## Archivi Categorie: NumPy

### NumPy – 89 – cos’è il machine learning – 1

Luciana Myriam Nicolosi

Continuo da qui, copio qui.

Before we take a look at the details of various machine learning methods, let’s start by looking at what machine learning is, and what it isn’t. Machine learning is often categorized as a subfield of artificial intelligence, but I find that categorization can often be misleading at first brush. The study of machine learning certainly arose from research in this context, but in the data science application of machine learning methods, it’s more helpful to think of machine learning as a means of building models of data.

Fundamentally, machine learning involves building mathematical models to help understand data. “Learning” enters the fray when we give these models tunable parameters that can be adapted to observed data; in this way the program can be considered to be “learning” from the data. Once these models have been fit to previously seen data, they can be used to predict and understand aspects of newly observed data. I’ll leave to the reader the more philosophical digression regarding the extent to which this type of mathematical, model-based “learning” is similar to the “learning” exhibited by the human brain.

Understanding the problem setting in machine learning is essential to using these tools effectively, and so we will start with some broad categorizations of the types of approaches we’ll discuss here.

OK 😎 un po’ più tranquillo 😁

Categorie del machine learning
At the most fundamental level, machine learning can be categorized into two main types: supervised learning and unsupervised learning.

• Supervised learning involves somehow modeling the relationship between measured features of data and some label associated with the data; once this model is determined, it can be used to apply labels to new, unknown data. This is further subdivided into classification tasks and regression tasks: in classification, the labels are discrete categories, while in regression, the labels are continuous quantities. We will see examples of both types of supervised learning in the following section.
• Unsupervised learning involves modeling the features of a dataset without reference to any label, and is often described as “letting the dataset speak for itself.” These models include tasks such as clustering and dimensionality reduction. Clustering algorithms identify distinct groups of data, while dimensionality reduction algorithms search for more succinct representations of the data. We will see examples of both types of unsupervised learning in the following section.

In addition, there are so-called semi-supervised learning methods, which falls somewhere between supervised learning and unsupervised learning. Semi-supervised learning methods are often useful when only incomplete labels are available.

Esempi qualitativi di applicazioni machine learning
To make these ideas more concrete, let’s take a look at a few very simple examples of a machine learning task. These examples are meant to give an intuitive, non-quantitative overview of the types of machine learning tasks we will be looking at in this chapter. In later sections, we will go into more depth regarding the particular models and how they are used. For a preview of these more technical aspects, you can find the Python source that generates the following figures in the Appendix: Figure Code.

classificazione: predire etichette discrete
We will first take a look at a simple classification task, in which you are given a set of labeled points and want to use these to classify some unlabeled points.

Per compltezza copio anche il codice disponibile all’URL indicato. Notare che manca l’istruizione import matplotlib.pyplot as plt.

from sklearn.datasets.samples_generator import make_blobs
from sklearn.svm import SVC

# create 50 separable points
X, y = make_blobs(n_samples=50, centers=2,
random_state=0, cluster_std=0.60)

# fit the support vector classifier model
clf = SVC(kernel='linear')
clf.fit(X, y)

# create some new points to predict
X2, _ = make_blobs(n_samples=80, centers=2,
random_state=0, cluster_std=0.80)
X2 = X2[50:]

# predict the labels
y2 = clf.predict(X2)

# plot the data
import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(8, 6))

# common plot formatting for below
def format_plot(ax, title):
ax.xaxis.set_major_formatter(plt.NullFormatter())
ax.yaxis.set_major_formatter(plt.NullFormatter())
ax.set_xlabel('feature 1', color='gray')
ax.set_ylabel('feature 2', color='gray')
ax.set_title(title, color='gray')

format_plot(ax, 'Input Data')
point_style = dict(cmap='Paired', s=50)
ax.scatter(X[:, 0], X[:, 1], c=y, **point_style)

ax.axis([-1, 4, -2, 7])

fig.savefig('no868.png')

Here we have two-dimensional data: that is, we have two features for each point, represented by the (x,y) positions of the points on the plane. In addition, we have one of two class labels for each point, here represented by the colors of the points. From these features and labels, we would like to create a model that will let us decide whether a new point should be labeled “blue” or “red.”

There are a number of possible models for such a classification task, but here we will use an extremely simple one. We will make the assumption that the two groups can be separated by drawing a straight line through the plane between them, such that points on each side of the line fall in the same group. Here the model is a quantitative version of the statement “a straight line separates the classes”, while the model parameters are the particular numbers describing the location and orientation of that line for our data. The optimal values for these model parameters are learned from the data (this is the “learning” in machine learning), which is often called training the model.

The following figure shows a visual representation of what the trained model looks like for this data:

from sklearn.datasets.samples_generator import make_blobs
from sklearn.svm import SVC
import numpy as np
import matplotlib.pyplot as plt

# common plot formatting for below
def format_plot(ax, title):
ax.xaxis.set_major_formatter(plt.NullFormatter())
ax.yaxis.set_major_formatter(plt.NullFormatter())
ax.set_xlabel('feature 1', color='gray')
ax.set_ylabel('feature 2', color='gray')
ax.set_title(title, color='gray')

# create 50 separable points
X, y = make_blobs(n_samples=50, centers=2,
random_state=0, cluster_std=0.60)

# fit the support vector classifier model
clf = SVC(kernel='linear')
clf.fit(X, y)

# Get contours describing the model
xx = np.linspace(-1, 4, 10)
yy = np.linspace(-2, 7, 10)
xy1, xy2 = np.meshgrid(xx, yy)
Z = np.array([clf.decision_function([t])
for t in zip(xy1.flat, xy2.flat)]).reshape(xy1.shape)

# plot points and model
fig, ax = plt.subplots(figsize=(8, 6))
point_style = dict(cmap='Paired', s=50)
line_style = dict(levels = [-1.0, 0.0, 1.0],
linestyles = ['dashed', 'solid', 'dashed'],
colors = 'gray', linewidths=1)
ax.scatter(X[:, 0], X[:, 1], c=y, **point_style)
ax.contour(xy1, xy2, Z, **line_style)

