Tagged: python

List Comprehension in Python

I stumbled across this nice tutorial on advanced design patterns in python today, and especially liked the following image that explains graphically what list comprehension is:


List comprehension in python is extremely flexible and powerful. Let us practice some more with further neat examples of it:

Square all non-negative numbers smaller than 10

[x**2 for x in range(10)]

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Non-negative multiples of 5 smaller than 100

[x for x in range(100) if x%5 == 0]

[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]

Non-negative multiples of 3 but not multiples of 6 smaller than 50

[x for x in range(50) if x%3 == 0 and x%6 != 0]

[3, 9, 15, 21, 27, 33, 39, 45]

All consonants in a given sentence (without repetition)

import string
punct = string.punctuation + ' '
vowels = "aeiou"
phrase = "On second thought, let's not go to Camelot. It is a silly place."
set([c for c in phrase.lower() if c not in vowels and c not in punct])

{'c', 'd', 'g', 'h', 'l', 'm', 'n', 'p', 's', 't', 'y'}

First character of every word in a sentence

[w[0] for w in phrase.split()]

['O', 's', 't', 'l', 'n', 'g', 't', 'C', 'I', 'i', 'a', 's', 'p']

Substitute all vowels in a sentence by the character ‘0’

"".join([c if c not in vowels else '0' for c in phrase])

"On s0c0nd th00ght, l0t's n0t g0 t0 C0m0l0t. It 0s 0 s0lly pl0c0."

Pairs of elements drawn from different lists

words1 = ['Lancelot', 'Robin', 'Galahad']
words2 = ['Camelot', 'Assyria']
[(w1,w2) for w1 in words1 for w2 in words2]

[('Lancelot', 'Camelot'), ('Lancelot', 'Assyria'), ('Robin', 'Camelot'), ('Robin', 'Assyria'), ('Galahad', 'Camelot'), ('Galahad', 'Assyria')]

I will update this list as more interesting and useful examples come to mind. What’s your favorite use of list comprehension and how many lines of code did it save you?

Finding the K in K-Means Clustering

A couple of weeks ago, here at The Data Science Lab we showed how Lloyd’s algorithm can be used to cluster points using k-means with a simple python implementation. We also produced interesting visualizations of the Voronoi tessellation induced by the clustering. At the end of the post we hinted at some of the shortcomings of this clustering procedure. The basic k-means is an extremely simple and efficient algorithm. However, it assumes prior knowledge of the data in order to choose the appropriate K. Other disadvantages are the sensitivity of the final clusters to the selection of the initial centroids and the fact that the algorithm can produce empty clusters. In today’s post, and by popular request, we are going to have a look at the first question, namely how to find the appropriate K to use in the k-means clustering procedure.

Meaning and purpose of clustering, and the elbow method

Clustering consist of grouping objects in sets, such that objects within a cluster are as similar as possible, whereas objects from different clusters are as dissimilar as possible. Thus, the optimal clustering is somehow subjective and dependent on the characteristic used for determining similarities, as well as on the level of detail required from the partitions. For the purpose of our clustering experiment we use clusters derived from Gaussian distributions, i.e. globular in nature, and look only at the usual definition of Euclidean distance between points in a two-dimensional space to determine intra- and inter-cluster similarity.

The following measure represents the sum of intra-cluster distances between points in a given cluster C_k containing n_k points:

\displaystyle D_k = \sum_{\mathrm{x}_i \in C_k} \sum_{\mathrm{x}_j \in C_k} ||\mathrm{x}_i - \mathrm{x}_j ||^2 = 2 n_k \sum_{\mathrm{x}_i \in C_k} ||\mathrm{x}_i - \mu_k ||^2.

Adding the normalized intra-cluster sums of squares gives a measure of the compactness of our clustering:

\displaystyle W_k = \sum_{k=1}^K \frac{1}{2n_k} D_k.

This variance quantity W_k is the basis of a naive procedure to determine the optimal number of clusters: the elbow method.DataClustering_ElbowCriterion

If you graph the percentage of variance explained by the clusters against the number of clusters, the first clusters will add much information (explain a lot of variance), but at some point the marginal gain will drop, giving an angle in the graph. The number of clusters are chosen at this point, hence the “elbow criterion”.

