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pythonathon.html
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<body>
<div id="content">
<div id="outline-container-orgcf4d2f5" class="outline-2">
<h2 id="orgcf4d2f5"><span class="todo TODO">TODO</span> In progress</h2>
<div class="outline-text-2" id="text-orgcf4d2f5">
</div><div id="outline-container-orge2f5972" class="outline-3">
<h3 id="orge2f5972"><span class="done DONE">DONE</span> List comprehensions</h3>
</div>
<div id="outline-container-orgf9750e0" class="outline-3">
<h3 id="orgf9750e0"><span class="done DONE">DONE</span> Namespaces and Imports</h3>
</div>
<div id="outline-container-orgccbd307" class="outline-3">
<h3 id="orgccbd307"><span class="todo TODO">TODO</span> Decorators</h3>
</div>
<div id="outline-container-org1a370a7" class="outline-3">
<h3 id="org1a370a7"><span class="todo TODO">TODO</span> Project Structure</h3>
</div>
</div>
<div id="outline-container-org14eb555" class="outline-2">
<h2 id="org14eb555">What Even</h2>
<div class="outline-text-2" id="text-org14eb555">
<p>
Some of us are born to python, some rise to python, and others have python
thrust upon 'em. Let's learn you a python.
</p>
<p>
This document assumes you've seen computer programming before, but tries to be
kind in how it is paced. Everything here is for Python 3.
</p>
</div>
<div id="outline-container-orgb8a57f2" class="outline-3">
<h3 id="orgb8a57f2">A note about Abstraction</h3>
<div class="outline-text-3" id="text-orgb8a57f2">
<p>
In some sense, the two core actions of computer programming are <i>abstraction</i> and
<i>naming</i>. That is: we're going to try and make code that expresses an idea; we
have to give that code and its constituents clear and meaningful names. This is,
I think, a tricky idea to get your head all the way around without thorough
exposure. My intention is that this document will point out some places where we
abstract ourselves from something so the ideas sink in well.
</p>
</div>
</div>
</div>
<div id="outline-container-orgc8a6614" class="outline-2">
<h2 id="orgc8a6614">Types</h2>
<div class="outline-text-2" id="text-orgc8a6614">
<p>
We've briefly covered the notion of a type. Python offers us some foundational
types to work with:
</p>
<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
<colgroup>
<col class="org-left" />
<col class="org-left" />
<col class="org-left" />
</colgroup>
<thead>
<tr>
<th scope="col" class="org-left">Type</th>
<th scope="col" class="org-left">Specification</th>
<th scope="col" class="org-left">Example</th>
</tr>
</thead>
<tbody>
<tr>
<td class="org-left">int</td>
<td class="org-left">Integer; effectively unlimited size</td>
<td class="org-left">1, 5, 12,487,129,420</td>
</tr>
<tr>
<td class="org-left">float</td>
<td class="org-left">Double-precision floating point number</td>
<td class="org-left">0.219, 50.6</td>
</tr>
<tr>
<td class="org-left">complex</td>
<td class="org-left">Complex numbers</td>
<td class="org-left">2i</td>
</tr>
<tr>
<td class="org-left">bool</td>
<td class="org-left">Boolean</td>
<td class="org-left">True/False</td>
</tr>
<tr>
<td class="org-left">str</td>
<td class="org-left">String</td>
<td class="org-left">'cat', 'house boat'</td>
</tr>
</tbody>
</table>
<p>
Python is a <i>strongly, dynamically typed</i> language. This means we almost never
have to care about what the type of a thing is when we declare or receive it,
but we <i>cannot</i> use types interchangeably in some contexts. For instance:
</p>
<pre class="example">
>>> 2 + 2
4
>>> 'cat' + 'dog'
'catdog'
>>> 2 + 'dog'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
</pre>
<p>
Between ints, <code>+</code> means "addition"; between strings, it means "concatenation". But
between an int and a string, python cannot and will not guess what <code>+</code> means, and
throws a type error. We must "cast", changing the type of one operand to match
the other, so that python knows how to <code>+</code> everything together correctly:
</p>
<pre class="example">
>>> str(2) + 'dog'
'2dog'
</pre>
</div>
</div>
<div id="outline-container-org04dfeeb" class="outline-2">
<h2 id="org04dfeeb">Identifiers</h2>
<div class="outline-text-2" id="text-org04dfeeb">
<p>
Ok, so we're going to name things. In python, we name things by giving them an
<i>identifier</i>. A valid identifier in python follows these rules:
</p>
<ol class="org-ol">
<li>It can be any combination of upper and lowercase letters, numbers, and the <code>_</code> character.</li>
<li>It must start with a letter.</li>
</ol>
<p>
So, <code>my_swe3t_l33t_IdEnTiFi3r</code> is valid (but don't ever do that); <code>3rd_item</code> is not.