# format plot
format_plot(ax, 'Model Learned from Input Data')
ax.axis([-1, 4, -2, 7])

fig.savefig('np869.png')

Now that this model has been trained, it can be generalized to new, unlabeled data. In other words, we can take a new set of data, draw this model line through it, and assign labels to the new points based on this model. This stage is usually called prediction. See the following figure:

from sklearn.datasets.samples_generator import make_blobs
from sklearn.svm import SVC
import numpy as np
import matplotlib.pyplot as plt

# common plot formatting for below
def format_plot(ax, title):
ax.xaxis.set_major_formatter(plt.NullFormatter())
ax.yaxis.set_major_formatter(plt.NullFormatter())
ax.set_xlabel('feature 1', color='gray')
ax.set_ylabel('feature 2', color='gray')
ax.set_title(title, color='gray')

# create 50 separable points
X, y = make_blobs(n_samples=50, centers=2,
random_state=0, cluster_std=0.60)

# fit the support vector classifier model
clf = SVC(kernel='linear')
clf.fit(X, y)

# Get contours describing the model
xx = np.linspace(-1, 4, 10)
yy = np.linspace(-2, 7, 10)
xy1, xy2 = np.meshgrid(xx, yy)
Z = np.array([clf.decision_function([t])
for t in zip(xy1.flat, xy2.flat)]).reshape(xy1.shape)

# create 50 separable points
X, y = make_blobs(n_samples=50, centers=2,
random_state=0, cluster_std=0.60)

# fit the support vector classifier model
clf = SVC(kernel='linear')
clf.fit(X, y)

# create some new points to predict
X2, _ = make_blobs(n_samples=80, centers=2,
random_state=0, cluster_std=0.80)
X2 = X2[50:]

# predict the labels
y2 = clf.predict(X2)

point_style = dict(cmap='Paired', s=50)
line_style = dict(levels = [-1.0, 0.0, 1.0],
linestyles = ['dashed', 'solid', 'dashed'],
colors = 'gray', linewidths=1)

# plot the results
fig, ax = plt.subplots(1, 2, figsize=(16, 6))

ax[0].scatter(X2[:, 0], X2[:, 1], c='gray', **point_style)
ax[0].axis([-1, 4, -2, 7])

ax[1].scatter(X2[:, 0], X2[:, 1], c=y2, **point_style)
ax[1].contour(xy1, xy2, Z, **line_style)
ax[1].axis([-1, 4, -2, 7])

format_plot(ax[0], 'Unknown Data')
format_plot(ax[1], 'Predicted Labels')

fig.savefig('np870.png')

This is the basic idea of a classification task in machine learning, where “classification” indicates that the data has discrete class labels. At first glance this may look fairly trivial: it would be relatively easy to simply look at this data and draw such a discriminatory line to accomplish this classification. A benefit of the machine learning approach, however, is that it can generalize to much larger datasets in many more dimensions.

For example, this is similar to the task of automated spam detection for email; in this case, we might use the following features and labels:

• feature 1, feature 2, etc. →  normalized counts of important words or phrases (“Viagra”, “Nigerian prince”, etc.)
• label → “spam” or “not spam”

For the training set, these labels might be determined by individual inspection of a small representative sample of emails; for the remaining emails, the label would be determined using the model. For a suitably trained classification algorithm with enough well-constructed features (typically thousands or millions of words or phrases), this type of approach can be very effective. We will see an example of such text-based classification in In Depth: Naive Bayes Classification [prossimamente].

Some important classification algorithms that we will discuss in more detail are Gaussian naive Bayes (see In Depth: Naive Bayes Classification [prossimamente]), support vector machines (see In-Depth: Support Vector Machines [prossimamente]), and random forest classification (see In-Depth: Decision Trees and Random Forests [prossimamente]).

Pausa 😯 poi si continua 😊

### NumPy – 88 – machine learning

Continuo da qui, copio qui.

L’ultimo capitolo del Notebook di Jake, diversi prossimi posts, è –come dire: sapete che sono vecchio; ma anche incosciente, avventuroso, parto 🤖

In many ways, machine learning is the primary means by which data science manifests itself to the broader world. Machine learning is where these computational and algorithmic skills of data science meet the statistical thinking of data science, and the result is a collection of approaches to inference and data exploration that are not about effective theory so much as effective computation.

The term “machine learning” is sometimes thrown around as if it is some kind of magic pill: apply machine learning to your data, and all your problems will be solved! As you might expect, the reality is rarely this simple. While these methods can be incredibly powerful, to be effective they must be approached with a firm grasp of the strengths and weaknesses of each method, as well as a grasp of general concepts such as bias and variance, overfitting and underfitting, and more.

This chapter will dive into practical aspects of machine learning, primarily using Python’s Scikit-Learn package. This is not meant to be a comprehensive introduction to the field of machine learning; that is a large subject and necessitates a more technical approach than we take here. Nor is it meant to be a comprehensive manual for the use of the Scikit-Learn package (for this, you can refer to the resources listed in Further Machine Learning Resources [prossimamente]). Rather, the goals of this chapter are:

• To introduce the fundamental vocabulary and concepts of machine learning.
• To introduce the Scikit-Learn API and show some examples of its use.
• To take a deeper dive into the details of several of the most important machine learning approaches, and develop an intuition into how they work and when and where they are applicable.

Much of this material is drawn from the Scikit-Learn tutorials and workshops I have given on several occasions at PyCon, SciPy, PyData, and other conferences. Any clarity in the following pages is likely due to the many workshop participants and co-instructors who have given me valuable feedback on this material over the years!

Finally, if you are seeking a more comprehensive or technical treatment of any of these subjects, I’ve listed several resources and references in Further Machine Learning Resources [prossimamente].

Panico? 😯 un po’😡  no, niente panico, si parte 😎

### NumPy – 87 – ulteriori risorse Matplotlib

Continuo da qui, copio qui.

