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<!DOCTYPE html>
<html>
<head>
<title>Style Guide</title>
<link
href="http://www-inst.eecs.berkeley.edu/~cs61a/su13/projects/ants/css/assignments.css" rel="stylesheet" type="text/css">
<style>
span.non-ess {
color:#b78727;
font-weight:bold;
}
span.python {
color:#0043c5;
font-weight:bold;
}
</style>
</head>
<body>
<div id='header'>
<div id ='logo'>
<h1>Style Guide</h1>
</div>
</div>
<ul><li><a href="#names-and-variables">Names and variables</a></li><li><a href="#spacing-and-indentation">Spacing and Indentation</a></li><li><a href="#repetition">Repetition</a></li><li><a href="#comments">Comments</a></li><li><a href="#control-structures">Control Structures</a></li><li><a href="#miscellaneous">Miscellaneous</a></li></ul>
<p>In 61A, each project has a composition score, worth 3 points, that is
graded on the <em>style</em> of your code. This document provides some
guidelines.</p>
<p>After your code works, you should strive to do two things:</p>
<ol>
<li>Make your code <strong>easier to read</strong></li>
<li>Make your code <strong>concise</strong></li>
<li>Make your code <strong>efficient</strong> (optional for this class)</li>
</ol>
<p>Sometimes these goals conflict with each other, and <strong>sometimes there
are exceptions to the rules</strong>. Whatever you do, you should always try
to make your code easy to read -- use your judgement.</p>
<p>Some of these guidelines will have one or more of the following marks:</p>
<ul>
<li><span class="non-ess">N</span>: a "non-essential" guideline; these
guidelines are not necessary for this class, but are generally good
practice</li>
<li><span class="python">P</span>: a Python-specific guideline; these
are "Pythonic" style conventions that don't necessarily apply to
other languages</li>
</ul>
<p>Finally, here is a link to to
<a href="http://www.python.org/dev/peps/pep-0008/">PEP-8</a>, the official Python
style guide.</p>
<h2 id="names-and-variables">Names and variables</h2>
<ol>
<li><strong>Meaningful names</strong>: variable and function names should be
<em>self-descriptive</em>:</p>
<pre class="prettyprint"># bad -- don't do this!
a, b, m = 100, 0, 0
thing = 'hello world'
stuff = lambda x: x % 2
# good
goal, score, opp_score = 100, 0, 0
greeting = 'hello world'
is_even = lambda x: x % 2
</pre></li>
<li><strong>Indices and mathematical symbols</strong>: using one-letter names and
abbreviations is okay for indices, mathematical symbols, or if it
is obvious what the variables are referring to.</p>
<pre class="prettyprint">i = 0 # a counter for a loop
x, y = 0, 0 # x and y coordinates
p, q = 5, 17 # mathematical names in the context of the question
</pre>
<p>In general, <code>i</code>, <code>j</code>, and <code>k</code> are the most common indices used.</li>
<li><strong>Avoid letters 'o' and 'l'</strong>: do not use them by themselves as
names:</p>
<pre class="prettyprint"># bad!
o = O + 4 # letter 'O' or number 0?
l = l + 5 # letter 'l' or number 1?
</pre></li>
<li><strong>Don't create unnecessary variables</strong>: for example,</p>
<pre class="prettyprint"># bad!
result = answer(argument)
return result
# good!
return answer(argument)
</pre>
<p>However, if it is unclear what your code is referring to, or if the
expression is too long, you <strong>should</strong> create a variable:</p>
<pre class="prettyprint"># bad!
do_something(lambda x: x % 49 == 0, (total + 1) // 7)
# good!
divisible_49 = lambda x: x % 49 == 0
score = (total + 1) // 7
do_somethig(divisible_49, result)
</pre></li>
<li><strong>Avoid profanity</strong>: don't leave it in your code. Even if you're
really frustrated.</p>
<pre class="prettyprint"># bad!
eff_this_class = 666
</pre></li>
<li><span class="non-ess">N</span><span class="python">P</span>: Use <code>lower_case_and_underscores</code> for variables and
functions:</p>
<pre class="prettyprint"># bad!
TotalScore = 0
finalScore = 1
def Mean_Strategy(score, opp):
...
# good!
total_score = 0
final_score = 1
def mean_strategy(score, opp):
...
</pre></li>
<li><span class="non-ess">N</span><span class="python">P</span>: Use <code>CamelCase</code> for classes:</p>
<pre class="prettyprint"># bad!
class example_class:
...
# good!
class ExampleClass:
...