</p>
<p>
Python usually follows these conventions:
</p>
<ol class="org-ol">
<li>Identifiers used for variables are in <i>snake case</i>: all lower-case letters with
words separated by the underscore character.
E.G. <code>a_variable_named_foo</code></li>
<li>Identifiers for classes are in <i>title case</i>: each word with its first letter
capitalized, no spaces or underscores.
E.G. <code>MyFooClass</code></li>
</ol>
</div>
</div>
<div id="outline-container-orgcaa57e1" class="outline-2">
<h2 id="orgcaa57e1">Statements</h2>
<div class="outline-text-2" id="text-orgcaa57e1">
<p>
Python code is made up of <i>statements</i>. A statement can be a whoooole lot of
things. A statement might be variable assignment, or creating a function. Simply
listing a type or an object <i>isn't</i> a statement.
</p>
<p>
So, in an interpreter:
</p>
<pre class="example">
>>> 5 # not a statement
5
>>> x = 5 # statement
</pre>
</div>
</div>
<div id="outline-container-orgf394436" class="outline-2">
<h2 id="orgf394436">Variables</h2>
<div class="outline-text-2" id="text-orgf394436">
<p>
Variable assignment is one of the most standard features a programming language
can have. In python, variable assignment is as simple as can be:
</p>
<div class="org-src-container">
<pre class="src src-python">x = 5
print(x)
</pre>
</div>
<pre class="example">
5
</pre>
<p>
Put another way: we're binding the <i>value</i> 5 to the <i>identifier</i> x. We can bind any
value we want to any valid identifier this way.
</p>
<p>
Now: it's important that you understand that there is a thing called <i>scope</i>,
which affects when and how variables can be accessed. We're going to get to
<a href="#orgbfa0668">scope</a> soon, but we need a few more ideas before we can fully explain it.
</p>
</div>
</div>
<div id="outline-container-org4580670" class="outline-2">
<h2 id="org4580670">Reserved Words</h2>
<div class="outline-text-2" id="text-org4580670">
<p>
Before we get too far, there's a thing about Python you should know – which is
a thing that's true of many programming languages, so it's useful to be clear
on. This is the notion of <i>reserved words</i>. It goes like this:
</p>
<p>
When we write code, we express to a computer what we want it to do. The language
we use to express ourselves is our programming language. That language has some
syntax, made of words and symbols, that allows us to get our ideas and
intentions written down. Certain words and symbols are baked in to the language,
very deeply – their meaning cannot be changed by us, and we have to respect and
use these words only in very specific ways.
</p>
<p>
(<i>Nota bene</i>: in python, "reserved words" are typically referred to as "keywords."
Same idea, slightly different name.)
</p>
<p>
What this means in practice is that we <i>cannot use a reserved word as an
identifier</i>. For instance:
</p>
<div class="org-src-container">
<pre class="src src-python">False = 5 # NOPE
import = 7 # SUPER NOPE
</pre>
</div>
<p>
The python keywords are:
<code>False</code>, <code>class</code>, <code>finally</code>, <code>is</code>, <code>return</code>, <code>None</code>, <code>continue</code>, <code>for</code>, <code>lambda</code>, <code>try</code>, <code>True</code>, <code>def</code>,
<code>from</code>, <code>nonlocal</code>, <code>while</code>, <code>and</code>, <code>del</code>, <code>global</code>, <code>not</code>, <code>with</code>, <code>as</code>, <code>elif</code>, <code>if</code>, <code>or</code>, <code>yield</code>,
<code>assert</code>, <code>else</code>, <code>import</code>, <code>pass</code>, <code>break</code>, <code>except</code>, <code>in</code>, <code>raise</code>
</p>
<p>
We will get in to what most of these do as we work through this document! Hang
in there.
