Python tutorial

Below is a tutorial with the basics of Python you need to know to use oTree.

The easiest way to run this file is in IDLE (which is usually bundled in the Python installation).

There are many other good python tutorials online, but note that some of the material covered in those tutorials is not necessary for oTree programming specifically.

Tutorial file

A downloadable version is here.

# Comments start with a # symbol.

####################################################
## 1. Basics
####################################################

# integer
3

# float (floating-point number)
3.14

# Math is what you would expect
1 + 1  # => 2
8 - 1  # => 7
10 * 2  # => 20
35 / 5  # => 7.0

# Enforce precedence with parentheses
(1 + 3) * 2  # => 8

# Boolean Operators
# Note they are
True and False # => False
False or True # => True

# negate with not
not True  # => False
not False  # => True

# Equality is ==
1 == 1  # => True
2 == 1  # => False

# Inequality is !=
1 != 1  # => False
2 != 1  # => True

# More comparisons
1 < 10  # => True
1 > 10  # => False
2 <= 2  # => True
2 >= 2  # => True

# Comparisons can be chained!
1 < 2 < 3  # => True
2 < 3 < 2  # => False

# A string (text) is created with " or '
"This is a string."
'This is also a string.'

# Strings can be added too!
"Hello " + "world!"  # => "Hello world!"

# None means an empty/nonexistent value
None  # => None


####################################################
## 2. Variables, lists, and dicts
####################################################

# print() displays the value in your command prompt window
print("I'm Python. Nice to meet you!") # => I'm Python. Nice to meet you!

# Variables
some_var = 5
some_var  # => 5

# Lists store sequences
li = []

# Add stuff to the end of a list with append
li.append(1)    # li is now [1]
li.append(2)    # li is now [1, 2]
li.append(3)    # li is now [1, 2, 3]

# Access a list like you would any array
# in Python, the first list index is 0, not 1.
li[0]  # => 1
# Assign new values to indexes that have already been initialized with =
li[0] = 42
li # => [42, 2, 3]


# You can add lists
other_li = [4, 5, 6]
li + other_li   # => [42, 2, 3, 4, 5, 6]

# Get the length with "len()"
len(li)   # => 6

# Dictionaries store mappings
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {
    'name': 'Lancelot',
    'quest': "To find the holy grail",
    'favorite_color': "Blue"
}

# Look up values with []
filled_dict['name']   # => 'Lancelot'

# Check for existence of keys in a dictionary with "in"
'name' in filled_dict   # => True
'age' in filled_dict   # => False

# set the value of a key with a syntax similar to lists
filled_dict["age"] = 30  # now, filled_dict["age"] => 30


####################################################
## 3. Control Flow
####################################################

# Let's just make a variable
some_var = 5

# Here is an if statement.
# prints "some_var is smaller than 10"
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.")

"""
SPECIAL NOTE ABOUT INDENTING
In Python, you must indent your code correctly, or it will not work.
All lines in a block of code must be aligned along the left edge.
When you're inside a code block (e.g. "if", "for", "def"; see below),
you need to indent by 4 spaces.

Examples of wrong indentation:

if some_var > 10:
print("bigger than 10." # error, this line needs to be indented by 4 spaces


if some_var > 10:
    print("bigger than 10.")
 else: # error, this line needs to be unindented by 1 space
    print("less than 10")

"""


"""
For loops iterate over lists
prints:
    1
    4
    9
"""
for x in [1, 2, 3]:
    print(x*x)

"""
"range(number)" returns a list of numbers
from zero to the given number MINUS ONE

the following code prints:
    0
    1
    2
    3
"""
for i in range(4):
    print(i)


####################################################
## 4. Functions
####################################################

# Use "def" to create new functions
def add(x, y):
    print('x is', x)
    print('y is', y)
    return x + y

# Calling functions with parameters
add(5, 6)   # => prints out "x is 5 and y is 6" and returns 11


####################################################
## 5. List comprehensions
####################################################

# We can use list comprehensions to loop or filter
numbers = [3,4,5,6,7]
[x*x for x in numbers]  # => [9, 16, 25, 36, 49]

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

####################################################
## 6. Modules
####################################################

# You can import modules
import random
print(random.random()) # random real between 0 and 1

Credits: This page’s tutorial is adapted from Learn Python in Y Minutes, and is released under the same license.