Jake è saggio, ecco qui… 😁
A single chapter in a book can never hope to cover all the available features and plot types available in Matplotlib. As with other packages we’ve seen, liberal use of IPython’s tab-completion and help functions (see Help and Documentation in IPython [qui]) can be very helpful when exploring Matplotlib’s API. In addition, Matplotlib’s online documentation can be a helpful reference. See in particular the Matplotlib gallery linked on that page: it shows thumbnails of hundreds of different plot types, each one linked to a page with the Python code snippet used to generate it. In this way, you can visually inspect and learn about a wide range of different plotting styles and visualization techniques.

For a book-length treatment of Matplotlib, I would recommend Interactive Applications Using Matplotlib, written by Matplotlib core developer Ben Root.

Altre librerie grafiche per Python
Although Matplotlib is the most prominent Python visualization library, there are other more modern tools that are worth exploring as well. I’ll mention a few of them briefly here:

• Bokeh is a JavaScript visualization library with a Python frontend that creates highly interactive visualizations capable of handling very large and/or streaming datasets. The Python front-end outputs a JSON data structure that can be interpreted by the Bokeh JS engine.
• Plotly is the eponymous open source product of the Plotly company, and is similar in spirit to Bokeh. Because Plotly is the main product of a startup, it is receiving a high level of development effort. Use of the library is entirely free.
• Vispy is an actively developed project focused on dynamic visualizations of very large datasets. Because it is built to target OpenGL and make use of efficient graphics processors in your computer, it is able to render some quite large and stunning visualizations.
• Vega and Vega-Lite are declarative graphics representations, and are the product of years of research into the fundamental language of data visualization. The reference rendering implementation is JavaScript, but the API is language agnostic. There is a Python API under development in the Altair package. Though as of summer 2016 it’s not yet fully mature, I’m quite excited for the possibilities of this project to provide a common reference point for visualization in Python and other languages.

The visualization space in the Python community is very dynamic, and I fully expect this list to be out of date as soon as it is published. Keep an eye out for what’s coming in the future!

Uh! 😯 devo indagare, con l’aiuto –se possibile– di Dirk 😜

### NumPy – 86 – visualizzazioni con Seaborn – 2

Gianni Krattli

Continuo da qui, copio qui.

Esempio: esplorare i tempi impiegati per la maratona
Here we’ll look at using Seaborn to help visualize and understand finishing results from a marathon. I’ve scraped the data from sources on the Web, aggregated it and removed any identifying information, and put it on GitHub where it can be downloaded (if you are interested in using Python for web scraping, I would recommend Web Scraping with Python by Ryan Mitchell). We will start by downloading the data from the Web, and loading it into Pandas:

Let’s fix this by providing a converter for the times:

Qui ci sono un po’ di funzioni deprecate, non vale come al solito ignorare i warnings, ho corretto il codice di Jake

That looks much better. For the purpose of our Seaborn plotting utilities, let’s next add columns that give the times in seconds:

To get an idea of what the data looks like, we can plot a jointplot over the data:

The dotted line shows where someone’s time would lie if they ran the marathon at a perfectly steady pace. The fact that the distribution lies above this indicates (as you might expect) that most people slow down over the course of the marathon. If you have run competitively, you’ll know that those who do the opposite—run faster during the second half of the race—are said to have “negative-split” the race.

Let’s create another column in the data, the split fraction, which measures the degree to which each runner negative-splits or positive-splits the race:

Where this split difference is less than zero, the person negative-split the race by that fraction. Let’s do a distribution plot of this split fraction:

Out of nearly 40,000 participants, there were only 250 people who negative-split their marathon.

Let’s see whether there is any correlation between this split fraction and other variables. We’ll do this using a pairgrid, which draws plots of all these correlations:

It looks like the split fraction does not correlate particularly with age, but does correlate with the final time: faster runners tend to have closer to even splits on their marathon time. (We see here that Seaborn is no panacea for Matplotlib’s ills when it comes to plot styles: in particular, the x-axis labels overlap. Because the output is a simple Matplotlib plot, however, the methods in Customizing Ticks [qui] can be used to adjust such things if desired.)

The difference between men and women here is interesting. Let’s look at the histogram of split fractions for these two groups:

The interesting thing here is that there are many more men than women who are running close to an even split! This almost looks like some kind of bimodal distribution among the men and women. Let’s see if we can suss-out what’s going on by looking at the distributions as a function of age.

A nice way to compare distributions is to use a violin plot

This is yet another way to compare the distributions between men and women.

Let’s look a little deeper, and compare these violin plots as a function of age. We’ll start by creating a new column in the array that specifies the decade of age that each person is in:

Looking at this, we can see where the distributions of men and women differ: the split distributions of men in their 20s to 50s show a pronounced over-density toward lower splits when compared to women of the same age (or of any age, for that matter).

Also surprisingly, the 80-year-old women seem to outperform everyone in terms of their split time. This is probably due to the fact that we’re estimating the distribution from small numbers, as there are only a handful of runners in that range:

Back to the men with negative splits: who are these runners? Does this split fraction correlate with finishing quickly? We can plot this very easily. We’ll use regplot, which will automatically fit a linear regression to the data:

Apparently the people with fast splits are the elite runners who are finishing within ~15,000 seconds, or about 4 hours. People slower than that are much less likely to have a fast second split.

### NumPy – 85 – visualizzazioni con Seaborn – 1

Continuo da qui, copio qui.

Matplotlib has proven to be an incredibly useful and popular visualization tool, but even avid users will admit it often leaves much to be desired. There are several valid complaints about Matplotlib that often come up: No non faccio l’elenco; tanto è vecchio 😊

An answer to these problems is Seaborn. Seaborn provides an API on top of Matplotlib that offers sane choices for plot style and color defaults, defines simple high-level functions for common statistical plot types, and integrates with the functionality provided by Pandas DataFrames.

To be fair, the Matplotlib team is addressing this: it has recently added the plt.style tools discussed in Customizing Matplotlib: Configurations and Style Sheets [qui], and is starting to handle Pandas data more seamlessly. The 2.0 release of the library will include a new default stylesheet that will improve on the current status quo. But for all the reasons just discussed, Seaborn remains an extremely useful addon.

