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Notes/01_Introduction/01_Python.md
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Notes/01_Introduction/01_Python.md
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# 1.1 Python
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In this part, we'll start with the absolute basics.
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## Notes
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### What is Python?
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Python is an interpreted high level programming language. It is often classified as a
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["scripting language"](https://en.wikipedia.org/wiki/Scripting_language) and
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is considered to be similar to languages such as Perl, Tcl, or Ruby. The syntax
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of Python is loosely inspired by elements of C programming.
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Python was created by Guido van Rossum around 1990 who named it in honor of Monty Python.
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### Where to get Python?
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[Python.org](https://www.python.org/) is where you obtain Python. For the purposes of this course, you
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only need a basic installation. We recommend installing Python 3.6 or newer. We will be using Python3 in
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our solutions and notes.
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### Why was Python created?
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According to Guido:
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> My original motivation for creating Python was the perceived need
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> for a higher level language in the Amoeba [Operating Systems]
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> project. I realized that the development of system administration
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> utilities in C was taking too long. Moreover, doing these things in
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> the Bourne shell wouldn't work for a variety of reasons. ... So,
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> there was a need for a language that would bridge the gap between C
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> and the shell.
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>
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> - Guido van Rossum
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### Where is Python on my Machine?
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Although there are many environments in which you might run Python,
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this course has you run Python programs from the terminal or command
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shell. From the terminal, you should be able to type a command such as
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this:
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```bash
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python --version
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# or
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python3 --version
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```
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If you are new to using the shell or a terminal, you should probably
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stop, finish a short tutorial on that first, and then return here.
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Just so you know, you will become a much better Python programmer if
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you are able to run, debug, and interact with Python at the terminal
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shell. This is Python's native environment. If you are able to use
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Python in the shell, you will be able to use virtually everywhere
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else.
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## Exercises
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### Using Python as a Calculator
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On your machine, start Python and use it as a calulator to solve the
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following problem.
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Lucky Larry bought 75 shares of Google stock at a price of $235.14 per
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share. Today, shares of Google are priced at $711.25. Using Python’s
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interactive mode as a calculator, figure out how much profit Larry would
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make if he sold all of his shares.
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```pycon
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>>> (711.25 - 235.14) * 75
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35708.25
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>>>
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```
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Pro-tip: Use the underscore (\_) variable to use the result of the last
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calculation. For example, how much profit does Larry make after his evil
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broker takes their 20% cut?
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```pycon
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>>> _ * 0.80
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28566.600000000002
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>>>
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```
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### Getting help
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Use the `help()` command to get help on the `abs()` function. Then use
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`help()` to get help on the `round()` function. Type `help()` just by
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itself with no value to enter the interactive help viewer.
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One caution with `help()` is that it doesn’t work for basic Python
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statements such as `for`, `if`, `while`, and so forth (i.e., if you type
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`help(for)` you’ll get a syntax error). You can try putting the help
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topic in quotes such as `help("for")` instead. If that doesn’t work,
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you’ll have to turn to an internet search.
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Followup: Go to <http://docs.python.org> and find the documentation for
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the `abs()` function (hint: it’s found under the library reference
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related to built-in functions).
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### Cutting and Pasting
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As you've noticed, this course is structured as a series of traditional
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web pages where you are encouraged to try interactive code samples by typing them
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by hand. If you are learning Python for the first time, this is encouraged.
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You will get a better feel for the language by slowing down, typing things in,
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and thinking about what you are doing.
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If you are inclined to use "cut and paste" in the editor, select code
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starting after the `>>>` prompt and going up to, but not any further
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than the first blank line or the next `>>>` prompt (whichever appears
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first). Select "copy" from the brower, go to the Python window, and
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select "paste" to copy it into the Python shell. To get the code to
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run, you may have to hit "Return" once after you’ve pasted it in.
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Use cut-and-paste to execute the Python statements in this session:
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```pycon
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>>> 12 + 20
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32
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>>> (3 + 4
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+ 5 + 6)
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18
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>>> for i in range(5):
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print(i)
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0
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1
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2
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3
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4
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>>>
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```
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Warning: It is never possible to paste more than one Python command
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(statements that appear after `>>>`) to the basic Python shell at a
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time. You have to paste each command one at a time.
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### Where is My Bus?
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If you’ve made it this far, try something more advanced and type these
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statements to find out how long people waiting on the corner of Clark
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street and Balmoral in Chicago will have to wait for the next
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northbound CTA \#22 bus:
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```pycon
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>>> import urllib.request
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>>> u = urllib.request.urlopen('http://ctabustracker.com/bustime/map/getStopPredictions.jsp?stop=14791&route=22')
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>>> from xml.etree.ElementTree import parse
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>>> doc = parse(u)
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>>> for pt in doc.findall('.//pt'):
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print(pt.text)
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6 MIN
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18 MIN
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28 MIN
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>>>
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```
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Yes, you just downloaded a web page, parsed an XML document, and
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extracted some useful information in about 6 lines of code. The data
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you accessed is actually feeding the website
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<http://ctabustracker.com/bustime/home.jsp>. Try it again and watch
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the predictions change.
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If the first import statement `import urllib.request` fails, you’re
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probably using Python 2. For this course, you need to make sure you’re
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using Python 3.6 or newer. Go to <https://www.python.org> to download
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it if you need it.
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If your work environment requires the use of an HTTP proxy server, you may need
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to set the `HTTP_PROXY` environment variable to make this part of the
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exercise work. For example:
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```pycon
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>>> import os
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>>> os.environ['HTTP_PROXY'] = 'http://yourproxy.server.com'
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>>>
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```
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\[ **[Solution](soln1_1.html)** | **[Next](ex1_2.html)** |
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**[Index](index.html)** \]
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