Python Coding Conventions and Standards

Note: this is a summary of the rules found in PEP 8, the official style guide for Python code, written by the authors of the Python language.

Naming

Choose your names carefully. As part of program documentation, names must clearly indicate both the kind of thing being named and its role in the program. Whenever possible, write short, obvious names for very local contexts (i.e., a for loop counter could be called i) and longer names for bigger contexts (examples to follow). Avoid cryptic abbreviations (e.g., use increment_event_count() rather than incevcnt()). Avoid choosing similar names for different things (e.g., index, indx, index2), or, on the other hand, similar names for very different things (example: if keys is a list of strings, then don’t use key for an integer).

attribute and local variable names should be noun phrases, written in lowercase, with multiple words distinguished by a single underscore (_)
function names (and method names) should be imperative verb phrases (if possible), lowercase, with words separated by underscores (e.g., get_student_name())
modules should have short, lowercase names, with underscores if necessary
class names should have capitalized words, without underscores to separate each word (e.g., ClassName)
variables or attributes that should be considered unchanging (i.e., “constants” in other languages like Java) should be written in all capital letters with underscores separating words

Indentation

Spyder is a software program that makes it easier to write and run Python code. We will use Spyder extensively in this course when we write programs, since it provides text editor features like code highlighting and automatic indentation.

In an Spyder Editor window, pressing the tab key will cause Spyder to increase the indent level. By default, Spyder will insert the appropriate number of ASCII space characters to get the text cursor to the next level of indentation.

In other text editors, pressing the tab key might insert a tab character (a special character indicating an indent). This is not ideal — if you aren’t using Spyder, you should configure your editor to use spaces.

Whitespace

Using blank lines

While the horizontal position of code on a line (i.e., indentation) conveys important information to the Python interpreter, the interpreter ignores the vertical spacing of code in a file. For example, this function definition

def distance(yards):
    print("yards: " + str(yards))
    feet = yards * 3
    print("feet: " + str(feet))
    inches = feet * 12
    print("inches: " + str(inches))
    return inches

is equivalent to this one:

def distance(yards):
    print("yards: " + str(yards))

    feet = yards * 3
    print("feet: " + str(feet))

    inches = feet * 12
    print("inches: " + str(inches))

    return inches

However, the second definition is more readable. Therefore putting blank lines between logical sections of code is encouraged, to increase readability.

Spaces in between elements on a line

The Python interpreter also ignores the spaces between binary operators like = or +. Therefore the expressions a=b and a = b are equivalent. You should get into the habit of using a single space between the operands in these expressions. However, a couple of notable exceptions to this whitespace rule can be found below:

Avoid spacing inside parentheses. For example, prefer convert(15.2, "inches", "miles") over convert( 15.2, "inches", "miles" ).
Avoid spacing between a function name and the parameter list. For example, prefer hello("Cody") over hello ("Cody").
Avoid spacing before a comma or colon. For example, prefer this
```
if x == 4:
    a, b = x, y; x, y = y, x
```
over the more confusing version:
```
if x == 4 :
    a, b = x , y ; x , y = y , x
```
Avoid spacing before indexing a dictionary. For example, prefer dict['key'] over dict ['key'].

Comments

Comments should help the reader of your code to understand it. They may help you to understand it if you need to rework your code at some later point. In the case of a program submitted for this course, they also serve to convince the grader that you grasp the what, how and why of your code (as opposed to having hacked it into working). Like any other form of expression, it should not be flowery or repetitive. In particular, please observe the following guidelines:

For each significant variable declaration, give a brief comment stating the use of the variable, and when useful, the units (e.g., feet or inches). Non-significant variables (not requiring explanation) would include temporary variables, loop counters, and so on.
When it helps to understand your code, before each major section of the program, give the purpose of that section. Indent the comments to line up with the code.

Documentation strings

Documentation strings (or docstrings) can be attached to any module (at the top of the .py file), function (after the def line), or class (after the class line). They are simple strings that are better than surrounding comments, since they can be reviewed using Python’s help() function, among other uses. (More information is available in PEP 257, Docstring Conventions.)

Use the triple-double quotation marks (""") to start and end a docstring. The docstring should begin on the line with the opening """, as follows:

def factorial(n):
    """Return the factorial of n."""

If the docstring can’t fit on one line, move the ending """ to its own line after the string and keep writing the docstring. Do not add an extra newline after the opening """. For example:

def replace(secret_word, display_word, letter):
    """Return a list of characters that results from taking the
    existing display word list and revealing any and all occurrences
    of the specified letter, as dictated by secret_word.
    """

Another point to note: PEP 257 states that function docstrings should be a phrase ending with a period that describes the effect of the function as a command, rather than describing it (see the examples above).

Miscellaneous

Keep line lengths under 80 characters. On many machines and in many software packages, this is a standard line length. It makes it easier for others (like graders) to read and/or print your programs. If your parameter list for a function or method is too long to fit within 80 characters, then insert line breaks and indent the remaining lines of parameters. For example, this function header
```
def calculate_average_daily_return(customer, balance, daily_interest_rate, discount_rate):
    ...
```
could be rewritten as:
```
def calculate_average_daily_return(customer, balance,
                                   daily_interest_rate,
                                   discount_rate):
    ...
```

Written by: Wayne Snyder, Aaron Stevens (Fall 2004)
Updated: Alexandra Stefan (Spring 2006), Dave Sullivan (Fall 2006)
Updated for Python: Alexander Breen (Fall 2014)