But as Wikipedia promptly explains, this “elbow” cannot always be unambiguously identified. In this post we will show a more sophisticated method that provides a statistical procedure to formalize the “elbow” heuristic.

The gap statistic

The gap statistic was developed by Stanford researchers Tibshirani, Walther and Hastie in their 2001 paper. The idea behind their approach was to find a way to standardize the comparison of \log W_k with a null reference distribution of the data, i.e. a distribution with no obvious clustering. Their estimate for the optimal number of clusters K is the value for which \log W_k falls the farthest below this reference curve. This information is contained in the following formula for the gap statistic:

\displaystyle \mathrm{Gap}_n(k) = E_n^*\{\log W_k\} - \log W_k.

The reference datasets are in our case generated by sampling uniformly from the original dataset’s bounding box (see green box in the upper right plot of the figures below). To obtain the estimate E_n^*\{\log W_k\} we compute the average of B copies \log W^*_k for B=10, each of which is generated with a Monte Carlo sample from the reference distribution. Those \log W^*_k from the B Monte Carlo replicates exhibit a standard deviation \mathrm{sd}(k) which, accounting for the simulation error, is turned into the quantity

\displaystyle s_k = \sqrt{1+1/B}\,\mathrm{sd}(k).

Finally, the optimal number of clusters K is the smallest k such that \mathrm{Gap}(k) \geq \mathrm{Gap}(k+1) - s_{k+1}.

A Python implementation of the algorithm

The computation of the gap statistic involves the following steps (see original paper):

  • Cluster the observed data, varying the number of clusters from k = 1, ..., k_{\mathrm{max}}, and compute the corresponding W_k.
  • Generate B reference data sets and cluster each of them with varying number of clusters k = 1, ..., k_{\mathrm{max}}. Compute the estimated gap statistic \mathrm{Gap}(k) = (1/B) \sum_{b=1}^B \log W^*_{kb} - \log W_k.
  • With \bar{w} = (1/B) \sum_b \log W^*_{kb}, compute the standard deviation \mathrm{sd}(k) = [(1/B) \sum_b (\log W^*_{kb} - \bar{w})^2]^{1/2} and define \displaystyle s_k = \sqrt{1+1/B}\,\mathrm{sd}(k).
  • Choose the number of clusters as the smallest k such that \mathrm{Gap}(k) \geq \mathrm{Gap}(k+1) - s_{k+1}.

Our python implementation makes use of the find_centers(X, K) function defined in this post. The quantity W_k is computed as follows:

def Wk(mu, clusters):
    K = len(mu)
    return sum([np.linalg.norm(mu[i]-c)**2/(2*len(c)) \
               for i in range(K) for c in clusters[i]])

The gap statistic is implemented in the following code snapshot. Note that we use B=10 for the reference datasets and we span values of K from 1 to 9.

def bounding_box(X):
    xmin, xmax = min(X,key=lambda a:a[0])[0], max(X,key=lambda a:a[0])[0]
    ymin, ymax = min(X,key=lambda a:a[1])[1], max(X,key=lambda a:a[1])[1]
    return (xmin,xmax), (ymin,ymax)

def gap_statistic(X):
    (xmin,xmax), (ymin,ymax) = bounding_box(X)
    # Dispersion for real distribution
    ks = range(1,10)
    Wks = zeros(len(ks))
    Wkbs = zeros(len(ks))
    sk = zeros(len(ks))
    for indk, k in enumerate(ks):
        mu, clusters = find_centers(X,k)
        Wks[indk] = np.log(Wk(mu, clusters))
        # Create B reference datasets
        B = 10
        BWkbs = zeros(B)
        for i in range(B):
            Xb = []
            for n in range(len(X)):
            Xb = np.array(Xb)
            mu, clusters = find_centers(Xb,k)
            BWkbs[i] = np.log(Wk(mu, clusters))
        Wkbs[indk] = sum(BWkbs)/B
        sk[indk] = np.sqrt(sum((BWkbs-Wkbs[indk])**2)/B)
    sk = sk*np.sqrt(1+1/B)
    return(ks, Wks, Wkbs, sk)

Finding the K

We shall now apply our algorithm to diverse distributions and see how it performs. Using the init_board_gauss(N, k) function defined in our previous post, we produce an ensemble of 200 data points normally distributed around 3 centers and run the gap statistic on them.

