Formatting

Indentation

Use 4 spaces per indentation level.

# Correct:

# Aligned with opening delimiter.
foo = long_function_name(var_one, var_two,
                         var_three, var_four)

# Add 4 spaces (an extra level of indentation) to distinguish arguments from the rest.
def long_function_name(
        var_one, var_two, var_three,
        var_four):
    print(var_one)

# Hanging indents should add a level.
foo = long_function_name(
    var_one, var_two,
    var_three, var_four)

# Wrong:

# Arguments on first line forbidden when not using vertical alignment.
foo = long_function_name(var_one, var_two,
    var_three, var_four)

# Further indentation required as indentation is not distinguishable.
def long_function_name(
    var_one, var_two, var_three,
    var_four):
    print(var_one)

IF - ELSE Statement

When the conditional part of an if-statement is long enough to require that it be written across multiple lines, it's worth noting that the combination of a two character keyword (i.e. if), plus a single space, plus an opening parenthesis creates a natural 4-space indent for the subsequent lines of the multiline conditional.

#Correct

if (this_is_one_thing
        and that_is_another_thing):
    do_something()

# Wrong

# No extra indentation.
if (this_is_one_thing and
    that_is_another_thing):
    do_something()

Functions

There must be at least 1 and no more than 2 spaces between functions.

# Correct
def A(self):
    # operations...

def B(self):
    # operations...

2 spaces can be left as below

# Correct
def A(self):
    # operations...


def B(self):
    # operations...

Imports

Imports should usually be on separate lines:

# Correct:
import os
import sys

# Wrong:
import sys, os

It's okay to say this though:

# Correct:
from subprocess import Popen, PIPE