</pre></li>
</ol>
<h2 id="spacing-and-indentation">Spacing and Indentation</h2>
<p>Whitespace style might seem superfluous, but using whitespace in
certain places (and omitting it in others) will often make it easier
to read code. In addition, since Python code depends on whitespace
(e.g. indentation), it requires some extra attention. For that
reason, this section has quite a few guidelines that you should
consider.</p>
<ol>
<li><strong>Use spaces, not tabs for indentation</strong>: our starter code always
uses 4 spaces instead of tabs. If you use both spaces and tabs,
Python will raise an <code>IndentationError</code>.</li>
<li><span class="python">P</span>: <strong>Use 4 spaces per indentation level</strong>: technically, Python
allows you to use any number of spaces as long as you are
consistent across an indentation level. The conventional style is
to use 4 spaces.</li>
<li><strong>Keep lines under 80 characters long</strong>: other conventions use 70
or 72 characters, but 80 is usually the upper limit.</li>
<li><strong>Don't double-space code</strong>: that is, do <em>not</em> insert a blank line
in between lines of code. Personally, I find that harder to read.</li>
<li><span class="non-ess">N</span>: <strong>Use spaces between primitive operators</strong>: always use spaces
between <code>+</code> and <code>-</code>. Depending on how illegible expressions get,
you can use your own judgement for <code>*</code>, <code>/</code>, and <code>**</code> (as long as
it's easy to read at a glance, it's fine).</p>
<pre class="prettyprint"># bad!
x=a+b*c*(a**2)/c-4
# good!
x = a + b*c*(a**2) / c - 4
</pre></li>
<li><span class="non-ess">N</span>: <strong>Spacing lists</strong>: When using tuples, lists, or function
operands, leave one space after each comma <code>,</code>:</p>
<pre class="prettyprint"># bad!
tup = (x,x/2,x/3,x/4)
# good!
tup = (x, x/2, x/3, x/4)
</pre></li>
<li><span class="non-ess">N</span><span class="python">P</span>: If a line gets two long, you have two options. If you are
using parentheses or braces with multiple elements, you can
continue them onto the next line:</p>
<pre class="prettyprint">def func(a, b, c, d, e, f,
g, h, i):
# body
tup = (1, 2, 3, 4, 5,
6, 7, 8)
names = ('alice',
'bob',
'eve')
</pre>
<p>Notice that the subsequent lines line up with the <em>start</em> of the
sequence. If the above rule does not apply, you can use Python's <code>\</code>
operator:</p>
<pre class="prettyprint">total = this_is(a, very, lengthy) + line + of_code \
+ so_it - should(be, separated) \
+ onto(multiple, lines)
</pre>
<p><em>Where</em> you put the <code>\</code> in relation to binary operators (e.g.
<code>hi \ + bye</code> versus <code>hi + \ bye</code>) will vary from person to person
-- for our class, it doesn't matter.</li>
<li><span class="non-ess">N</span><span class="python">P</span>: Leave a blank line between the end of a function or class and the
next line:</p>
<pre class="prettyprint">def example():
return 'stuff'
x = example() # notice the space above
</pre></li>
<li><span class="non-ess">N</span>: Don't leave whitespace at the end of a line.</li>
</ol>
<h2 id="repetition">Repetition</h2>
<p>In general, <strong>don't repeat yourself</strong> (DRY). It wastes space and can
be computationally inefficient.</p>
<ol>
<li><strong>DON'T</strong> repeat complex expressions:</p>
<pre class="prettyprint">if a + b - 3 * h / 2 % 47 == 4:
total += a + b - 3 * h / 2 % 47
return total
</pre>
<p><strong>Instead</strong>, store the expression in a variable:</p>
<pre class="prettyprint">turn_score = a + b - 3 * h / 2 % 47
if turn_score == 4:
total += turn_score
return total
</pre>
<p>This will also make your code more readable.</li>
<li><strong>Don't repeat computationally-heavy function calls</strong>:</p>
<pre class="prettyprint">if takes_one_minute_to_run(x) != ():
first = takes_one_minute_to_run(x)[0]
second = takes_one_minute_to_run(x)[1]
third = takes_one_minute_to_run(x)[2]
</pre>
<p><strong>Instead</strong>, store the expression in a variable:</p>
<pre class="prettyprint">result = takes_one_minute_to_run(x)
if result != ():
first = result[0]
second = result[0]
third = result[0]
</pre></li>
<li><strong>DON'T</strong> have the same code in both the <code>if</code> and the <code>else</code> clause
of a conditional:</p>
<pre class="prettyprint">if pred: # bad!
print('stuff')
x += 1
return x
else:
x += 1
return x
</pre>
<p><strong>Instead</strong>, pull the line(s) out of the conditional:</p>
<pre class="prettyprint">if pred: # good!
print('stuff')
x += 1
return x
</pre></li>
</ol>
<h2 id="comments">Comments</h2>
<p>Recall that Python comments begin with the <code>#</code> sign. Keep in mind that
the triple-quotes are technically strings, not comments. Comments can
be helpful for explaining ambiguous code, but there are some
guidelines for when to use them.</p>
<ol>
<li><span class="python">P</span>: <strong>Put docstrings only at the top of functions</strong>: docstrings are
denoted by triple-quotes at the beginning of a function or class:</p>
<pre class="prettyprint">def average(fn, samples):
"""Calls a 0-argument function SAMPLES times, and takes
the average of the outcome."""