X = init_board_gauss(200,3)
ks, logWks, logWkbs, sk = gap_statistic(X)

The following plot gives an idea of what is happening:

The upper left plot shows the target distribution with 3 clusters. On the right is its bounding box and one Monte Carlo sample drawn from a uniform reference distribution within that rectangle. In the middle left we see the plot of W_k that is used to determine K with the elbow method. Indeed a knee-like feature is observed at K=3, however the gap statistic is a better way of formalizing this phenomenon. On the right is the comparison of \log W_k for the original and averaged reference distributions. Finally, the bottom plots show the gap quantity on the left, with a clear peak at the correct K=3 and the criteria for choosing it on the right. The correct K is the smallest for which the quantity plotted in blue bars becomes positive. The optimal number is correctly guessed by the algorithm as K=3.

Let us now have a look at another example with 400 points around 5 clusters:

In this case, the elbow method would not have been conclusive, however the gap statistic correctly shows a peak in the gap at K=5 and the bar plot changes sign at the same correct value.

Similarly, we can study what happens when the data points are clustered around a single centroid:

It is clear in the above figures that the original and the reference distributions in the middle right plot follow the same decay law, so that no abrupt fall-off of the blue curve with respect to the red one is observed at any K. The bar plot shows positive values for the entire K range. We conclude that K=1 is the correct clustering.

Finally, let us have a look at a uniform, non-clustered distribution of 200 points, generated with the init_board(N) function defined in our previous post:

In this case, the algorithm also guesses K=1 correctly, and it is clear from the middle right plot that both the original and the reference distributions follow exactly the same decay law, since they are essentially different samples from the same uniform distribution on [-1,1] x [-1,1]. The gap curve on the bottom left oscillates between local maxima and minima, indicating certain structures within the original distribution originated by statistical fluctuations.

Table-top data experiment take-away message

The estimation of the optimal number of clusters within a set of data points is a very important problem, as most clustering algorithms need that parameter as input in order to group the data. Many methods have been proposed to find the proper K, among which the “elbow” method offers a very clear and naive solution based on intra-cluster variance. The gap statistic, proposed by Tobshirani et al. formalizes this approach and offers an easy-to-implement algorithm that successfully finds the correct K in the case of globular, Gaussian-distributed, mildly disjoint data distributions.

Update: For a proper initialization of the centroids at the start of the k-means algorithm, we implement the improved k-means++ seeding procedure.

Update: For a comparison of this approach with an alternative method for finding the K in k-means clustering, read this article.

Beautiful Plots With Pandas and Matplotlib

[Click here to see the final plot described in this article.]

Data visualization plays a crucial role in the communication of results from data analyses, and it should always help transmit insights in an honest and clear way. Recently, the highly recommendable blog Flowing Data posted a review of data visualization highlights during 2013, and at The Data Science Lab we felt like doing a bit of pretty plotting as well.

For Python lovers, matplotlib is the library of choice when it comes to plotting. Quite conveniently, the data analysis library pandas comes equipped with useful wrappers around several matplotlib plotting routines, allowing for quick and handy plotting of data frames. Nice examples of plotting with pandas can be seen for instance in this ipython notebook. Still, for customized plots or not so typical visualizations, the panda wrappers need a bit of tweaking and playing with matplotlib’s inside machinery. If one is willing to devote a bit of time to google-ing and experimenting, very beautiful plots can emerge.

Visualizing demographic data

For this pre-Christmas data visualization table-top experiment we are going to use demographic data from countries in the European Union obtained from Wolfram|Alpha. Our data set contains information on population, extension and life expectancy in 24 European countries. We create a pandas data frame from three series that we simply construct from lists, setting the countries as index for each series, and consequently for the data frame.

import pandas as pd
import matplotlib as mpl
from matplotlib.colors import LinearSegmentedColormap
from matplotlib.lines import Line2D 

countries = ['France','Spain','Sweden','Germany','Finland','Poland','Italy',
             'United Kingdom','Romania','Greece','Bulgaria','Hungary',
             'Portugal','Austria','Czech Republic','Ireland','Lithuania','Latvia',
extensions = [547030,504782,450295,357022,338145,312685,301340,243610,238391,
populations = [63.8,47,9.55,81.8,5.42,38.3,61.1,63.2,21.3,11.4,7.35,
life_expectancies = [81.8,82.1,81.8,80.7,80.5,76.4,82.4,80.5,73.8,80.8,73.5,
data = {'extension' : pd.Series(extensions, index=countries), 
        'population' : pd.Series(populations, index=countries),
        'life expectancy' : pd.Series(life_expectancies, index=countries)}

df = pd.DataFrame(data)
df = df.sort('life expectancy')

