The document provides an introduction to Python, focusing on using the IDLE tool for basic data manipulation and arithmetic operations. It explains how to define variables, perform mathematical operations, and utilize strings with examples. Additionally, it covers error messages and how to use different types of quotes for strings, emphasizing the importance of attention to detail in coding.

1. 3-1: Introduction to Python
Objectives:
 Basic familiarity with the IDLE tool
 Basic data manipulation using Python syntax
All the software for this sequence should already be on the Pi, as part of
Raspbian. Verify that the IDLE tool is available in the X Windows System.
What's the point? Giving the computer instructions, to tell it to do exactly what
you want. It can't do what it doesn't know, so we humans have to decide how
to teach it, and craft our programs appropriately. Games, web, documents,
security, music, virtually anything is possible.
Basic data manipulation:
Type a number into IDLE and press enter. It examines what you told it, does it,
and tell you the result. In this case, it tells you what number you
entered...because there is nothing else to do.
Try some basic math, e.g. 1 + 1. It tells you the answer is 2. Some basic
operators are as follows:
 + (addition)
 - (subtraction)
 * (multiplication)
 / (division)
 % (modulo or remainder, as in 10 mod 3 == 1)
 ** (power, as in 4**2 = 4 squared = 16)
Try 10 / 3. Notice that the answer is 3. Now try 10 / 3.0. Notice that the answer is
3.3333333. If you tell it that you care about decimal places, you get decimal
numbers. Otherwise, you get integers.
Take a few minutes to use Python as a calculator. There are some editing
shortcuts that might also help at this point.
 Ctrl-End will always get you back to the current prompt, if you move the
cursor elsewhere.
 Alt-p will put the previous entry at the prompt again. If you hit it again, it
will cycle through entries previous to that as well.
 Alt-n will go to the next entry. This is particularly useful if you are looking for
something in particular with Alt-p, but you pass it.
 Ctrl-c will interrupt what is currently happening, clear the current
programming entry, and put your cursor at the prompt. If you don't know

2. what is happening, this can help you reset to a known state.
 Copy and paste do work. Be careful of indentation if you choose to
paste, and also be careful of the >>> characters.
Variables:
(Attendees who have done some algebra will find this much easier.)
In its basic form, a variable is a box. You can put things in the box, you can take
things out of the box, you can forget what's inside, and then look to see what it
is. When you carry it around, though, it's still a box. Boxes come in different
shapes and sizes, but all of ours are going to be the same size for now.
Let's put the number 12 in a variable, by telling Python “x = 12” – notice that
Python responds by telling us absolutely nothing. Now, let's ask Python what x is:
>>> x
12
>>>
Next, let's use our variable in some arithmetic expressions. Try things like x + 1,
x**x, 42 / x, etc. Notice that it works just like the number 12.
Now, let's change x to something else. Try the following:
>>> x = x - 3
>>>
We can change what's in the box without actually looking at it, which is
convenient. Have a look to see what its value currently is.
>>> x
9
>>>
You can call variables pretty much whatever you like, as long as you use letters
and no spaces. Numbers are okay within a variable name, and multi-word
variable names are usually distinguished by capitalizing all but the first one. So,
the following are all valid: x, y, xy, george, john, paul, ringo, octopus,
thisIsAVeryLongVariableName, theSong, grocerylist. Be descriptive, and try to
label your boxes so you know what's inside them.

3. Strings:
We've seen how Python can act as a calculator with numbers, and how we can
use words to name variables. What if we want Python to talk to us? If you try to
tell it “hello” you get a nasty-looking error:
>>> hello
Traceback (most recent call last):
File "<pyshell#22>", line 1, in <module>
hello
NameError: name 'hello' is not defined
>>>
Such errors are commonplace and not to be feared. They simply indicate that
something went wrong, and usually provide useful information for fixing the
problem. In this case, Python doesn't know anything about hello – it is trying to
interpret it as a variable or other identifier. We want it to be treated as a literal
string of characters, so we have to put it inside quotation marks:
>>> "hello"
'hello'
>>>
Notice that when Python tells us 'hello' it is between a pair of apostrophes, also
called single quotes. Either single or double quotation marks are acceptable, as
long as they match.
We can also add strings together, and we can put them in variables. For
example:
>>> y = 'hello'

4. >>> y
'hello'
>>> y + " world”
'hello world'
Remember how we have the number 9 in x at the moment? It is often useful to
insert the values of numeric variables into strings, but to do so we must use a
different character: the backtick. It points toward the right, while the
apostrophe On US keyboards, it is usually on the same key as the tilde.
>>> y + " " + 'world' + `x`
'hello world9'
>>>
This may be a reasonable stopping point – the attention to detail required for
success throughout these examples may take some effort for attendees,
especially if you have some who do not have experience with algebra. Pack
up the Pi kits as normal.