</p>
</div>
</div>
<div id="outline-container-orgcf5ebf8" class="outline-2">
<h2 id="orgcf5ebf8">Boolean comparisons</h2>
<div class="outline-text-2" id="text-orgcf5ebf8">
<p>
Let's say we want to make a logical statement about the comparison of two
values. If we're dealing with numbers, python provides a set of built-in
operators to help us do precisely this. We can explore this in the python
interpreter:
</p>
<pre class="example">
>>> 5 < 6
True
>>> 1 > 100
False
</pre>
<p>
Note our first two keywords: <code>True</code> and <code>False</code>.
</p>
<p>
Python also supports greater-than-or-equal to, so:
</p>
<pre class="example">
>>> 5 >= 9
False
>>> 9 >= 9
True
</pre>
<p>
Or we can test equality:
</p>
<pre class="example">
>>> 10 == 10
True
</pre>
<p>
Common in many languages, exclamation point captures the idea of negation in a
symbol. So, "not equal" is written:
</p>
<pre class="example">
>>> 4 != 5
True
>>> 4 != 4
False
</pre>
<p>
Python also provides the keyword <code>not</code>, which, as with <code>!</code>, negates any Boolean
expression following it:
</p>
<pre class="example">
>>> not True
False
>>> not 4 == 5
True
</pre>
<p>
Note that python also has nice English keywords for Boolean operators: <code>and</code> and
<code>or</code>:
</p>
<pre class="example">
>>> False or True
True
>>> False and False
False
>>> False and True
False
>>> True and True
True
</pre>
</div>
<div id="outline-container-org05b82b8" class="outline-3">
<h3 id="org05b82b8">Equality versus Identity</h3>
<div class="outline-text-3" id="text-org05b82b8">
<p>
Along with equality operators (e.g. <code>==</code>), python provides an <i>identity</i> operator.
While extremely useful, the identity operator can also lead to some very subtle
bugs. This is in part because the identity operator is <code>is</code>, and thus has a much
more natural language syntax than <code>==</code>. However, observe:
</p>
<pre class="example">
>>> a = 19998989890
>>> b = 19998989889 + 1
>>> a == b
True
>>> a is b
False
</pre>
<p>
<i>Equality</i> compares the <i>value</i> of two things; <i>identity</i> checks to see if two things
are literally the same object in memory.
</p>
<p>
As a general rule, <code>is</code> can always be used to compare with <code>True</code>, <code>False</code>, and <code>None</code>.
This is because these three values (all keywords, notice) are <i>singleton objects</i>
– there is only one <code>True</code> object, ever, period, so equality and identity are
effectively interchangeable. For more complex kinds of values, it's often better
to stick to <code>==</code>. Thus:
</p>
<pre class="example">
>>> x = True
>>> x is True
True
>>> x is not False
True
>>> y = 10
>>> y == 10
True
</pre>
</div>
</div>
</div>
<div id="outline-container-org1c4ddd1" class="outline-2">
<h2 id="org1c4ddd1">Control Flow</h2>
<div class="outline-text-2" id="text-org1c4ddd1">
<p>
If we have a notion of Boolean values and truthiness, we can now decide to
change the way our program works based on some Boolean condition. This is called
<code>control flow</code>, and it is very nice.
</p>
<p>
The single most common control flow structure is the <code>if / else</code> block. Python
elides the common <code>else if</code> phrase in to <code>elif</code>, for no reason in particular.
