scifi_corr.values[np.triu_indices_from(scifi_corr.values)] = np.nan

scifi_corr.values[:5, :5]
```

```
## array([[        nan,         nan,         nan,         nan,         nan],
##        [ 0.62769528,         nan,         nan,         nan,         nan],
##        [ 0.2975283 ,  0.68760573,         nan,         nan,         nan],
##        [ 0.69310571,  0.3303204 ,  0.0037037 ,         nan,         nan],
##        [-0.2111842 ,  0.23530427,  0.48989795, -0.04082483,         nan]])
```

---

Convert this to a long edgelist, filter out the `Nan`:

```python
scifi_edgelist = scifi_corr.reset_index().melt(id_vars='book',
  value_vars=scifi_fil.index, var_name='book2', value_name='corr')

scifi_edgelist = scifi_edgelist[~pd.isna(scifi_edgelist['corr'])]
print(scifi_edgelist.shape)
```

```
## (300, 3)
```

```python
scifi_edgelist.head(5)
```

```
##                                     book         book2      corr
## 1  Twenty Thousand Leagues Under the Sea  Frankenstein  0.627695
## 2     Journey to the Center of the Earth  Frankenstein  0.297528
## 3                        Brave New World  Frankenstein  0.693106
## 4   The Lion, the Witch and the Wardrobe  Frankenstein -0.211184
## 5                               I, Robot  Frankenstein  0.700617
```

---

Filter all correlations above 0.5 and convert to a `Graph()`:

```python
G = nx.from_pandas_edgelist(scifi_edgelist[scifi_edgelist['corr'] > 0.5],
  'book', 'book2', ['corr'])

nx.draw_networkx(G)
limits = plt.axis('off')
plt.show()
```

---

# Let NetworkX work

---

### Adjacency Matrix

You can get a Numpy array with `to_numpy_array()`:

```python
A = nx.to_numpy_array(G)

print(A.shape)
```

```
## (23, 23)
```

```python
A[:4, :4]
```

```
## array([[0., 1., 0., 0.],
##        [1., 0., 1., 1.],
##        [0., 1., 0., 1.],
##        [0., 1., 1., 0.]])
```

---

But a realistic network would be large and sparse so the `adjacency_matrix()` method returns a SciPy sparse matrix:

```python
A = nx.adjacency_matrix(G)

print(A.shape)
```

```
## (23, 23)
```

```python
print(A[:4, :4])
```

```
##   (0, 1)	1
##   (1, 0)	1
##   (1, 2)	1
##   (1, 3)	1
##   (2, 1)	1
##   (2, 3)	1
##   (3, 1)	1
##   (3, 2)	1
```

---

### Easy to check if...

```python
print(nx.is_directed(G))
```

```
## False
```

```python
print(nx.is_directed_acyclic_graph(G))
```

```
## False
```

```python
print(nx.is_bipartite(G))
```

```
## False
```

```python
print(nx.is_connected(G))
```

```
## True
```

---

### Degree and Density

Degree is a big deal, there's an attribute *and* a method for that:

```python
G.degree
```

```
## DegreeView({'Twenty Thousand Leagues Under the Sea': 4, 'Frankenstein': 5, 'Brave New World': 5, 'I, Robot': 6, 'Solaris': 5, 'The Hunger Games': 5, 'Journey to the Center of the Earth': 5, 'Flowers for Algernon': 2, 'The Dune Chronicles': 3, 'The Princess Bride': 6, 'A Song of Ice and Fire': 6, 'Harry Potter Series': 5, '2001: A Space Odyssey': 6, "Ender's Game": 6, 'American Gods': 1, 'The Lord of the Rings': 5, 'The Lion, the Witch and the Wardrobe': 3, "The Hitchhiker's Guide to the Galaxy": 6, 'Contact': 6, 'Watership Down': 3, 'Babel 17': 4, 'World War Z': 2, "The Handmaid's Tale": 3})
```

