Introduction to Python

Experimental Economics Data Workflow

• A very short introduction to Python
  • Based on Learn X in Y minutes, where X=Python Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
  • Lutz, M. (2013). Learning python: Powerful object-oriented programming. " O'Reilly Media, Inc.".

Numbers and logical operators

3+3

6

10 * 3

30

35 / 5

7.0

35 % 6

5

(1 + 3) * 2

8

1 + 3 * 2

7

Boolean Operators

True and False 

False

False or True   

True

• True and False are actually 1 and 0 but with different keywords

True + True 

2

True * 8    

8

Comparisons

• Equality is ==

1 == 1

True

2 == 1

False

• Inequality is !=

1 != 1  

False

2 != 1 

True

More comparisons

1 > 10 

False

1 > 10 or 1 < 10 

True

2 <= 2

True

Strings and variables

Strings are an ordered collection of characters

'This is a string.'

'This is a string.'

"Hello " + "world!"

'Hello world!'

• Variables can be declared

name = "oTree"

• Variables (strings) can be sliced and indexed

name[0] 

'o'

name[-3]  

'r'

Lists and tuples

li = []

li

[]

• Add stuff to the end of a list with append

li = [2,3,4]

li

[2, 3, 4]

• Add stuff to the end of a list with append

li.append(5)

li

[2, 3, 4, 5]

• Access a list like you would any array

li[0] 

2

len(li)

4

li [1:3]

[3, 4]

del li[2]

li

[2, 3, 5]

• Check for existence in a list with "in"

1 in li

False

Dictionaries

• Useful way to collect and organize information
  • As an example, payoffs of players
  • They are used in settings.py to control apps
• Storage of mappimngs from keys to values

empty_dict = {}

filled_dict = {"one": 1, "two": 2, "three": 3}

filled_dict["one"]

1

Get all keys as an iterable with "keys()". We need to wrap the call in list()

list(filled_dict.keys())

['one', 'two', 'three']

Get all values as an iterable with "values()". Once again we need to wrap it

list(filled_dict.values())

[1, 2, 3]

Check for existence of keys in a dictionary with "in"

"one" in filled_dict

True

1 in filled_dict

False

• Adding to an existing dictionary

filled_dict.update({"four": 4})

Control flow and iteration

• Often you need to iterate through variables
  • As an example, collect all choice sof participants in a group

Iterations

some_var = 5


if some_var > 10:
    print("some_var is totally bigger than 10.")
elif some_var < 10:    # This elif clause is optional.
    print("some_var is smaller than 10.")
else:                  # This is optional too.
    print("some_var is indeed 10.")

some_var is smaller than 10.

Loops

for i in range(4):
    print(i)

0
1
2
3

for animal in ["dog", "cat", "mouse"]:
    # You can use format() to interpolate formatted strings
    print("{} is a mammal".format(animal))

dog is a mammal
cat is a mammal
mouse is a mammal

• We can use list comprehensions to loop or filter

numbers = [3, 4, 5, 6, 7]
[x for x in numbers if x > 5]   # => [6, 7]

[6, 7]

Indenting is important!

Functions

• Use "def" to create new functions

def add(x, y):
    print("x is {} and y is {}".format(x, y))
    return x+y  # Return values with a return statement

add(5, 6)

x is 5 and y is 6

11

Classes

• Classes are main object-oriented-programming (OOP) in Python
  • Class objects provide default behavior
    • The class statement creates a class object and assigns it a name
    • Assignments inside class statements make class attributes
    • Class attributes export object state and behavior
      • def statements inside class generate a method
  • Instance objects are generated from classes
    • Calling a class object makes a new instance object
    • Each instance object inherits class attributes and gets its own namespace
    • Assignments to self in methods make per-instance attributes
      • self refers to the instance being processed

Classes: An Example

• Pokemon is a class with some properties

  • Height
  • Weight
  • Category

• Two instances of the class →

Charmander:

• Height: 2' 00";

• Weight 18.7 lbs;

• Category: Lizard

Bulbasaur:

• Height: 2' 04";

• Weight: 15.2 lbs;

• Category: Seed

class Pokemon:
    nature = "Pokemon"  # this property is shared by all instances

    def __init__(self, name, height, weight, category):
        # Assign the argument to the instance's name attribute
        self.name = name
        self.height = height
        self.weight = weight
        self.category = category
        self.comment = []
    # to add a comment

    def add_comment(self, comment):
        self.comment.append(comment)
    # convert height and weight into metric

    def convert_metric(self):
        # conversion rates
        feet_conv_cm = 30.5
        inch_conv_cm = 2.54
        lbs_conv_kg = 0.453592
        self.height_cm = int(self.height.split(
            "'")[0])*feet_conv_cm+int(self.height.split("'")[1])*inch_conv_cm
        self.weight_kg = self.weight*lbs_conv_kg
        # print the result
        print("Height (cm): %f, Weight (Kg): %f" %
              (self.height_cm, self.weight_kg))

• Create two instances of the class

p_1 = Pokemon("Charmander", "2' 5''", 18.7, "Lizard")
p_2 = Pokemon("Bulbasaur", "2' 04''", 15.2, "Seed")

p_1.name
p_2.name

'Bulbasaur'

• Add a comment

p_1.add_comment("Its ability is Blaze")
p_2.add_comment("Its ability is Overgrow")

• Convert their measures in metric system

p_1.convert_metric()
p_2.convert_metric()

Height (cm): 73.700000, Weight (Kg): 8.482170
Height (cm): 71.160000, Weight (Kg): 6.894598

• Who is taller?

if p_1.height > p_2.height:
    print(p_1.name + " is taller")
elif p_1.height < p_2.height:
    print(p_2.name + " is taller")
else:
    print(p_2.name + " and" + p_1.name + "are equally taller")

Charmander is taller

Further material

• Take a look at the official Beginner's Guide for suggestions/ideas