Boston Python Workshop 8/ColorWall handout

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ColorWall goals[edit]

  • practice for loops
  • practice using functions
  • practice implementing functions
  • have fun experimenting with Python code that produces graphical results

Concept review[edit]

Indentation reminder[edit]

In Python, indentation matters. Everything is indented by a multiple of some number of spaces, often 4.

In if statements, you indent everything you want to be run if the if conditional is True. For example:

>>> James = 35
>>> Alice = 30
>>> if James > Alice:
...     print "James is older than Alice."
...
James is older than Alice.
>>>

Because James really is older than Alice, the if conditional is True, so Python does execute the code indented under the if line. In this case we print "James is older than Alice."

>>> James = 35
>>> Alice = 30
>>> if James < Alice:
...     print "James is younger than Alice."
...
>>>

Because James is not older than Alice, the if conditional is False, so Python does not execute the code indented under the if line.

In for loops, you indent everything you want to be run each loop For example:

>>> names = ["Jessica", "Adam", "Liz"]
>>> for name in names:
...     print "Hello", name
...
Hello Jessica
Hello Adam
Hello Liz

The print line is indented 4 spaces under the for. That's how Python knows to execute the print line for every name in names.

dictionaries[edit]

  • dictionaries are a way to map keys to values
  • if you used a Python dictionary to store the information from a real dictionary, the keys would be words and the values would be definitions
  • dictionaries aren't ordered (lists are ordered)
>>> flavors = {"Jessica" : "Chocolate", "Adam" : "Vanilla", "Liz" : "Strawberry"}
>>> flavors["Jessica"]
'Chocolate'
>>> flavors["Eve"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Eve'
>>> flavors.get("Eve")
>>> flavors["Eve"] = "Rocky Road"
>>> flavors
{'Eve': 'Rocky Road', 'Jessica': 'Chocolate', 'Liz': 'Strawberry', 'Adam': 'Vanilla'}

getting keys and values from dictionaries[edit]

  • use keys() to get a list of keys
  • use values() to get a list of values
>>> flavors = {"Jessica" : "Chocolate", "Adam" : "Vanilla", "Liz" : "Strawberry"}
>>> for person in flavors.keys():
...     print person
... 
Jessica
Liz
Adam
>>> for flavor in flavors.values():
...     print flavor
... 
Chocolate
Strawberry
Vanilla

range[edit]

>>> range(5)
[0, 1, 2, 3, 4]
>>> for i in range(5):
...     print "Hi" * i
...

Hi
HiHi
HiHiHi
HiHiHiHi

if statements inside for loops[edit]

>>> for name in ["Alice", "Bob", "Cassie", "Deb", "Ellen"]:
...     if name[0] in "AEIOU":
...         print name + " starts with a vowel."
... 
Alice starts with a vowel.
Ellen starts with a vowel.

for loops inside for loops[edit]

>>> letters = ["a", "b", "c"]
>>> numbers = [1, 2, 3]
>>> for letter in letters:
...     for number in numbers:
...         print letter * number
...
a
aa
aaa
b
bb
bbb
c
cc
ccc
>>> for number in numbers:
...     for letter in letters:
...         print number * letter
...
a
b
c
aa
bb
cc
aaa
bbb
ccc

Imports[edit]

Imports look like this:

>>> import random
>>> import time

In the above example, random and time are both Python modules. Modules are Python files outside of the current Python file that contain Python code, like functions and variables. You can use code from modules by first importing the module. Here's an example from the random module:

>>> import random
>>> random.randint(0, 10)
7
>>> random.randint(0, 10)
6
>>> random.randint(0, 10)
1
>>> random.randint(0, 10)
3
>>> random.randint(0, 10)
4
>>> random.randint(0, 10)
9

randint is a function in the random module. It takes a lower bound as the first argument and an upper bound as the second argument and returns a random integer between those bounds.

Possibly useful links[edit]

http://www.yafla.com/yaflaColor/ColorRGBHSL.aspx for HSV color picking.
See the slides used to teach this project.


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