</pre>
<p>You should <strong>not</strong> put docstrings in the middle of the function --
only put them at the beginning.</li>
<li><strong>Remove commented-out code from final version</strong>: you can comment
lines out when you are debugging. When you are turning in your
project, take all commented lines out (including <code>TODO</code>s) -- this
makes it easier for readers to read your code.</li>
<li><strong>Don't write unnecessary comments</strong>: for example, the
following is bad:</p>
<pre class="prettyprint">def example(y):
x += 1 # increments x by 1
return square(x) # returns the square of x
</pre>
<p>Your actual code should be <em>self-documenting</em> -- try to make it as
obvious as possible what you are doing without resorting to
comments. Only use comments if something is not obvious or needs to
be explicitly emphasized</li>
</ol>
<h2 id="control-structures">Control Structures</h2>
<ol>
<li><strong>DON'T</strong> compare a boolean variable to <code>True</code> or <code>False</code>:</p>
<pre class="prettyprint">if pred == True: # bad!
...
if pred == False: # bad!
...
</pre>
<p><strong>Instead</strong>, do this:</p>
<pre class="prettyprint">if pred: # good!
...
if not pred: # good!
...
</pre></li>
<li><strong>DON'T</strong> do this:</p>
<pre class="prettyprint">if pred: # bad!
return True
else:
return False
</pre>
<p><strong>Instead</strong>, do this:</p>
<pre class="prettyprint">return pred # good!
</pre></li>
<li>(related to the previous:) <strong>DON'T</strong> do this:</p>
<pre class="prettyprint">if num != 49:
total += example(4, 5, True)
else:
total += example(4, 5, False)
</pre>
<p>In the example above, the only thing that changes between the
conditionals is the boolean at the end. <strong>Instead</strong>, do this:</p>
<pre class="prettyprint">total += example(4, 5, num != 49)
</pre></li>
<li><strong>DON'T</strong> use a <code>while</code> loop when you should use an <code>if</code>:</p>
<pre class="prettyprint">while pred:
x += 1
return x
</pre>
<p><strong>Instead</strong>, use an <code>if</code></p>
<pre class="prettyprint">if pred:
x += 1
return x
</pre></li>
<li><span class="python">P</span>: <strong>DON'T</strong> use parentheses with conditional statements:</p>
<pre class="prettyprint">if (x == 4):
...
elif (x == 5):
...
while (x < 10):
...
</pre>
<p>Parentheses are not necessary in Python conditionals (they are in
other languages though).</li>
</ol>
<h2 id="miscellaneous">Miscellaneous</h2>
<ol>
<li><span class="python">P</span>: <strong>Do not use semicolons</strong>. This is not C/C++/Java/etc.</li>
<li><span class="python">P</span>: Use <code>is</code> and <code>is not</code> for <code>None</code>, not <code>==</code> and <code>!=</code>.</li>
<li><span class="python">P</span>: <strong>Use the "implicit" <code>False</code> value when possible</strong>. Examples
include empty containers like <code>[]</code>, <code>()</code>, <code>{}</code>, <code>set()</code>.</p>
<pre class="prettyprint">if lst: # if lst is not empty
...
if not tup: # if tup is empty
...
</pre></li>
<li><span class="python">P</span>: <strong>Generator expressions are okay for simple expressions</strong>:
this includes list comprehensions, dictionary comprehensions, set
comprehensions, etc. Generator expressions are neat ways to
concisely create lists. Simple ones are fine and even encouraged:</p>
<pre class="prettyprint">ex = [x*x for x in range(10)]
L = [pair[0] + pair[1] for pair in pairs if len(pair) == 2]
</pre>
<p>However, complex generator expressions are very hard to read, even
illegible. As such, do not use generator expressions for complex
expressions.</p>
<pre class="prettyprint">L = [x + y + z for x in nums if x > 10 for y in nums2 for z in nums3 if y > z]
</pre>
<p>Use your best judgement.</li>
</ol>
</body>
</html>