In this part, we'll start with the absolute basics.

Python is an interpreted high level programming language. It is often classified as a

["scripting language"](https://en.wikipedia.org/wiki/Scripting_language) and

is considered to be similar to languages such as Perl, Tcl, or Ruby. The syntax

of Python is loosely inspired by elements of C programming.

Python was created by Guido van Rossum around 1990 who named it in honor of Monty Python.

[Python.org](https://www.python.org/) is where you obtain Python. For the purposes of this course, you

only need a basic installation. We recommend installing Python 3.6 or newer. We will be using Python3 in

our solutions and notes.

According to Guido:

Although there are many environments in which you might run Python,

this course has you run Python programs from the terminal or command

shell. From the terminal, you should be able to type a command such as

this:

python --version

python3 --version

If you are new to using the shell or a terminal, you should probably

stop, finish a short tutorial on that first, and then return here.

Just so you know, you will become a much better Python programmer if

you are able to run, debug, and interact with Python at the terminal

shell. This is Python's native environment. If you are able to use

Python in the shell, you will be able to use virtually everywhere

else.

On your machine, start Python and use it as a calulator to solve the

following problem.

Lucky Larry bought 75 shares of Google stock at a price of $235.14 per

share. Today, shares of Google are priced at $711.25. Using Python’s

interactive mode as a calculator, figure out how much profit Larry would

make if he sold all of his shares.

>>> (711.25 - 235.14) * 75

35708.25

>>>

Pro-tip: Use the underscore (\_) variable to use the result of the last

calculation. For example, how much profit does Larry make after his evil

broker takes their 20% cut?

>>> _ * 0.80

28566.600000000002

>>>

Use the `help()` command to get help on the `abs()` function. Then use

`help()` to get help on the `round()` function. Type `help()` just by

itself with no value to enter the interactive help viewer.

One caution with `help()` is that it doesn’t work for basic Python

statements such as `for`, `if`, `while`, and so forth (i.e., if you type

`help(for)` you’ll get a syntax error). You can try putting the help

topic in quotes such as `help("for")` instead. If that doesn’t work,

you’ll have to turn to an internet search.

Followup: Go to <http://docs.python.org> and find the documentation for

the `abs()` function (hint: it’s found under the library reference

related to built-in functions).

As you've noticed, this course is structured as a series of traditional

web pages where you are encouraged to try interactive code samples by typing them

by hand. If you are learning Python for the first time, this is encouraged.

You will get a better feel for the language by slowing down, typing things in,

and thinking about what you are doing.

If you are inclined to use "cut and paste" in the editor, select code

starting after the `>>>` prompt and going up to, but not any further

than the first blank line or the next `>>>` prompt (whichever appears

first). Select "copy" from the brower, go to the Python window, and

select "paste" to copy it into the Python shell. To get the code to

run, you may have to hit "Return" once after you’ve pasted it in.

Use cut-and-paste to execute the Python statements in this session:

>>> 12 + 20

32

>>> (3 + 4

+ 5 + 6)

18

>>> for i in range(5):

print(i)

0

1

2

3

4

>>>

Warning: It is never possible to paste more than one Python command

(statements that appear after `>>>`) to the basic Python shell at a

time. You have to paste each command one at a time.

If you’ve made it this far, try something more advanced and type these

statements to find out how long people waiting on the corner of Clark

street and Balmoral in Chicago will have to wait for the next

northbound CTA \#22 bus:

>>> import urllib.request

>>> u = urllib.request.urlopen('http://ctabustracker.com/bustime/map/getStopPredictions.jsp?stop=14791&route=22')

>>> from xml.etree.ElementTree import parse

>>> doc = parse(u)

>>> for pt in doc.findall('.//pt'):

print(pt.text)

6 MIN

18 MIN

28 MIN

>>>

Yes, you just downloaded a web page, parsed an XML document, and

extracted some useful information in about 6 lines of code. The data

you accessed is actually feeding the website

<http://ctabustracker.com/bustime/home.jsp>. Try it again and watch

the predictions change.

If the first import statement `import urllib.request` fails, you’re

probably using Python 2. For this course, you need to make sure you’re

using Python 3.6 or newer. Go to <https://www.python.org> to download

it if you need it.

If your work environment requires the use of an HTTP proxy server, you may need

to set the `HTTP_PROXY` environment variable to make this part of the

exercise work. For example:

>>> import os

>>> os.environ['HTTP_PROXY'] = 'http://yourproxy.server.com'

>>>

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