</p>
<div class="org-src-container">
<pre class="src src-python">x = 5
if x > 10:
print('X is greater than 10!')
elif x == 10:
print('X is exactly 10')
else:
print('X must be less than ten')
</pre>
</div>
<pre class="example">
X must be less than ten
</pre>
<p>
These checks can get quite complex:
</p>
<div class="org-src-container">
<pre class="src src-python">if x < 5 or y is 'cow':
print('woah')
elif (x is 5 and y is 5 and z is 5) or skip_the_fives:
print('okay double woah')
else:
print('whew')
</pre>
</div>
<p>
A thing to notice: instead of doing an explicit comparison, we can use the
<a href="#org79ffe11">Truthiness</a> of a term directly:
</p>
<div class="org-src-container">
<pre class="src src-python">if 5:
print('it must be 5')
</pre>
</div>
<pre class="example">
it must be 5
</pre>
<p>
Seen slightly less frequently, but still fairly common, is the <code>while</code> construct,
which loops "while" some term is truthy:
</p>
<div class="org-src-container">
<pre class="src src-python">x = 0
while x < 10:
print(x)
x = x + 1
</pre>
</div>
<pre class="example">
0
1
2
3
4
5
6
7
8
9
</pre>
<p>
Note two things:
</p>
<ol class="org-ol">
<li>If <code>x</code> weren't mutated, the loop would loop forever.</li>
<li>You can use a <code>while</code> loop to loop forever, on purpose.</li>
</ol>
<p>
#2 is not uncommonly seen for the "main loop" of a program. That is: if we
#consider a computer "program" to be a thing that sits idle until some action
#occurs, then goes back to being idle, we could express that idea like so:
</p>
<pre class="example">
while True:
if check_for_user_input():
respond_appropriately()
</pre>
</div>
<div id="outline-container-org79ffe11" class="outline-3">
<h3 id="org79ffe11">Truthiness</h3>
<div class="outline-text-3" id="text-org79ffe11">
<p>
Python has a broad notion of what we often call "truthiness". That is: certain
values are <i>implicitly</i> considered to be roughly equivalent to <code>True</code> or <code>False</code> when
used in control flow expressions.
</p>
<p>
So:
</p>
<dl class="org-dl">
<dt>Truthy Values are</dt><dd><ul class="org-ul">
<li><code>True</code></li>
<li>Any string with length greater than 0</li>
<li>All numbers</li>
<li>All non-empty collections</li>
<li>Most object instances (we'll get in to what this is in a little bit)</li>
</ul></dd>
<dt>Falsy Values are</dt><dd><ul class="org-ul">
<li><code>False</code></li>
<li>Empty string</li>
<li>Empty collections</li>
<li><code>None</code></li>
</ul></dd>
</dl>
<p>
We use them like:
</p>
<div class="org-src-container">
<pre class="src src-python">a_list = []
if not a_list:
print('it is empty!')
else:
print('it is full')
</pre>
</div>
<pre class="example">
it is empty!
</pre>
<p>
Or:
</p>
<div class="org-src-container">
<pre class="src src-python">full_string = 'this is a string'
empty_string = ''
if full_string:
print('there was some string!')
if empty_string:
print('you should be surprised if this prints')
</pre>
</div>
<pre class="example">
there was some string!
</pre>
</div>
</div>
</div>
<div id="outline-container-org9e01adf" class="outline-2">
<h2 id="org9e01adf">Collections</h2>
<div class="outline-text-2" id="text-org9e01adf">
<p>
A "collection" is, as the name implies, a kind of container or group of Things.
Python comes with four main collection types built-in; in practice, we use two
of them vastly more than the others. For every collection, python provides a
<i>literal</i> syntax, which is a shorthand way of creating a new collection.
</p>
<p>
<b>Note</b>: all collections in python are <i>zero indexed</i>. This means that the very first
element in a collection is the 0 element, the second is the 1 element, etc.
This takes a little getting used to, but is also very common.
</p>
<p>
Also note: all python collections are <i>heterogeneous</i> – they can contain Things
of any combination of types, including other collections.
</p>
</div>
<div id="outline-container-org9dba229" class="outline-3">
<h3 id="org9dba229">Tuples</h3>
<div class="outline-text-3" id="text-org9dba229">
<p>
A tuple is an immutable, and usually small, collection. It is used to group
together a small number of things we implicitly assert are related to one
another. The tuple literal is a set of parens <code>()</code>. We access the elements of a
tuple by their index.
</p>
<div class="org-src-container">
<pre class="src src-python">x = ('cat', 'dog', 'phone')
print(x[0])
print(x[1])
print(x[2])
</pre>
</div>
<pre class="example">
cat
dog
phone
</pre>
<p>
Note a python oddity: to make a single-element tuple, a comma is needed after
the first element – e.g. <code>('cat',)</code>.