```python
list(G.degree)
```

```
## [('Twenty Thousand Leagues Under the Sea', 4), ('Frankenstein', 5), ('Brave New World', 5), ('I, Robot', 6), ('Solaris', 5), ('The Hunger Games', 5), ('Journey to the Center of the Earth', 5), ('Flowers for Algernon', 2), ('The Dune Chronicles', 3), ('The Princess Bride', 6), ('A Song of Ice and Fire', 6), ('Harry Potter Series', 5), ('2001: A Space Odyssey', 6), ("Ender's Game", 6), ('American Gods', 1), ('The Lord of the Rings', 5), ('The Lion, the Witch and the Wardrobe', 3), ("The Hitchhiker's Guide to the Galaxy", 6), ('Contact', 6), ('Watership Down', 3), ('Babel 17', 4), ('World War Z', 2), ("The Handmaid's Tale", 3)]
```

Average degree:

```python
np.mean(list(dict(G.degree).values()))
```

```
## 4.434782608695652
```

---

Get the degree of a specific node:

```python
G.degree['Contact']
```

```
## 6
```

Get the density of a network:

```python
nx.density(G)
```

```
## 0.2015810276679842
```

For directed graphs, use `in_degree` and `out_degree`:

```python
D = nx.DiGraph(G)

D.in_degree()
D.out_degree()
```

---

### Paths and Diameter

Is there a path between two nodes?

```python
nx.has_path(G, 'Frankenstein', 'Contact')
```

```
## True
```

What is the shortest path distance between two nodes:

```python
nx.shortest_path_length(G, 'Frankenstein', 'Contact')
```

```
## 2
```

Give me (one, there could be more) shortest path:

```python
nx.shortest_path(G, 'Frankenstein', 'Contact')
```

```
## ['Frankenstein', 'I, Robot', 'Contact']
```

---

Give me all shortest paths between two nodes:

```python
all_sps = nx.all_shortest_paths(G, 'Frankenstein', 'Contact') # generator!
list(all_sps)
```

```
## [['Frankenstein', 'I, Robot', 'Contact'], ['Frankenstein', 'Solaris', 'Contact']]
```

Get the diameter of a network:

```python
nx.diameter(G)
```

```
## 5
```

---

### Components

How many connected components?

```python
nx.number_connected_components(G)
```

```
## 1
```

All connected components, each is a set of nodes:

```python
cc = nx.connected_components(G) # generator!
list(cc)
```

```
## [{'The Hunger Games', 'Solaris', 'World War Z', 'Frankenstein', 'Babel 17', 'The Princess Bride', 'Flowers for Algernon', 'The Lion, the Witch and the Wardrobe', 'Watership Down', 'A Song of Ice and Fire', 'Journey to the Center of the Earth', 'Harry Potter Series', 'I, Robot', 'The Lord of the Rings', 'Contact', "Ender's Game", 'The Dune Chronicles', 'Brave New World', "The Handmaid's Tale", 'Twenty Thousand Leagues Under the Sea', '2001: A Space Odyssey', "The Hitchhiker's Guide to the Galaxy", 'American Gods'}]
```

Get component of a specific node:

```python
nx.node_connected_component(G, 'Contact')
```

```
## {'The Hunger Games', 'Solaris', 'World War Z', 'Frankenstein', 'Babel 17', 'The Princess Bride', 'Flowers for Algernon', 'Watership Down', 'The Lion, the Witch and the Wardrobe', 'A Song of Ice and Fire', 'Journey to the Center of the Earth', 'Harry Potter Series', 'I, Robot', 'The Lord of the Rings', "Ender's Game", 'Contact', 'The Dune Chronicles', 'Brave New World', "The Handmaid's Tale", 'Twenty Thousand Leagues Under the Sea', '2001: A Space Odyssey', "The Hitchhiker's Guide to the Galaxy", 'American Gods'}
```

---

Get largest component (a.k.a Giant Connected Component)

```python
gcc = max(nx.connected_components(G), key=len)
len(gcc)
```

```
## 23
```

For directed graphs use `strongly_`/`weakly_` prefixes with previous functions:

```python
D = nx.DiGraph(G)

nx.number_strongly_connected_components(D)
nx.strongly_connected_components(D)
```

---

### Laplacian

```python
L = nx.laplacian_matrix(G)

L.todense()[:5, :5]
```

```
## array([[ 4, -1,  0,  0,  0],
##        [-1,  5, -1, -1, -1],
##        [ 0, -1,  5, -1,  0],
##        [ 0, -1, -1,  6, -1],
##        [ 0, -1,  0, -1,  5]], dtype=int32)
```