Confronto tra Seaborn e Matplotlib
Here is an example of a simple random-walk plot in Matplotlib, using its classic plot formatting and colors.

Although the result contains all the information we’d like it to convey, it does so in a way that is not all that aesthetically pleasing, and even looks a bit old-fashioned in the context of 21st-century data visualization.

Now let’s take a look at how it works with Seaborn. As we will see, Seaborn has many of its own high-level plotting routines, but it can also overwrite Matplotlib’s default parameters and in turn get even simple Matplotlib scripts to produce vastly superior output. We can set the style by calling Seaborn’s set() method. By convention, Seaborn is imported as sns:

Ah, much better!

Esplorazione dei grafici di Seaborn
The main idea of Seaborn is that it provides high-level commands to create a variety of plot types useful for statistical data exploration, and even some statistical model fitting.

Let’s take a look at a few of the datasets and plot types available in Seaborn. Note that all of the following could be done using raw Matplotlib commands (this is, in fact, what Seaborn does under the hood) but the Seaborn API is much more convenient.

istogrammi, KDE (finestre di Parzen) e grafici di densità
Often in statistical data visualization, all you want is to plot histograms and joint distributions of variables. We have seen that this is relatively straightforward in Matplotlib:

Rather than a histogram, we can get a smooth estimate of the distribution using a kernel density estimation, which Seaborn does with sns.kdeplot:

Histograms and KDE can be combined using distplot:

If we pass the full two-dimensional dataset to kdeplot, we will get a two-dimensional visualization of the data:

We can see the joint distribution and the marginal distributions together using sns.jointplot. For this plot, we’ll set the style to a white background:

There are other parameters that can be passed to jointplot —for example, we can use a hexagonally based histogram instead:

grafici di coppie (pair)

When you generalize joint plots to datasets of larger dimensions, you end up with pair plots. This is very useful for exploring correlations between multidimensional data, when you’d like to plot all pairs of values against each other.

We’ll demo this with the well-known Iris dataset, which lists measurements of petals and sepals of three iris species:

Visualizing the multidimensional relationships among the samples is as easy as calling sns.pairplot:

istogrammi sfaccettati
Sometimes the best way to view data is via histograms of subsets. Seaborn’s FacetGrid makes this extremely simple. We’ll take a look at some data that shows the amount that restaurant staff receive in tips based on various indicator data:

grafici factor
Factor plots can be useful for this kind of visualization as well. This allows you to view the distribution of a parameter within bins defined by any other parameter:

unione di distribuzioni
Similar to the pairplot we saw earlier, we can use sns.jointplot to show the joint distribution between different datasets, along with the associated marginal distributions:

The joint plot can even do some automatic kernel density estimation and regression:

grafici a barre
Time series can be plotted using sns.factorplot. In the following example, we’ll use the Planets data that we first saw in Aggregation and Grouping [qui]:

We can learn more by looking at the method of discovery of each of these planets:

OK, ci vorrebbe un monitor più largo 😊

For more information on plotting with Seaborn, see the Seaborn documentation, a tutorial, and the Seaborn gallery.

Non so voi ma io sono thunderstrucked, very assay 😯

### NumPy – 84 – grafici geografici con dati Basemap – 2

Continuo da qui, copio qui.

Tipi di mappe
Earlier we saw the bluemarble() and shadedrelief() methods for projecting global images on the map, as well as the drawparallels() and drawmeridians() methods for drawing lines of constant latitude and longitude. The Basemap package contains a range of useful functions for drawing borders of physical features like continents, oceans, lakes, and rivers, as well as political boundaries such as countries and US states and counties. The following are some of the available drawing functions that you may wish to explore using IPython’s help features:

confini fisici e bacini d’acqua

• drawcoastlines(): Draw continental coast lines
• drawlsmask(): Draw a mask between the land and sea, for use with projecting images on one or the other
• drawmapboundary(): Draw the map boundary, including the fill color for oceans.
• drawrivers(): Draw rivers on the map
• fillcontinents(): Fill the continents with a given color; optionally fill lakes with another color

confini politici

• drawcountries(): Draw country boundaries
• drawstates(): Draw US state boundaries
• drawcounties(): Draw US county boundaries
• Caratteristiche delle mappe
• drawgreatcircle(): Draw a great circle between two points
• drawparallels(): Draw lines of constant latitude
• drawmeridians(): Draw lines of constant longitude
• drawmapscale(): Draw a linear scale on the map

immagini globali

• bluemarble(): Project NASA’s blue marble image onto the map
• shadedrelief(): Project a shaded relief image onto the map
• etopo(): Draw an etopo relief image onto the map
• warpimage(): Project a user-provided image onto the map

For the boundary-based features, you must set the desired resolution when creating a Basemap image. The resolution argument of the Basemap class sets the level of detail in boundaries, either 'c' (crude), 'l' (low), 'i' (intermediate), 'h' (high), 'f' (full), or None if no boundaries will be used. This choice is important: setting high-resolution boundaries on a global map, for example, can be very slow.

Here’s an example of drawing land/sea boundaries, and the effect of the resolution parameter. We’ll create both a low- and high-resolution map of Scotland’s beautiful Isle of Skye. It’s located at 57.3°N, 6.2°W, and a map of 90,000 × 120,000 kilometers shows it well:

Notice that the low-resolution coastlines are not suitable for this level of zoom, while high-resolution works just fine. The low level would work just fine for a global view, however, and would be much faster than loading the high-resolution border data for the entire globe! It might require some experimentation to find the correct resolution parameter for a given view: the best route is to start with a fast, low-resolution plot and increase the resolution as needed.

Disegnare dati sulle mappe
Perhaps the most useful piece of the Basemap toolkit is the ability to over-plot a variety of data onto a map background. For simple plotting and text, any plt function works on the map; you can use the Basemap instance to project latitude and longitude coordinates to (x, y) coordinates for plotting with plt, as we saw earlier in the Seattle example.