</p>
</div>
</div>
<div id="outline-container-orgf87c0ea" class="outline-3">
<h3 id="orgf87c0ea">Lists</h3>
<div class="outline-text-3" id="text-orgf87c0ea">
<p>
A <code>list</code> is one of the data structures we interact with alllllll the time in
python. We can make a list with the <code>list</code> function, but it's more common to do it
with the list literal, which is a set of square braces <code>[]</code>.
</p>
<p>
Lists are ordered and mutable. We access the elements of a list by their index.
</p>
<div class="org-src-container">
<pre class="src src-python">a_list = [5, False, 'gazpacho']
print(a_list[2])
</pre>
</div>
<pre class="example">
gazpacho
</pre>
</div>
</div>
<div id="outline-container-orgf968600" class="outline-3">
<h3 id="orgf968600">Dicts</h3>
<div class="outline-text-3" id="text-orgf968600">
<p>
A <code>dict</code> captures the notion of key-value pairs in python; the name is short for
<i>dictionary</i>, which gives us a very good hit about its use. <code>Dicts</code> offer us <i>very
fast</i> lookup of elements. There is a <code>dict</code> function, but we more commonly use the
curly-brace literal, <code>{}</code>, with the internal format keyname, colon, space, value
of key (E.G. <code>{name_of_key: value}</code>.)
</p>
<p>
The key of a <code>dict</code> is typically a string, but sometimes, tuples or integers are
used.<sup><a id="fnr.1" class="footref" href="#fn.1">1</a></sup>
</p>
<p>
We access a list of the keys in a <code>dict</code> using an instance<sup><a id="fnr.2" class="footref" href="#fn.2">2</a></sup> method called
<code>keys()</code>. We access values by the name of their key. Like so:
</p>
<div class="org-src-container">
<pre class="src src-python">the_dict = {'googoo': 'cachoo',
'hocus': 'pocus',
'Marlon': 'Brando'}
print(the_dict.keys())
print(the_dict['hocus'])
</pre>
</div>
<pre class="example">
['googoo', 'Marlon', 'hocus']
pocus
</pre>
</div>
</div>
<div id="outline-container-org9169887" class="outline-3">
<h3 id="org9169887">Sets</h3>
<div class="outline-text-3" id="text-org9169887">
<p>
A set is a very handy data type with a special property: <i>every element of the
set is guaranteed unique</i>. Sets are, thus, used for uniquing, and for
maintaining collections of unique elements. You can use the <code>set</code> function, or
you can use the set literal, which is, slightly confusingly, also curly braces
<code>{}</code>. (If there are no colons inside the braces, python knows it's a <code>set</code>, not a
<code>dict.</code>)
</p>
<p>
When you create a set, all of the elements will be uniqued correctly. This is
done by… wait for it… hashing each element, which means each element in a
set must be hashable.
</p>
<div class="org-src-container">
<pre class="src src-python">list_with_duplicates = [1, 1, 1, 2, 2, 3, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5, 5]
the_set = set(list_with_duplicates)
print(the_set)
</pre>
</div>
<pre class="example">
set([1, 2, 3, 4, 5])
</pre>
<p>
For those of you with a math bent, you might be thinking, "I wonder if we can
take the union, difference, and intersection of Python's sets?" Good news! You
absolutely can. The interface is exposed as instance methods on a given set.
</p>
<div class="org-src-container">
<pre class="src src-python">first_set = {1, 2, 3}
second_set = {3, 4, 5}
# The union of two sets is all the unique elements of both sets together in one
print(first_set.union(second_set))
# The intersection is only those elements found in both sets
print(first_set.intersection(second_set))
# The difference is all the elements from the calling set not found in the
# argument set -- in this case, all the elements in first_set not found in
# second_set
print(first_set.difference(second_set))
</pre>
</div>
<pre class="example">
set([1, 2, 3, 4, 5])
set([3])
set([1, 2])
</pre>
</div>
</div>
</div>
<div id="outline-container-org0b22fa7" class="outline-2">
<h2 id="org0b22fa7">Iteration and Comprehension</h2>
<div class="outline-text-2" id="text-org0b22fa7">
<p>