# CS 61A

Structure and Interpretation of Computer Programs, Fall 2014

Instructor: John DeNero

## Introduction

In 61A, each project has a composition score, worth 3 points, that is graded on the style of your code. This document provides some guidelines.

After your code works, you should strive to do the following things:

3. Make your code efficient (optional for this class)

Sometimes these goals conflict with each other, and sometimes there are exceptions to the rules. Whatever you do, you should always try to make your code easy to read — use your judgement.

Some of these guidelines have one or more of the following marks:

• N: a "non-essential" guideline; these guidelines are not necessary for this class, but are generally good practice
• P: a Python-specific guideline; these are "Pythonic" style conventions that don't necessarily apply to other languages

Finally, here is a link to to PEP-8, the official Python style guide.

## Names and variables

### Meaningful names

Variable and function names should be self-descriptive:

a, b, m = 100, 0, 0
thing = 'hello world'
stuff = lambda x: x % 2
goal, score, opp_score = 100, 0, 0
greeting = 'hello world'
is_even = lambda x: x % 2

### Indices and mathematical symbols

Using one-letter names and abbreviations is okay for indices, mathematical symbols, or if it is obvious what the variables are referring to.

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

In general, i, j, and k are the most common indices used.

### 'o' and 'l'

Do not use the letters 'o' and 'l' by themselves as names:

o = O + 4     # letter 'O' or number 0?
l = l + 5     # letter 'l' or number 1?

### Unnecessary variables

Don't create unnecessary variables. For example,

return result

However, if it is unclear what your code is referring to, or if the expression is too long, you should create a variable:

do_something(lambda x: x % 49 == 0, (total + 1) // 7)
divisible_49 = lambda x: x % 49 == 0
score = (total + 1) // 7
do_somethig(divisible_49, score)

### Profanity

Don't leave profanity in your code. Even if you're really frustrated.

eff_this_class = 666

### Naming convention:

NP: Use lower_case_and_underscores for variables and functions:
TotalScore = 0
finalScore = 1

def Mean_Strategy(score, opp):
...
total_score = 0
final_score = 1

def mean_strategy(score, opp):
...

On the other hand, use CamelCase for classes:

class example_class:
...
class ExampleClass:
...

## Spacing and Indentation

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.

### Spaces vs. tabs

Use spaces, not tabs for indentation. Our starter code always uses 4 spaces instead of tabs. If you use both spaces and tabs, Python will raise an IndentationError.

### Indent size

P: Use 4 spaces to denote an indent. 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.

### Line Length

Keep lines under 80 characters long. Other conventions use 70 or 72 characters, but 80 is usually the upper limit.

### Double-spacing

Don't double-space code. That is, do not insert a blank line in between lines of code. Personally, I find that harder to read.

### Spaces with operators

N: Use spaces between + and -. Depending on how illegible expressions get, you can use your own judgement for *, /, and ** (as long as it's easy to read at a glance, it's fine).
x=a+b*c*(a**2)/c-4
x = a + b*c*(a**2) / c - 4

### Spacing lists

N: When using tuples, lists, or function operands, leave one space after each comma ,:
tup = (x,x/2,x/3,x/4)
tup = (x, x/2, x/3, x/4)

### Line wrapping

NP: If a line gets too long, you have two options. If you are using parentheses or braces with multiple elements, you can continue them onto the next line:
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')

Notice that the subsequent lines line up with the start of the sequence. If the above rule does not apply, you can use Python's \ operator:

total = this_is(a, very, lengthy) + line + of_code \
+ so_it - should(be, separated) \
+ onto(multiple, lines)

Where you put the \ in relation to binary operators (e.g. hi \ + bye versus hi + \ bye) will vary from person to person — for our class, it doesn't matter.

### Blank lines

NP: Leave a blank line between the end of a function or class and the next line:
def example():
return 'stuff'

x = example() # notice the space above

### Trailing whitespace

N: Don't leave whitespace at the end of a line.

## Repetition

In general, don't repeat yourself (DRY). It wastes space and can be computationally inefficient.

### Complex expressions

Do not repeat complex expressions:

if a + b - 3 * h / 2 % 47 == 4:
total += a + b - 3 * h / 2 % 47

Instead, store the expression in a variable:

turn_score = a + b - 3 * h / 2 % 47
if turn_score == 4:
total += turn_score

### Computation-heavy function calls

Don't repeat computationally-heavy function calls:

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]

Instead, store the expression in a variable:

result = takes_one_minute_to_run(x)
if result != ():
first = result[0]
second = result[1]
third = result[2]

### if/else conditions

DON'T have the same code in both the if and the else clause of a conditional:

print('stuff')
x += 1
return x
else:
x += 1
return x

Instead, pull the line(s) out of the conditional:

if pred:            # good!
print('stuff')
x += 1
return x

Recall that Python comments begin with the # 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.

### Docstrings

P: Put docstrings only at the top of functions. Docstrings are denoted by triple-quotes at the beginning of a function or class:
def average(fn, samples):
"""Calls a 0-argument function SAMPLES times, and takes
the average of the outcome.
"""

You should not put docstrings in the middle of the function — only put them at the beginning.

### Remove commented-out code

Remove commented-out code from final version. You can comment lines out when you are debugging. When you are turning in your project, take all commented lines out (including TODOs) — this makes it easier for readers to read your code.

def example(y):
x += 1            # increments x by 1
return square(x)  # returns the square of x

Your actual code should be self-documenting — 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

## Control Structures

### Boolean comparisons

Don't compare a boolean variable to True or False:

if pred == True:   # bad!
...
if pred == False:  # bad!
...

if pred:           # good!
...
if not pred:       # good!
...

### Redundant if/else

Don't do this:

return True
else:
return False

return pred         # good!

### Similar if/else suites

(related to the previous:) Don't do this:

if num != 49:
total += example(4, 5, True)
else:
total += example(4, 5, False)

In the example above, the only thing that changes between the conditionals is the boolean at the end. Instead, do this:

total += example(4, 5, num != 49)

### while vs. if

Don't use a while loop when you should use an if:

while pred:
x += 1
return x

if pred:
x += 1
return x

### Parentheses

P: Don't use parentheses with conditional statements:
if (x == 4):
...
elif (x == 5):
...
while (x < 10):
...

Parentheses are not necessary in Python conditionals (they are in other languages though).

## Miscellaneous

### Semicolons

P: Do not use semicolons. This is not C/C++/Java/etc.

### Checking None

P: Use is and is not for None, not == and !=.

### Implicit False

P: Use the "implicit" False value when possible. Examples include empty containers like [], (), {}, set().
if lst:       # if lst is not empty
...
if not tup:   # if tup is empty
...

### Generator expressions

P: Generator expressions are okay for simple expressions. This includes list comprehensions, dictionary comprehensions, set comprehensions, etc. Generator expressions are neat ways to concisely create lists. Simple ones are fine:
ex = [x*x for x in range(10)]
L = [pair[0] + pair[1]
for pair in pairs
if len(pair) == 2]

However, complex generator expressions are very hard to read, even illegible. As such, do not use generator expressions for complex expressions.

L = [x + y + z for x in nums if x > 10 for y in nums2 for z in nums3 if y > z]