In addition to this, there are many map-specific functions available as methods of the Basemap instance. These work very similarly to their standard Matplotlib counterparts, but have an additional Boolean argument latlon, which if set to True allows you to pass raw latitudes and longitudes to the method, rather than projected (x, y) coordinates.

Some of these map-specific methods are:

• contour()/contourf() : Draw contour lines or filled contours
• imshow(): Draw an image
• pcolor()/pcolormesh(): Draw a pseudocolor plot for irregular/regular meshes
• plot(): Draw lines and/or markers.
• scatter(): Draw points with markers.
• quiver(): Draw vectors.
• barbs(): Draw wind barbs.
• drawgreatcircle(): Draw a great circle.

We’ll see some examples of a few of these as we continue. For more information on these functions, including several example plots, see the online Basemap documentation.

Esempio: le città della California
Recall that in Customizing Plot Legends [qui], we demonstrated the use of size and color in a scatter plot to convey information about the location, size, and population of California cities. Here, we’ll create this plot again, but using Basemap to put the data in context.

Serve la directory data contenente il DB csv già usato per il post indicato.
Siccome lo script è decisamente lungo non metto lo screenshot ma il codice (cal-cities.py).

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

import pandas as pd

# Extract the data we're interested in
lat = cities['latd'].values
lon = cities['longd'].values
population = cities['population_total'].values
area = cities['area_total_km2'].values

# Next, we set up the map projection, scatter the data,
#       and then create a colorbar and legend:

# 1. Draw the map background
fig = plt.figure(figsize=(8, 8))
m = Basemap(projection='lcc', resolution='h',
lat_0=37.5, lon_0=-119,
width=1E6, height=1.2E6)
m.drawcoastlines(color='gray')
m.drawcountries(color='gray')
m.drawstates(color='gray')

# 2. scatter city data, with color reflecting population
# and size reflecting area
m.scatter(lon, lat, latlon=True,
c=np.log10(population), s=area,
cmap='Reds', alpha=0.5)

# 3. create colorbar and legend
plt.colorbar(label=r'$\log_{10}({\rm population})$')
plt.clim(3, 7)

# make legend with dummy points
for a in [100, 300, 500]:
plt.scatter([], [], c='k', alpha=0.5, s=a,
label=str(a) + ' km$^2$')
plt.legend(scatterpoints=1, frameon=False,
labelspacing=1, loc='lower left');

fig.savefig('np813.png')

Al solito alcune funzioni sono deprecate, se si dovesse usare ci sarebbero aggiornamenti da considerare, il mondo avanza, non si ferma mai 😯:

This shows us roughly where larger populations of people have settled in California: they are clustered near the coast in the Los Angeles and San Francisco areas, stretched along the highways in the flat central valley, and avoiding almost completely the mountainous regions along the borders of the state.

A questo punto Jake propone un altro esempio, Example: Surface Temperature Data ma i dati richiesti non sono più disponibili; ho tentato di recuperare i nuovi dati sul sito della NASA, qui, ma non sono riuscito a riprodurre l’esempio. Che peraltro è qualcosa diverso dai miei interessi informatici. Il riscaldamento globale invece è un grosso problema; per tutti.

### NumPy – 83 – grafici geografici con dati Basemap – 1

Continuo da qui, copio qui.

One common type of visualization in data science is that of geographic data. Matplotlib’s main tool for this type of visualization is the Basemap toolkit, which is one of several Matplotlib toolkits which lives under the mpl_toolkits namespace. Admittedly, Basemap feels a bit clunky to use, and often even simple visualizations take much longer to render than you might hope. More modern solutions such as leaflet or the Google Maps API may be a better choice for more intensive map visualizations. Still, Basemap is a useful tool for Python users to have in their virtual toolbelts. In this section, we’ll show several examples of the type of map visualization that is possible with this toolkit.

Installation of Basemap is straightforward; if you’re using conda you can type this and the package will be downloaded:

We add just a single new import to our standard boilerplate:

Once you have the Basemap toolkit installed and imported, geographic plots are just a few lines away (the graphics in the following also requires the PIL package in Python 2, or the pillow package in Python 3):

OK, ci sarebbe qualche aggiornamento da considerare 😊

The meaning of the arguments to Basemap will be discussed momentarily.

The useful thing is that the globe shown here is not a mere image; it is a fully-functioning Matplotlib axes that understands spherical coordinates and which allows us to easily overplot data on the map! For example, we can use a different map projection, zoom-in to North America and plot the location of Seattle. We’ll use an etopo image (which shows topographical features both on land and under the ocean) as the map background:

This gives you a brief glimpse into the sort of geographic visualizations that are possible with just a few lines of Python. We’ll now discuss the features of Basemap in more depth, and provide several examples of visualizing map data. Using these brief examples as building blocks, you should be able to create nearly any map visualization that you desire.

Tipi di proiezione
The first thing to decide when using maps is what projection to use. You’re probably familiar with the fact that it is impossible to project a spherical map, such as that of the Earth, onto a flat surface without somehow distorting it or breaking its continuity. These projections have been developed over the course of human history, and there are a lot of choices! Depending on the intended use of the map projection, there are certain map features (e.g., direction, area, distance, shape, or other considerations) that are useful to maintain.

The Basemap package implements several dozen such projections, all referenced by a short format code. Here we’ll briefly demonstrate some of the more common ones.

We’ll start by defining a convenience routine to draw our world map along with the longitude and latitude lines:

proiezioni cilindriche
The simplest of map projections are cylindrical projections, in which lines of constant latitude and longitude are mapped to horizontal and vertical lines, respectively. This type of mapping represents equatorial regions quite well, but results in extreme distortions near the poles. The spacing of latitude lines varies between different cylindrical projections, leading to different conservation properties, and different distortion near the poles. In the following figure we show an example of the equidistant cylindrical projection, which chooses a latitude scaling that preserves distances along meridians. Other cylindrical projections are the Mercator (projection='merc') and the cylindrical equal area (projection='cea') projections.

