A soft intro to Python (part I)

Setup

Setting up a local python environment is a pain…

Source: http://xkcd.com/1987

Setup

Easiest way : install Anaconda Navigator…

Editors:

  • IDLE = default

  • Anaconda > Spyder, PyCharm, …

and RStudio too! File > New file > Python script

Python prompt: >>>

Setup

Online options:

  • Jupyter notebooks: open from Anaconda

  • google colab : (+) has google installed libraries and GPU/TPU power (limited GO). (-) to make one you need a google account

Basics

Same programming rules:

  • named variables:

>>> a=50 - use += to increment/concatenate

>>> a += 1

  • a list = multiple values in a variable. Each value is identified by its key (name).

>>> mylist = ['tic', 'tac', 'toc']

  • in Python, index starts at 0, not 1 (so NOT like in R): in mylist, the key for value ‘tic’ is 0.

>>> print(mylist[2])

Basics

A common object to manipulate in Python are dictionaries: they’re a pair of KEY:Value just like in lists, but here keys in dictionaries aren’t ordered. Use {} to initiate them (but still use [] to manipulate them).

>>> mydico = {} or

>>> mydico = dict(Lastname = "Petit", Firstname = "Cathleen", Pronounciation = "Kétline")

Same conditional rules as in R: if, else, elseif… (–> NO !! it’s elif instead of elseif… but who uses those anyways ?)

Arithmetic operators : the usual, but also: %, which means ‘modulo’ = is a multiple of. Returns 1 or 0 (True/False).

Logical operators : ==, !=, >, >= …

For loops

Syntax:

string = 'Hello sunshine !'
for a in string :
  print(a)

Try it !

A few commun built-in functions:

random(list) → choose random element of a list

len(list) → number of elements in list

max / min(list)

str.split(separator) → transform str into list according to separator

list.append(value) → add a value to list (like c() in R)

del list[key], NOT the same as list.remove(value)

list.count(value) → counts number of occurrences of that value in the list

list[-1] → shows the last value of the list

list[a:b] → show values from a to b

x.extend(y) → concatenate lists x and y

str.find(string) → counts occurrences of str in string

str.replace(where, what)

input(string) → prints string and asks user for feedback (VERY INTERESTING)

range(int) → useful in for loops: for i in range(1, 10):

Libraries

Just like in R: built-in functions, but also packages of functions, called libraries.

Use import library to load them. Colab has many pre-installed libraries, but sometimes you need to activate them at the beginning of your session with !pip install library or pip install library or pip3 install library if you’re using a local environment.

Common libraries:

  • math, random, numpy, …

  • scikit-learn, pytorch, SciPy, TensorFlow, Keras….: import sklearn as skl → data science.

  • matplotlib : data viz. You can also choose to only import a function instead of the whole library: import matplotlib.pyplot as plt

  • pandas : Python’s tidyverse ;) Handles dataframes.

Your turn !!

  • play around with the different functions

Exercise:

Write a script to ask the user their age, and give different answers depending on whether you think it’s old or young.

Answer:

limit = 99

age = input('How old are you ?')

if float(age) > limit:
  print("Wow, you're a living fossil !")
else:
  print("Oh yeah, you're alright.")

Take home :

  • indents !

  • “:” after ifs and elses, rather than brackets like in R

  • code execution by individual cells (on Colab/Jupyter) or entire script (Spyder, etc).

  • python errors are usually quite precise: read the error messages carefully. Otherwise, absolute rule is: the internet has looooaaads of python forums, help pages, tutorials, bug fixes. A very active community.

Homework :)

Generate a list of integers, and then show:

  • the list

  • the list in column format, displaying matching keys and values

  • the sum of all elements of the list

  • create a new list made of 5 multiples of the original list

  • give min and max values of the list

  • count occurrences of pair and odd numbers and sum them

Quick games

Write a program to…

  • throw a dice

  • throw 2 dice

  • Rock paper cisors

  • guess a number between 1 and 1000 by indicating when the value is higher or lower

  • hangman (without the graphical aspect)