Boston Python Workshop 4/ColorWall handout

ColorWall goals

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

Concept review

Indentation reminder

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.

Range

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

Hi
HiHi
HiHiHi
HiHiHiHi

if statements inside for loops

>>> for i in range(80):
...     if i % 9 == 0:
...         print i, "is divisible by 9."
...
0 is divisible by 9.
9 is divisible by 9.
18 is divisible by 9.
27 is divisible by 9.
36 is divisible by 9.
45 is divisible by 9.
54 is divisible by 9.
63 is divisible by 9.
72 is divisible by 9.

for loops inside for loops

>>> 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

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.