Now, thanks to the pandas plotting machinery, it is extremely straightforward to show the contents of this data frame by calling the pd.plot function. The code below generates a figure with three subplots displayed vertically, each of which shows a bar plot for a particular column of the data frame. The plots are automatically labelled with the column names of the data frame, and the whole procedure takes literally no time.

fig, axes = plt.subplots(nrows=3, ncols=1)
for i, c in enumerate(df.columns):
    df[c].plot(kind='bar', ax=axes[i], figsize=(12, 10), title=c)
plt.savefig('EU1.png', bbox_inches='tight')

The output figure looks like this:


Customization with matplotlib directives

While this is an acceptable plot for the first steps of data exploration, the figure is not really publication-ready. It also looks very much “academic” and lacks that subtle flair that infographics in mainstream media have. Over the next paragraphs we will turn this plot into a much more beautiful object by playing around with the options that matplotlib supplies.

Let us first start by creating a figure and an axis object that will contain our subfigure:

# Create a figure of given size
fig = plt.figure(figsize=(16,12))
# Add a subplot
ax = fig.add_subplot(111)
# Set title
ttl = 'Population, size and age expectancy in the European Union'

Colors are very important for data visualizations. By default, the matplotlib color palette offers solid hues, which can be softened by applying transparencies. Similarly, the default colorbars can be customized to match our taste (see below how one can define a custom-made color map with a gradient that softly changes from orange to gray-blue hues).

# Set color transparency (0: transparent; 1: solid)
a = 0.7
# Create a colormap
customcmap = [(x/24.0,  x/48.0, 0.05) for x in range(len(df))]

The main plotting instruction in our figure uses the pandas plot wrapper. In the initialization options, we specify the type of plot (horizontal bar), the transparency, the color of the bars following the above-defined custom color map, the x-axis limits and the figure title. We also set the color of the bar borders to white for a cleaner look.

# Plot the 'population' column as horizontal bar plot
df['population'].plot(kind='barh', ax=ax, alpha=a, legend=False, color=customcmap,
                      edgecolor='w', xlim=(0,max(df['population'])), title=ttl)

After this simple pandas plot directive, the figure already looks very promising. Note that, because we sorted the data frame by life expectancy and applied a gradient color map, the color of the different bars in itself carries information. We will explicitly label that information below when constructing a color bar. For now we want to remove the grid, frame and axes lines from our plot, as well as customize its title and x,y axes labels.

# Remove grid lines (dotted lines inside plot)
# Remove plot frame
# Pandas trick: remove weird dotted line on axis

# Customize title, set position, allow space on top of plot for title
ax.set_title(ax.get_title(), fontsize=26, alpha=a, ha='left')

# Set x axis label on top of plot, set label text
xlab = 'Population (in millions)'
ax.set_xlabel(xlab, fontsize=20, alpha=a, ha='left')
ax.xaxis.set_label_coords(0, 1.04)

# Position x tick labels on top
# Remove tick lines in x and y axes

# Customize x tick lables
xticks = [5,10,20,50,80]
ax.set_xticklabels(xticks, fontsize=16, alpha=a)

# Customize y tick labels
yticks = [item.get_text() for item in ax.get_yticklabels()]
ax.set_yticklabels(yticks, fontsize=16, alpha=a)

So far, the lenghts of our horizontal bars display the population (in millions) of the EU countries. All bars have the same height (which is set to 50% of the total space between bars by default by pandas). An interesting idea is to use the height of the bars to display further data. If we could made the bar height dependent on, say, the countries’ extension, we would be adding an supplementary piece of information to the plot. This is possible in matplotlib by accessing the elements that contain the bars and assigning them a specific height in a for loop. Each bar is an element of the class Rectangle, and all the corresponding class methods can be applied to it. For assigning a given height according to each country’s extension, we code a simple linear interpolation and create a lambda function to apply it.