The additional arguments to Basemap for this view specify the latitude (lat) and longitude (lon) of the lower-left corner (llcrnr) and upper-right corner (urcrnr) for the desired map, in units of degrees.

proiezioni pseudo-cilindriche
Pseudo-cylindrical projections relax the requirement that meridians (lines of constant longitude) remain vertical; this can give better properties near the poles of the projection. The Mollweide projection (projection='moll') is one common example of this, in which all meridians are elliptical arcs. It is constructed so as to preserve area across the map: though there are distortions near the poles, the area of small patches reflects the true area. Other pseudo-cylindrical projections are the sinusoidal (projection='sinu‘) and Robinson (projection='robin') projections.

The extra arguments to Basemap here refer to the central latitude (lat_0) and longitude (lon_0) for the desired map.

proiezioni prospettive
Perspective projections are constructed using a particular choice of perspective point, similar to if you photographed the Earth from a particular point in space (a point which, for some projections, technically lies within the Earth!). One common example is the orthographic projection (projection='ortho'), which shows one side of the globe as seen from a viewer at a very long distance. As such, it can show only half the globe at a time. Other perspective-based projections include the gnomonic projection (projection='gnom') and stereographic projection (projection='stere'). These are often the most useful for showing small portions of the map.

Here is an example of the orthographic projection:

proiezioni coniche
A Conic projection projects the map onto a single cone, which is then unrolled. This can lead to very good local properties, but regions far from the focus point of the cone may become very distorted. One example of this is the Lambert Conformal Conic projection (projection='lcc'), which we saw earlier in the map of North America. It projects the map onto a cone arranged in such a way that two standard parallels (specified in Basemap by lat_1 and lat_2) have well-represented distances, with scale decreasing between them and increasing outside of them. Other useful conic projections are the equidistant conic projection (projection='eqdc') and the Albers equal-area projection (projection='aea'). Conic projections, like perspective projections, tend to be good choices for representing small to medium patches of the globe.

altre proiezioni
If you’re going to do much with map-based visualizations, I encourage you to read up on other available projections, along with their properties, advantages, and disadvantages. Most likely, they are available in the Basemap package. If you dig deep enough into this topic, you’ll find an incredible subculture of geo-viz geeks who will be ready to argue fervently in support of their favorite projection for any given application!

Pausa 😊

### NumPy – 82 – grafici tridimensionali

Continuo da qui, copio qui.

Matplotlib was initially designed with only two-dimensional plotting in mind. Around the time of the 1.0 release, some three-dimensional plotting utilities were built on top of Matplotlib’s two-dimensional display, and the result is a convenient (if somewhat limited) set of tools for three-dimensional data visualization. three-dimensional plots are enabled by importing the mplot3d toolkit, included with the main Matplotlib installation:

With this three-dimensional axes enabled, we can now plot a variety of three-dimensional plot types. Three-dimensional plotting is one of the functionalities that benefits immensely from viewing figures interactively rather than statically in the notebook; recall that to use interactive figures, you can use %matplotlib notebook rather than %matplotlib inline when running this code. Oppure usando show e mettendo lo script in pausa e operando sulla finestra del grafico (Grazie a s_ per l’osservazione).

Punti e liee tridimensionali
The most basic three-dimensional plot is a line or collection of scatter plot created from sets of (x, y, z) triples. In analogy with the more common two-dimensional plots discussed earlier, these can be created using the ax.plot3D and ax.scatter3D functions. The call signature for these is nearly identical to that of their two-dimensional counterparts, so you can refer to Simple Line Plots [qui] and Simple Scatter Plots [qui] for more information on controlling the output. Here we’ll plot a trigonometric spiral, along with some points drawn randomly near the line:

Notice that by default, the scatter points have their transparency adjusted to give a sense of depth on the page. While the three-dimensional effect is sometimes difficult to see within a static image, an interactive view can lead to some nice intuition about the layout of the points.

Grafici di superfici (contour) tridimensionali
Analogous to the contour plots we explored in Density and Contour Plots [qui], mplot3d contains tools to create three-dimensional relief plots using the same inputs. Like two-dimensional ax.contour plots, ax.contour3D requires all the input data to be in the form of two-dimensional regular grids, with the Z data evaluated at each point. Here we’ll show a three-dimensional contour diagram of a three-dimensional sinusoidal function:

Sometimes the default viewing angle is not optimal, in which case we can use the view_init method to set the elevation and azimuthal angles. In the following example, we’ll use an elevation of 60 degrees (that is, 60 degrees above the x-y plane) and an azimuth of 35 degrees (that is, rotated 35 degrees counter-clockwise about the z-axis):

Again, note that this type of rotation can be accomplished interactively by clicking and dragging when using one of Matplotlib’s interactive backends.

Grafici wireframes e di superficie
Two other types of three-dimensional plots that work on gridded data are wireframes and surface plots. These take a grid of values and project it onto the specified three-dimensional surface, and can make the resulting three-dimensional forms quite easy to visualize. Here’s an example of using a wireframe:

A surface plot is like a wireframe plot, but each face of the wireframe is a filled polygon. Adding a colormap to the filled polygons can aid perception of the topology of the surface being visualized:

Note that though the grid of values for a surface plot needs to be two-dimensional, it need not be rectilinear. Here is an example of creating a partial polar grid, which when used with the surface3D plot can give us a slice into the function we’re visualizing:

Triangolazioni di superfici
For some applications, the evenly sampled grids required by the above routines is overly restrictive and inconvenient. In these situations, the triangulation-based plots can be very useful. What if rather than an even draw from a Cartesian or a polar grid, we instead have a set of random draws?

We could create a scatter plot of the points to get an idea of the surface we’re sampling from:

This leaves a lot to be desired. The function that will help us in this case is ax.plot_trisurf, which creates a surface by first finding a set of triangles formed between adjacent points (remember that x, y, and z here are one-dimensional arrays):

The result is certainly not as clean as when it is plotted with a grid, but the flexibility of such a triangulation allows for some really interesting three-dimensional plots. For example, it is actually possible to plot a three-dimensional Möbius strip using this, as we’ll see next.

