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245 lines
8.4 KiB
Markdown
245 lines
8.4 KiB
Markdown
---
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category: tool
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tool: Statistical Computing with Python
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contributors:
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- ["e99n09", "https://github.com/e99n09"]
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---
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This is a tutorial on how to do some typical statistical programming tasks using Python. It's intended for people basically familiar with Python and experienced at statistical programming in a language like R, Stata, SAS, SPSS, or MATLAB.
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```python
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# 0. Getting set up ====
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""" Get set up with IPython and pip install the following: numpy, scipy, pandas,
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matplotlib, seaborn, requests.
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Make sure to do this tutorial in the IPython notebook so that you get
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the inline plots and easy documentation lookup.
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"""
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# 1. Data acquisition ====
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""" One reason people choose Python over R is that they intend to interact a lot
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with the web, either by scraping pages directly or requesting data through
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an API. You can do those things in R, but in the context of a project
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already using Python, there's a benefit to sticking with one language.
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"""
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import requests # for HTTP requests (web scraping, APIs)
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import os
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# web scraping
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r = requests.get("https://github.com/adambard/learnxinyminutes-docs")
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r.status_code # if 200, request was successful
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r.text # raw page source
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print(r.text) # prettily formatted
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# save the page source in a file:
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os.getcwd() # check what's the working directory
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f = open("learnxinyminutes.html", "wb")
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f.write(r.text.encode("UTF-8"))
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f.close()
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# downloading a csv
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fp = "https://raw.githubusercontent.com/adambard/learnxinyminutes-docs/master/"
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fn = "pets.csv"
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r = requests.get(fp + fn)
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print(r.text)
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f = open(fn, "wb")
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f.write(r.text.encode("UTF-8"))
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f.close()
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""" for more on the requests module, including APIs, see
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http://docs.python-requests.org/en/latest/user/quickstart/
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"""
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# 2. Reading a CSV file ====
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""" Wes McKinney's pandas package gives you 'DataFrame' objects in Python. If
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you've used R, you will be familiar with the idea of the "data.frame" already.
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"""
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import pandas as pd
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import numpy as np
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import scipy as sp
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pets = pd.read_csv(fn)
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pets
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# name age weight species
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# 0 fluffy 3 14 cat
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# 1 vesuvius 6 23 fish
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# 2 rex 5 34 dog
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""" R users: note that Python, like most normal programming languages, starts
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indexing from 0. R is the unusual one for starting from 1.
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"""
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# two different ways to print out a column
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pets.age
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pets["age"]
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pets.head(2) # prints first 2 rows
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pets.tail(1) # prints last row
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pets.name[1] # 'vesuvius'
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pets.species[0] # 'cat'
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pets["weight"][2] # 34
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# in R, you would expect to get 3 rows doing this, but here you get 2:
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pets.age[0:2]
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# 0 3
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# 1 6
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sum(pets.age) * 2 # 28
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max(pets.weight) - min(pets.weight) # 20
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""" If you are doing some serious linear algebra and number-crunching, you may
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just want arrays, not DataFrames. DataFrames are ideal for combining columns
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of different types.
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"""
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# 3. Charts ====
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import matplotlib as mpl
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import matplotlib.pyplot as plt
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%matplotlib inline
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# To do data visualization in Python, use matplotlib
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plt.hist(pets.age);
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plt.boxplot(pets.weight);
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plt.scatter(pets.age, pets.weight)
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plt.xlabel("age")
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plt.ylabel("weight");
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# seaborn sits atop matplotlib and makes plots prettier
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import seaborn as sns
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plt.scatter(pets.age, pets.weight)
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plt.xlabel("age")
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plt.ylabel("weight");
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# there are also some seaborn-specific plotting functions
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# notice how seaborn automatically labels the x-axis on this barplot
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sns.barplot(pets["age"])
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# R veterans can still use ggplot
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from ggplot import *
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ggplot(aes(x="age",y="weight"), data=pets) + geom_point() + labs(title="pets")
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# source: https://pypi.python.org/pypi/ggplot
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# there's even a d3.js port: https://github.com/mikedewar/d3py
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# 4. Simple data cleaning and exploratory analysis ====
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""" Here's a more complicated example that demonstrates a basic data
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cleaning workflow leading to the creation of some exploratory plots
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and the running of a linear regression.
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The data set was transcribed from Wikipedia by hand. It contains
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all the Holy Roman Emperors and the important milestones in their lives
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(birth, death, coronation, etc.).
