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Tag: Python

Python: Getting page title

# Get the HTML page content
from urllib import request
html = request.urlopen("https://silveiraneto.net").read()

# Get the title tag
from bs4 import BeautifulSoup
title = BeautifulSoup(html, 'html.parser').find('title')

print(title.string)

This uses the XML/HTML library BeautifulSoup . This could also be done using regex but playing with HTML and regex is usually a bad idea.

Telephone keypad combinations

Problem: Given a sequence of numbers, show all possible letter combinations in a telephone keypad.

Recursive solution in Python:

keyboard = {
  '1': [],
  '2': ['a','b','c'],
  '3': ['d','e','f'],
  '4': ['g','h','i'],
  '5': ['j','k','l'],
  '6': ['m','n','o'],
  '7': ['p','q','r','s'],
  '8': ['t','u','v'],
  '9': ['w','x','y','z'],
  '0': []
}

def printkeys(numbers, prefix=""):
    if len(numbers)==0:
        print prefix
        return

    for letter in keyboard[numbers[0]]:
        printkeys(numbers[1:], prefix+letter)

printkeys("234")

Output:

adg
adh
adi
aeg
aeh
aei
afg
afh
afi
bdg
bdh
bdi
beg
beh
bei
bfg
bfh
bfi
cdg
cdh
cdi
ceg
ceh
cei
cfg
cfh
cfi

permutations implemented in Python

In case you can’t use Python’s itertools or in case you want a simple, recursive python implementation for a permutation of a list:

def perm(a,k=0):
   if(k==len(a)):
      print a
   else:
      for i in xrange(k,len(a)):
         a[k],a[i] = a[i],a[k]
         perm(a, k+1)
         a[k],a[i] = a[i],a[k]

perm([1,2,3])

Output:

[1, 2, 3]
[1, 3, 2]
[2, 1, 3]
[2, 3, 1]
[3, 2, 1]
[3, 1, 2]

This Python implementation is based in the algorithm presented in the book Computer Algorithms by Horowitz, Sahni and Rajasekaran.

GWU Computer Science Graduate Classes Graph

In order to help me to take decisions about which class to take every semester I did a web scrapping from the graduate and undergraduate bulletin. For every class I could get classe name, prerequisites, credits, teacher, program, description, etc, in a formated tabular document.

Using Python CSV library I could read the tables and parse the data to other formats. One format very useful to handle graph structures is the DOT language script (included in the Graphviz project), in which you can describe both the graph structure and the elements of the graph layout.

Here is the Python source-code to convert the tables to graphs at Github.

The final result (click to view in full size):

Limitations and comments:

  • Prerequisites are only displayed using AND logic. It’s not showing other logics as OR (equivalent classes).
  • Errors may exists due to the scrapping process, conversions, or in the errors in the original source.
  • In the sources there is also a function to convert the graph in Dracula (a JavaScript interactive graph representation) but the current result is too tangled.

 

Substitutions in a phylogenetic tree file

The newick tree

The Newick tree format is a way of representing a graph trees with edge lengths using parentheses and commas.

A newick tree example:

(((Espresso:2,(Milk Foam:2,Espresso Macchiato:5,((Steamed Milk:2,Cappucino:2,(Whipped Cream:1,Chocolate Syrup:1,Cafe Mocha:3):5):5,Flat White:2):5):5):1,Coffee arabica:0.1,(Columbian:1.5,((Medium Roast:1,Viennese Roast:3,American Roast:5,Instant Coffee:9):2,Heavy Roast:0.1,French Roast:0.2,European Roast:1):5,Brazilian:0.1):1):1,Americano:10,Water:1);

A graphical representation for the newick tree above (using the http://www.jsphylosvg.com/ library):

The Newick format is commonly used for store phylogenetic trees.

The problem

A phylogenetic tree can be highly branched and dense and even using proper visualization software can be difficult to analyse it. Additionally, as a tree are produced by a chain of different software with data from the laboratory, the label for each leaf/node can be something not meaningful for a human reader.

For this particular problem, an example of a node label could be SXS_3014_Albula_vulpes_id_30.

There was a spreadsheet with more meaningful information where a node label could be used as a primary key. Example for the node above:

Taxon Order Family Genus Species ID
Albuliformes Albulidae Albula vulpes SXS_3014_Albula_vulpes_id_30

The problem consists in using the tree and the spreadsheet to produce a new tree with the same structure, where each node have a more meaningful label.

The approach

The new tree can be mounted by substituting each label of the initial tree with the respective information from the spreadsheet. A script can be used to automate this process.

The solution

After converting the spreadsheet to a CSV file that could be more easily handled by a CSV Python library the problem is reduced to a file handling and string substitution. Fortunately, due the simplicity of the Newick format and its limited vocabulary, a tree parser is not necessary.

Source-code at Github.

Difficulties found

The spreadsheet was originally in a Microsoft Office Excel 2007 (.xlsx) and the conversion to CSV provided by Excel was not good and there was no configuration option available. Finally, the conversion provided by LibreOffice Productivity Suite was more configurable and was easier to read by the CSV library.

In the script, the DictReader class showed in the the long-term much more reliable and tolerant to changes in the spreadsheet as long the names of the columns remain the same.

P.S. due to the nature of the original sources for the tree and spreadsheet I don’t have the authorization for public publishing their complete and original content. The artificial data displayed here is merely illustrative.