Notes of PEP 8

Code Layout

Indentation

(a1)

foo = long_function_name(var_one, var_two,
                         var_three, var_four)  

(a2)

foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

def long_function_name(
        var_one, var_two, var_three,
        var_four):
    print(var_one)

(b1)

if (this_is_one_thing and
    that_is_another_thing):
    do_something()

(b2)

if (this_is_one_thing and
    that_is_another_thing):
    # Since both conditions are true, we can frobnicate
    do_something()

(b3)

if (this_is_one_thing
        and that_is_another_thing):
    do_something()

(c1)

my_list = [
    1, 2, 3,
    4, 5, 6,  # Notice the ',' here
    ]

result = some-function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f'
    )

(c2)

my_list = [
    1, 2, 3,
    4, 5, 6,
]

result = some-function_that_takes_arguments(
    'a', 'b', 'c',
    'd', 'e', 'f'
)

Tabs or Spaces?

Use 4 spaces.

Maximum Line Length

Code: 79 characters
Docstrings or comments: 72 characters
Preferred way of wrapping long lines: implied line continuation inside parentheses, brackets and braces.

Source File Encodings:

Files using UTF-8 in Python 3 should not have an encoding declaration.

Imports

(a1)

import os
import sys

(a2)

from subprocess import Popen, PIPE

Import group order:

1. Std library Imports
2. Related third party imports
3. Local application/library specific imports

Each group should be separated by a blank line.

String Quotes

Pick a rule and stick to it.
Always use double quote characters for triple quoted strings.

Whitespace in Expressions and Statements

Pet Peeves

In a slice the colon should have equal amounts on either side.
When a slice parameter is omitted, the space is omitted.

(a1)

ham[:9:3]
ham[lower:upper]

ham[lower+offset : upper+offset]
ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]

ham[lower + offset : upper + offset]
# Wrong: ham[lower + offset:upper + offset]

Other Recommendations

If operators with different priorities are used, consider adding whitespaces around the operators with the lowest priority(ies).

(a1)

x = x*2 - 1
# Not recommended: x = x * 2 - 1
hypot2 = x*x + y*y
# Not recommended: hypot2 = x * x + y * y
c = (a+b) * (a-b)
# Not recommended: c = (a + b) * (a - b)

Comments

Always make a priority of keeping the comments up-to-date when the code changes.
Comments should be complete sentences.
The first word should be capitalized.
Block comments generally consists of one or more paras built out of complete sentences, with each sentence ending in a period.
You should use 2 spaces after a sentence-ending period in multi-sentence comments.

Block Comments

Each line of a block comment starts with a # and a single space.
Paras inside a block comment are separated by a line containing a single
#.

Inline Comments

Use inline comments sparingly.

Documentation Strings

Write docstrings for all public modules, functions, classes, and methods.
Docstrings are not necessary for non-public methods, but you should have a comment that describes what the method does.

(a1)

"""Return a foobang  # Capitalize the first character

Optional plotz says to frobnicate the bizbaz first.
"""

For one liner docstrings, keep the closing """ on the same line.

Naming Conventions

Descriptive: Naming Styles

When using acronyms in CapWords, capitalize all the letters of the acronym, thus HTTPServerError is better than HttpServer.
_single_leading_underscore: weak internal use indicator. from M import * does not import objs whose names start with an underscore.
single_trailing_underscore_: used by convention to avoid conflicts with Python keyword: Tkinter.Toplevel(master, class_='ClassName')
__double_leading_underscore: when naming a class attr, invokes name mangling (inside class FooBar, __boo becomes _FooBar__boo)

Packages and Modules Names

Modules: short, all lowercase names, underscores can be used.
Packages: short, all lowercase names, the use of underscores is discouraged.

Global Variable Names

Modules that are designed for use via from M import * should use __all___ mechanism to prevent exporting globals, or use the older convention of prefixing such globals with an underscore.

Method Names and Instance Variables

Use one leading underscore only for non-public methods and instance vars.

Constants

All capital letters.

Programming Recommendations

1. Don't use a += bor a = a + b for string concatenation, use ''.join() instead.
2. Beware of writing if x when you really mean if x is not None.
3. When implementing ordering operations with rich comparisons, it's best to implement all six operations(__eq__ and so on) rather than relying on other code to only exercise a particular comparison.
4. Derive exceptions from Exception rather than BaseException.
5. When catching exceptions, mention specific exceptions whenever possible instead of using a bare except: clause. If you want to catch all exceptions that signal program errors, use except Exception:.
6. For all try/except clauses, limit the try clause to the absolute minimum amount of code necessary.
7. Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any statement returns an expression, any return statement where no value is returned should explicitly state this as return None, and an explicit return statement should be present at the end of the function (if reachable).
8. Use ''.startswith() and ''.endswith() instead of string slicing to check for prefixes or suffixes.
9. Object type comparison should always use isinstance() instead of comparing types directly. if isinstance(obj, int):
10. For sequences, use the fact that empty sequences are False instead of using if len(seq):.
11. Use of the flow control statements return/break/continue within the finally suite of try...finally, where the flow control statement would jump outside the finally suite, is discouraged.

---

Reference:
PEP 8 -- Style Guide for Python Code