Esempio: la striscia di Möbius
A Möbius strip is similar to a strip of paper glued into a loop with a half-twist. Topologically, it’s quite interesting because despite appearances it has only a single side! Here we will visualize such an object using Matplotlib’s three-dimensional tools. The key to creating the Möbius strip is to think about it’s parametrization: it’s a two-dimensional strip, so we need two intrinsic dimensions. Let’s call them θ, which ranges from 0 to 2π around the loop, and w which ranges from -1 to 1 across the width of the strip:

Now from this parametrization, we must determine the (x, y, z) positions of the embedded strip.

Thinking about it, we might realize that there are two rotations happening: one is the position of the loop about its center (what we’ve called θ), while the other is the twisting of the strip about its axis (we’ll call this ϕ). For a Möbius strip, we must have the strip makes half a twist during a full loop, or Δϕ=Δθ/2.

Now we use our recollection of trigonometry to derive the three-dimensional embedding. We’ll define r, the distance of each point from the center, and use this to find the embedded (x,y,z) coordinates:

Finally, to plot the object, we must make sure the triangulation is correct. The best way to do this is to define the triangulation within the underlying parametrization, and then let Matplotlib project this triangulation into the three-dimensional space of the Möbius strip. This can be accomplished as follows:

Combining all of these techniques, it is possible to create and display a wide variety of three-dimensional objects and patterns in Matplotlib.

Jake & Matplotlib rockzs 🚀

### NumPy – 81 – personalizzare Matplotlib: configurazioni e stylesheets

Continuo da qui, copio qui.

Matplotlib’s default plot settings are often the subject of complaint among its users. While much is slated to change in the 2.0 Matplotlib release [uh! ormai…] in late 2016, the ability to customize default settings helps bring the package inline with your own aesthetic preferences.

Here we’ll walk through some of Matplotlib’s runtime configuration (rc) options, and take a look at the newer stylesheets feature, which contains some nice sets of default configurations.

Personalizzazioni al volo del grafico
Through this chapter, we’ve seen how it is possible to tweak individual plot settings to end up with something that looks a little bit nicer than the default. It’s possible to do these customizations for each individual plot. For example, here is a fairly drab default histogram:

We can adjust this by hand to make it a much more visually pleasing plot:

OK, ci sarebbe da aggiornarsi 😯

This looks better, and you may recognize the look as inspired by the look of the R language’s ggplot visualization package. But this took a whole lot of effort! We definitely do not want to have to do all that tweaking each time we create a plot. Fortunately, there is a way to adjust these defaults once in a way that will work for all plots.

Cambiare i defaults: rcParams
Each time Matplotlib loads, it defines a runtime configuration (rc) containing the default styles for every plot element you create. This configuration can be adjusted at any time using the plt.rc convenience routine. Let’s see what it looks like to modify the rc parameters so that our default plot will look similar to what we did before.

We’ll start by saving a copy of the current rcParams dictionary, so we can easily reset these changes in the current session:

Now we can use the plt.rc function to change some of these settings:

Let’s see what simple line plots look like with these rc parameters:

I find this much more aesthetically pleasing than the default styling. If you disagree with my aesthetic sense, the good news is that you can adjust the rc parameters to suit your own tastes! These settings can be saved in a .matplotlibrc file, which you can read about in the Matplotlib documentation. That said, I prefer to customize Matplotlib using its stylesheets instead.

Stylesheets
The version 1.4 release of Matplotlib in August 2014 added a very convenient style module, which includes a number of new default stylesheets, as well as the ability to create and package your own styles. These stylesheets are formatted similarly to the .matplotlibrc files mentioned earlier, but must be named with a .mplstyle extension.

Even if you don’t create your own style, the stylesheets included by default are extremely useful. The available styles are listed in plt.style.available —here I’ll list only the first five for brevity:

The basic way to switch to a stylesheet is to call

plt.style.use('stylename')

But keep in mind that this will change the style for the rest of the session! Alternatively, you can use the style context manager, which sets a style temporarily:

with plt.style.context('stylename'):
make_a_plot()

Let’s create a function that will make two basic types of plot:

We’ll use this to explore how these plots look using the various built-in styles.

stile di default
The default style is what we’ve been seeing so far throughout the book; we’ll start with that. First, let’s reset our runtime configuration to the notebook default:

stile FiveThiryEight
The fivethirtyeight style mimics the graphics found on the popular FiveThirtyEight website. As you can see here, it is typified by bold colors, thick lines, and transparent axes:

stile ggplot
The ggplot package in the R language is a very popular visualization tool. Matplotlib’s ggplot style mimics the default styles from that package:

stile Bayesian Methods for Hackers
There is a very nice short online book called Probabilistic Programming and Bayesian Methods for Hackers; it features figures created with Matplotlib, and uses a nice set of rc parameters to create a consistent and visually-appealing style throughout the book. This style is reproduced in the bmh stylesheet:

stile dark background
For figures used within presentations, it is often useful to have a dark rather than light background. The dark_background style provides this:

stile Grayscale
Sometimes you might find yourself preparing figures for a print publication that does not accept color figures. For this, the grayscale style, shown here, can be very useful:

stile Seaborn
Matplotlib also has stylesheets inspired by the Seaborn library (discussed more fully in Visualization With Seaborn [prossimamente]). As we will see, these styles are loaded automatically when Seaborn is imported into a notebook. I’ve found these settings to be very nice, and tend to use them as defaults in my own data exploration.

With all of these built-in options for various plot styles, Matplotlib becomes much more useful for both interactive visualization and creation of figures for publication. Throughout this book, I will generally use one or more of these style conventions when creating plots.

### NumPy – 80 – personalizzare le scale

Continuo da qui, copio qui.

Matplotlib’s default tick locators and formatters are designed to be generally sufficient in many common situations, but are in no way optimal for every plot. This section will give several examples of adjusting the tick locations and formatting for the particular plot type you’re interested in.