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The goal of the analysis will be to explore whether a relationship
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exists between emperor birth year and emperor lifespan.
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data source: https://en.wikipedia.org/wiki/Holy_Roman_Emperor
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"""
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# load some data on Holy Roman Emperors
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url = "https://raw.githubusercontent.com/e99n09/R-notes/master/data/hre.csv"
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r = requests.get(url)
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fp = "hre.csv"
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with open(fp, "wb") as f:
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f.write(r.text.encode("UTF-8"))
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hre = pd.read_csv(fp)
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hre.head()
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"""
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Ix Dynasty Name Birth Death Election 1
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0 NaN Carolingian Charles I 2 April 742 28 January 814 NaN
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1 NaN Carolingian Louis I 778 20 June 840 NaN
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2 NaN Carolingian Lothair I 795 29 September 855 NaN
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3 NaN Carolingian Louis II 825 12 August 875 NaN
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4 NaN Carolingian Charles II 13 June 823 6 October 877 NaN
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Election 2 Coronation 1 Coronation 2 Ceased to be Emperor
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0 NaN 25 December 800 NaN 28 January 814
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1 NaN 11 September 813 5 October 816 20 June 840
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2 NaN 5 April 823 NaN 29 September 855
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3 NaN Easter 850 18 May 872 12 August 875
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4 NaN 29 December 875 NaN 6 October 877
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Descent from whom 1 Descent how 1 Descent from whom 2 Descent how 2
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0 NaN NaN NaN NaN
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1 Charles I son NaN NaN
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2 Louis I son NaN NaN
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3 Lothair I son NaN NaN
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4 Louis I son NaN NaN
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"""
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# clean the Birth and Death columns
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import re # module for regular expressions
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rx = re.compile(r'\d+$') # match trailing digits
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""" This function applies the regular expression to an input column (here Birth,
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Death), flattens the resulting list, converts it to a Series object, and
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finally converts the type of the Series object from string to integer. For
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more information into what different parts of the code do, see:
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- https://docs.python.org/2/howto/regex.html
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- http://stackoverflow.com/questions/11860476/how-to-unlist-a-python-list
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- http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.html
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"""
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def extractYear(v):
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return(pd.Series(reduce(lambda x, y: x + y, map(rx.findall, v), [])).astype(int))
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hre["BirthY"] = extractYear(hre.Birth)
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hre["DeathY"] = extractYear(hre.Death)
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# make a column telling estimated age
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hre["EstAge"] = hre.DeathY.astype(int) - hre.BirthY.astype(int)
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# simple scatterplot, no trend line, color represents dynasty
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sns.lmplot("BirthY", "EstAge", data=hre, hue="Dynasty", fit_reg=False);
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# use scipy to run a linear regression
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from scipy import stats
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(slope, intercept, rval, pval, stderr) = stats.linregress(hre.BirthY, hre.EstAge)
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# code source: http://wiki.scipy.org/Cookbook/LinearRegression
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# check the slope
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slope # 0.0057672618839073328
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# check the R^2 value:
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rval**2 # 0.020363950027333586
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# check the p-value
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pval # 0.34971812581498452
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# use seaborn to make a scatterplot and plot the linear regression trend line
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sns.lmplot("BirthY", "EstAge", data=hre);
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""" For more information on seaborn, see
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- http://web.stanford.edu/~mwaskom/software/seaborn/
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- https://github.com/mwaskom/seaborn
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For more information on SciPy, see
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- http://wiki.scipy.org/SciPy
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- http://wiki.scipy.org/Cookbook/
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To see a version of the Holy Roman Emperors analysis using R, see
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- http://github.com/e99n09/R-notes/blob/master/holy_roman_emperors_dates.R
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"""
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```
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If you want to learn more, get _Python for Data Analysis_ by Wes McKinney. It's a superb resource and I used it as a reference when writing this tutorial.
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You can also find plenty of interactive IPython tutorials on subjects specific to your interests, like Cam Davidson-Pilon's <a href="http://camdavidsonpilon.github.io/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers/" Title="Probabilistic Programming and Bayesian Methods for Hackers">Probabilistic Programming and Bayesian Methods for Hackers</a>.
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Some more modules to research:
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- text analysis and natural language processing: nltk, http://www.nltk.org
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- social network analysis: igraph, http://igraph.org/python/
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