UNB/ CS/ David Bremner/ teaching/ cs2613/ books/ practical-python/ 01 Introduction/ 01 Python

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1.1 Python

What is Python?

Python is an interpreted high level programming language. It is often classified as a "scripting language" and is considered 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.

Where to get Python?

Python.org is where you obtain Python. For the purposes of this course, you only need a basic installation. I recommend installing Python 3.6 or newer. Python 3.6 is used in the notes and solutions.

Why was Python created?

In the words of Python's creator:

My original motivation for creating Python was the perceived need for a higher level language in the Amoeba [Operating Systems] project. I realized that the development of system administration utilities in C was taking too long. Moreover, doing these things in the Bourne shell wouldn't work for a variety of reasons. ... So, there was a need for a language that would bridge the gap between C and the shell.

Where is Python on my Machine?

Although there are many environments in which you might run Python, Python is typically installed on your machine as a program that runs from the terminal or command shell. From the terminal, you should be able to type python like this:

bash $ python
Python 3.8.1 (default, Feb 20 2020, 09:29:22)
[Clang 10.0.0 (clang-1000.10.44.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print("hello world")
hello world

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.

Although there are many non-shell environments where you can code Python, you will be a stronger Python programmer if you are able to run, debug, and interact with Python at the terminal. This is Python's native environment. If you are able to use Python here, you will be able to use it everywhere else.


Exercise 1.1: Using Python as a Calculator

On your machine, start Python and use it as a calculator 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

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

Exercise 1.2: Getting help

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).

Exercise 1.3: Cutting and Pasting

This course is structured as a series of traditional web pages where you are encouraged to try interactive Python code samples by typing them out by hand. If you are learning Python for the first time, this "slow approach" 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 must "cut and paste" code samples, 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 browser, 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
>>> (3 + 4
         + 5 + 6)
>>> for i in range(5):


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.

Now that you've done this, just remember that you will get more out of the class by typing in code slowly and thinking about it--not cut and pasting.

Exercise 1.4: Where is My Bus?

Note: This was a whimsical example that was a real crowd-pleaser when I taught this course in my office. You could query the bus and then literally watch it pass by the window out front. Sadly, APIs rarely live forever and it seems that this one has now ridden off into the sunset. --Dave

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'):

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.

Note: This service only reports arrival times within the next 30 minutes. If you're in a different timezone and it happens to be 3am in Chicago, you might not get any output. You use the tracker link above to double check.

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'

If you can't make this work, don't worry about it. The rest of this course has nothing to do with parsing XML.

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