Before we go into examples, it will be best for us to understand further the object hierarchy of Matplotlib plots. Matplotlib aims to have a Python object representing everything that appears on the plot: for example, recall that the figure is the bounding box within which plot elements appear. Each Matplotlib object can also act as a container of sub-objects: for example, each figure can contain one or more axes objects, each of which in turn contain other objects representing plot contents.

The tick marks are no exception. Each axes has attributes xaxis and yaxis, which in turn have attributes that contain all the properties of the lines, ticks, and labels that make up the axes.

Indicatori principali e secondari
Within each axis, there is the concept of a major tick mark, and a minor tick mark. As the names would imply, major ticks are usually bigger or more pronounced, while minor ticks are usually smaller. By default, Matplotlib rarely makes use of minor ticks, but one place you can see them is within logarithmic plots:

ho settato i limiti del grafico che altrimenti erano troppo stretti.

We see here that each major tick shows a large tickmark and a label, while each minor tick shows a smaller tickmark with no label.

These tick properties—locations and labels—that is, can be customized by setting the formatter and locator objects of each axis. Let’s examine these for the x axis of the just shown plot:

We see that both major and minor tick labels have their locations specified by a LogLocator (which makes sense for a logarithmic plot). Minor ticks, though, have their labels formatted by a NullFormatter: this says that no labels will be shown.

We’ll now show a few examples of setting these locators and formatters for various plots.

Nascondere indicatori o etichette
Perhaps the most common tick/label formatting operation is the act of hiding ticks or labels. This can be done using plt.NullLocator() and plt.NullFormatter(), as shown here:

Notice that we’ve removed the labels (but kept the ticks/gridlines) from the x axis, and removed the ticks (and thus the labels as well) from the y axis. Having no ticks at all can be useful in many situations—for example, when you want to show a grid of images. For instance, consider the following figure, which includes images of different faces, an example often used in supervised machine learning problems (see, for example, In-Depth: Support Vector Machines [prossimamente]):

Notice that each image has its own axes, and we’ve set the locators to null because the tick values (pixel number in this case) do not convey relevant information for this particular visualization.

Ridurre o aumentare il numero degli indicatori
One common problem with the default settings is that smaller subplots can end up with crowded labels. We can see this in the plot grid shown here:

Particularly for the x ticks, the numbers nearly overlap and make them quite difficult to decipher. We can fix this with the plt.MaxNLocator(), which allows us to specify the maximum number of ticks that will be displayed. Given this maximum number, Matplotlib will use internal logic to choose the particular tick locations:

This makes things much cleaner. If you want even more control over the locations of regularly-spaced ticks, you might also use plt.MultipleLocator, which we’ll discuss in the following section.

Indicatori in formato personalizzato (fancy)
Matplotlib’s default tick formatting can leave a lot to be desired: it works well as a broad default, but sometimes you’d like do do something more. Consider this plot of a sine and a cosine:

There are a couple changes we might like to make. First, it’s more natural for this data to space the ticks and grid lines in multiples of π. We can do this by setting a MultipleLocator, which locates ticks at a multiple of the number you provide. For good measure, we’ll add both major and minor ticks in multiples of π/4:

But now these tick labels look a little bit silly: we can see that they are multiples of π, but the decimal representation does not immediately convey this. To fix this, we can change the tick formatter. There’s no built-in formatter for what we want to do, so we’ll instead use plt.FuncFormatter, which accepts a user-defined function giving fine-grained control over the tick outputs:

troppe istruzioni, le riunisco nello script fancy-ind.py

import matplotlib.pyplot as plt
plt.style.use('classic')
import numpy as np

# Plot a sine and cosine curve
fig, ax = plt.subplots()
x = np.linspace(0, 3 * np.pi, 1000)
ax.plot(x, np.sin(x), lw=3, label='Sine')
ax.plot(x, np.cos(x), lw=3, label='Cosine')

# Set up grid, legend, and limits
ax.grid(True)
ax.legend(frameon=False)
ax.axis('equal')
ax.set_xlim(0, 3 * np.pi);

def format_func(value, tick_number):
# find number of multiples of pi/2
N = int(np.round(2 * value / np.pi))
if N == 0:
return "0"
elif N == 1:
return r"$\pi/2$"
elif N == 2:
return r"$\pi$"
elif N % 2 > 0:
return r"${0}\pi/2$".format(N)
else:
return r"${0}\pi$".format(N // 2)

ax.xaxis.set_major_formatter(plt.FuncFormatter(format_func))

fig.savefig("np746.png")

This is much better! Notice that we’ve made use of Matplotlib’s LaTeX support, specified by enclosing the string within dollar signs. This is very convenient for display of mathematical symbols and formulae: in this case, “$\pi$” is rendered as the Greek character π.

The plt.FuncFormatter() offers extremely fine-grained control over the appearance of your plot ticks, and comes in very handy when preparing plots for presentation or publication.

Sommario dei formattatori e localizzatori
We’ve mentioned a couple of the available formatters and locators. We’ll conclude this section by briefly listing all the built-in locator and formatter options. For more information on any of these, refer to the docstrings or to the Matplotlib online documentaion. Each of the following is available in the plt namespace:

Locator class    Description
NullLocator      No ticks
FixedLocator     Tick locations are fixed
IndexLocator     Locator for index plots (e.g., where x = range(len(y)))
LinearLocator    Evenly spaced ticks from min to max
LogLocator       Logarithmically ticks from min to max
MultipleLocator  Ticks and range are a multiple of base
MaxNLocator      Finds up to a max number of ticks at nice locations
AutoLocator      (Default.) MaxNLocator with simple defaults.
AutoMinorLocator Locator for minor ticks

Formatter Class    Description
NullFormatter      No labels on the ticks
IndexFormatter     Set the strings from a list of labels
FixedFormatter     Set the strings manually for the labels
FuncFormatter      User-defined function sets the labels
FormatStrFormatter Use a format string for each value
ScalarFormatter    (Default.) Formatter for scalar values
LogFormatter       Default formatter for log axes

We’ll see further examples of these through the remainder of  the book  [prossimi posts].