Python PEP 8: Style guide / Best Practices

What's PEP 8?

It contains some conventions in coding with Python. They make the codes clearer and more beautiful. Read the full doc here. Below are just some of them in my choice.

Naming styles

Package & module names & function & variable: all_lower_case or short with underscore.

Class names: use CapWords.

Constant: ALL_CAPITAL_LETTERS.

Avoid these:

1l = 1 # lowercase letter el
2O = 1 # uppercase letter oh
3I = 1 # uppercase letter eye
4
5Capitalized_Words_With_Underscores = 1 # ugly

Code layout

Indentation

Use 4 spaces per indentation level. Note that, in this site, for a better view, I use 2 spaces for the code highlight.

1# Do
2def func(...):
3    commands # 4 spaces

1# Don't
2def func(...):
3  commands # 2 spaces

Vertical align when break a continuous line:

1# Do
2foo = long_function_name(var_one, var_two,
3                         var_three, var_four)

1# Don't
2foo = long_function_name(var_one, var_two,
3    var_three, var_four)

Distinguish arguments from the rest:

1# Do
2def long_function_name(
3        var_one, var_two, var_three,
4        var_four):
5    print(var_one)

1# Don't
2def long_function_name(
3    var_one, var_two, var_three,
4    var_four):
5    print(var_one)

Tabs or spaces?

Spaces are preferred. Don't mix tabs and spaces (not allowed in Python 3).

Max line lenght

Max of 79 characters.

Line break with operator

Operators should go with operands

1# Do
2income = (salary
3          + sale)

1# Don't
2income = (salary +
3          sale)

Import

Imports should usually be on separate lines:

1# Do
2import os
3import sys

1# Don't
2import os, sys

1# But yes
2from subprocess import Popen, PIPE

Whitespace

Avoid extraneous whitespace:

1# Do
2spam(ham[1], {eggs: 2})
3foo = (0,)
4if x == 4: print x, y; x, y = y, x

1# Don't
2spam( ham[ 1 ], { eggs: 2 } )
3bar = (0, )
4if x == 4 : print x , y ; x , y = y , x

For slices

1# Do
2ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
3ham[lower:upper], ham[lower:upper:], ham[lower::step]
4ham[lower+offset : upper+offset]
5ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
6ham[lower + offset : upper + offset]

1# Don't
2ham[lower + offset:upper + offset]
3ham[1: 9], ham[1 :9], ham[1:9 :3]
4ham[lower : : upper]
5ham[ : upper]

Add open parenthesis/bracket right after:

1# Do
2spam(1)
3dct['key'] = lst[index]

1# Don't
2spam (1)
3dct ['key'] = lst [index]

No need to have verticle alignment:

1# Do
2x = 1
3y = 2
4long_variable = 3

1# Don't
2x             = 1
3y             = 2
4long_variable = 3

With operators:

1# Do
2i = i + 1
3submitted += 1
4x = x*2 - 1
5hypot2 = x*x + y*y
6c = (a+b) * (a-b)

1# Don't
2i=i+1
3submitted +=1
4x = x * 2 - 1
5hypot2 = x * x + y * y
6c = (a + b) * (a - b)

Def of a function:

1# Do
2def complex(real, imag=0.0):
3    return magic(r=real, i=imag)

1# Don't
2def complex(real, imag = 0.0):
3    return magic(r = real, i = imag)

Programming Recommendations

Using not inside if:

1# Do
2if foo is not None:

1# Don't
2if not foo is None:

Using Use .startswith() and .endswith() instead of string slicing:

1# Do
2if foo.startswith('bar'):

1# Don't
2if foo[:3] == 'bar':

For sequences, (strings, lists, tuples), use the fact that empty sequences are false:

1# Do
2if not seq:
3if seq:

1# Don't
2if len(seq):
3if not len(seq):

Don't compare boolean values to True or False using ==:

1# Do
2if greeting:

1# Don't
2if greeting == True:

1# Worse
2if greeting is True: