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[Contents](../Contents.md) \| [Next (1.2 A First Program)](02_Hello_world.md)
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# 1.1 Python
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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 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. I recommend installing Python 3.6 or newer. Python 3.6 is used in the notes
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and solutions.
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### Why was Python created?
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In the words of Python's creator:
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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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Python is typically installed on your machine as a program that runs
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from the terminal or command shell. From the terminal, you should be
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able to type `python` like this:
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```
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bash $ python
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Python 3.8.1 (default, Feb 20 2020, 09:29:22)
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[Clang 10.0.0 (clang-1000.10.44.4)] on darwin
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Type "help", "copyright", "credits" or "license" for more information.
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>>> print("hello world")
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hello world
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>>>
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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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Although there are many non-shell environments where you can code
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Python, you will be a stronger Python programmer if you are able to
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run, debug, and interact with Python at the terminal. This is
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Python's native environment. If you are able to use Python here, you
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will be able to use it everywhere else.
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## Exercises
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### Exercise 1.1: Using Python as a Calculator
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On your machine, start Python and use it as a calculator 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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```python
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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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```python
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>>> _ * 0.80
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28566.600000000002
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>>>
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```
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### Exercise 1.2: 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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### Exercise 1.3: Cutting and Pasting
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This course is structured as a series of traditional web pages where
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you are encouraged to try interactive Python code samples **by typing
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them out by hand.** If you are learning Python for the first time,
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this "slow approach" is encouraged. You will get a better feel for
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the language by slowing down, typing things in, and thinking about
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what you are doing.
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If you must "cut and paste" code samples, 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 browser, 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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```python
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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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Now that you've done this, just remember that you will get more out of
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the class by typing in code slowly and thinking about it--not cut and pasting.
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### Exercise 1.4: Where is My Bus?
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Note: This was a whimsical example that was a real crowd-pleaser when
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I taught this course in my office. You could query the bus and then
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literally watch it pass by the window out front. Sadly, APIs rarely live
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forever and it seems that this one has now ridden off into the sunset. --Dave
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Update: GitHub user @asett has suggested the following modified code might work,
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but you'll have to provide your own API key (available [here](https://www.transitchicago.com/developers/bustracker/)).
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```python
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import urllib.request
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u = urllib.request.urlopen('http://www.ctabustracker.com/bustime/api/v2/getpredictions?key=REDACTED_PLACEHOLDER&rt=22&stpid=14791')
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from xml.etree.ElementTree import parse
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doc = parse(u)
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print("Arrival time in minutes:")
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for pt in doc.findall('.//prdctdn'):
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print(pt.text)
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```
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(Original exercise example follows below)
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Try something more advanced and type these statements to find out how
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long people waiting on the corner of Clark street and Balmoral in
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Chicago will have to wait for the next northbound CTA \#22 bus:
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```python
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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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Note: This service only reports arrival times within the next 30 minutes.
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If you're in a different timezone and it happens to be 3am in Chicago, you
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might not get any output. You use the tracker link above to double check.
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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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```python
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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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If you can't make this work, don't worry about it. The rest of this course
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has nothing to do with parsing XML.
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[Contents](../Contents.md) \| [Next (1.2 A First Program)](02_Hello_world.md)
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