# Set bar height dependent on country extension
# Set min and max bar thickness (from 0 to 1)
hmin, hmax = 0.3, 0.9
xmin, xmax = min(df['extension']), max(df['extension'])
# Function that interpolates linearly between hmin and hmax
f = lambda x: hmin + (hmax-hmin)*(x-xmin)/(xmax-xmin)
# Make array of heights
hs = [f(x) for x in df['extension']]

# Iterate over bars
for container in ax.containers:
    # Each bar has a Rectangle element as child
    for i,child in enumerate(container.get_children()):
        # Reset the lower left point of each bar so that bar is centered
        child.set_y(child.get_y()- 0.125 + 0.5-hs[i]/2)
        # Attribute height to each Recatangle according to country's size
        plt.setp(child, height=hs[i])

Having added this “dimension” to the plot, we need a way of labelling the information so that the countries’ extension is understandable. A legend would be the ideal solution, but since our plotting directive was set to display the column ['population'], we can not use the default. We can construct a “fake” legend though, and custom-made its handles to roughly match the height of the bars. We position the legend in the lower right part of our plot.

# Legend
# Create fake labels for legend
l1 = Line2D([], [], linewidth=6, color='k', alpha=a) 
l2 = Line2D([], [], linewidth=12, color='k', alpha=a) 
l3 = Line2D([], [], linewidth=22, color='k', alpha=a)

# Set three legend labels to be min, mean and max of countries extensions 
# (rounded up to 10k km2)
rnd = 10000
labels = [str(int(round(l/rnd)*rnd)) for l in min(df['extension']), 
          mean(df['extension']), max(df['extension'])]

# Position legend in lower right part
# Set ncol=3 for horizontally expanding legend
leg = ax.legend([l1, l2, l3], labels, ncol=3, frameon=False, fontsize=16, 
                bbox_to_anchor=[1.1, 0.11], handlelength=2, 
                handletextpad=1, columnspacing=2, title='Size (in km2)')

# Customize legend title
# Set position to increase space between legend and labels
plt.setp(leg.get_title(), fontsize=20, alpha=a)
leg.get_title().set_position((0, 10))
# Customize transparency for legend labels
[plt.setp(label, alpha=a) for label in leg.get_texts()]

Finally, there is another piece of information in the plot that needs to be labelled, and that is the color map indicating the average life expectancy in the EU countries. Since we used a custom-made color map, the regular call to plt.colorbar() would not work. We need to create a LinearSegmentedColormap instead and “trick” matplotlib to display it as a colorbar. Then we can use the usual customization methods from colorbar to set fonts, transparency, position and size of the diverse elements in the color legend.

# Create a fake colorbar
ctb = LinearSegmentedColormap.from_list('custombar', customcmap, N=2048)
# Trick from http://stackoverflow.com/questions/8342549/
# matplotlib-add-colorbar-to-a-sequence-of-line-plots
sm = plt.cm.ScalarMappable(cmap=ctb, norm=plt.normalize(vmin=72, vmax=84))
# Fake up the array of the scalar mappable
sm._A = []

# Set colorbar, aspect ratio
cbar = plt.colorbar(sm, alpha=0.05, aspect=16, shrink=0.4)
# Remove colorbar container frame
# Fontsize for colorbar ticklabels
# Customize colorbar tick labels
mytks = range(72,86,2)
cbar.ax.set_yticklabels([str(a) for a in mytks], alpha=a)

# Colorbar label, customize fontsize and distance to colorbar
cbar.set_label('Age expectancy (in years)', alpha=a, 
               rotation=270, fontsize=20, labelpad=20)
# Remove color bar tick lines, while keeping the tick labels
cbarytks = plt.getp(cbar.ax.axes, 'yticklines')
plt.setp(cbarytks, visible=False)

The final and most rewarding step consists of saving the figure in our preferred format.

# Save figure in png with tight bounding box
plt.savefig('EU.png', bbox_inches='tight', dpi=300)

The final result looks this beautiful:


Table-top data experiment take-away message

When producing a plot based on multidimensional data, it is a good idea to resort to shapes and colors that visually guide us through the variables on display. Matplotlib offers a high level of customization for all details of a plot, albeit the truth is that finding exactly which knob to tweak might be at times bewildering. Beautiful plots can be created by experimenting with various settings, among which hues, transparencies and simple layouts are the focal points. The results are publication-ready figures with open-source software that can be easily replicated by means of structured python code.