Python Programming Essentials - M31 - PEP 8

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Coding Style & Pep 8
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
1
Python Programming Essentials

© SkillBrew http://skillbrew.com
Contents
 Code layout
 Indentation, tabs and spaces
 Imports
 Blank lines
 Whitespace in expressions and statements
 Pep8
2

Readability counts
One of Guido's key
insights is that code is
read much more often
than it is written.
"Readability counts"
3

Code Layout
 Indentation
 Tabs or spaces
 Maximum length lines
 Blank lines
 imports
4

Indentation
def get(self, request,
post_id, comment_id):
pass
my_list = [
1, 2, 3,
4, 5, 6,
]
def get(self, request,
post_id, comment_id):
pass
my_list = [
1, 2, 3,
4, 5, 6,]
5

Tabs or spaces
Spaces are the preferred indentation method
Maximum Line Length
Limit all lines to maximum 79 characters
6

Blank lines
def get_index(val):
pass
def get_value(index):
pass
class DoSomething():
def __init__(self):
pass
def do(self):
pass
• Separate top-level function and
class definitions with two blank
lines
• Method definitions inside a
class are separated by a single
blank line
7

imports
Imports should usually be
on separate lines
import os
import sys
import sys, os
8

imports
Recommended
from testapp.views import CreateView
Not Recommended
from .views import CreateView
Absolute imports are
recommended
Better readability
myproject
views.py utils.py models.py
testapp
utils.py
9

Blank lines
def get_index(val):
pass
def get_value(index):
pass
class DoSomething():
def __init__(self):
pass
def do(self):
pass
• Separate top-level function and
class definitions with two blank
lines
• Method definitions inside a
class are separated by a single
blank line
10

Whitespace in Expressions and Statements
Avoid extra whitespaces in following situations
spam(ham[1], {eggs: 2})
spam( ham[ 1 ], { eggs: 2 } )
Immediately inside parentheses, brackets or
braces
11

Whitespace in Expressions and Statements (2)
if x == 4: print x, y; x, y = y, x
if x == 4 : print x , y ; x , y = y , x
Immediately before a comma, semicolon, or
colon
12

spam(1)
spam (1)
Immediately before the open parenthesis
that starts the argument list of a function
call
13

dict['key'] = list[index]
dict ['key'] = list [index]
Immediately before the open parenthesis
that starts an indexing or slicing
14

x = 1
long_variable = 3
x = 1
long_variable = 3
More than one space around an assignment
(or other) operator to align it with another
15

def complex(real, imag=0.0):
return magic(r=real, i=imag)
def complex(real, imag = 0.0):
return magic(r = real, i = imag)
Don't use spaces around the = sign when used to
indicate a keyword argument or a default
parameter value
16

Documentation Strings
17
PEP 257: The """ that ends a multiline
docstring should be on a line by itself
"""Return a foobang
Optional plotz says to frobnicate the bizbaz first.
"""

Naming Conventions
18
Class names should normally use the CapWords
convention. Eg. MyClass
Function names should be lowercase, with words
separated by underscores as necessary to improve
readability. Eg. this_is_a_function()
Constants are usually defined on a module level and
written in all capital letters with underscores separating
words. Eg. DEFAULT_SENDER_EMAIL

Naming Conventions (2)
19
Always use self for the first argument to instance
methods.
Always use cls for the first argument to class
methods.
Public attributes should have no leading underscores.

Naming Conventions (3)
20
Since module names are mapped to file names,
and some file systems are case insensitive and
truncate long names, it is important that module
names be chosen to be fairly short and
lowercase.

Leading Underscores
21
__double_leading_and_trailing_underscore__:
"magic" objects or attributes that live in user-controlled
namespaces. E.g. __init__, __import__ or __file__.
Never invent such names; only use them as
documented.
_single_leading_underscore: weak "internal
use" indicator. E.g. from M import * does not import
objects whose name starts with an underscore.
single_trailing_underscore_: used by
convention to avoid conflicts with Python keyword. Eg.
Tkinter.Toplevel(master, class_='ClassName')

Recommended
Please go through the pep 8 style guide
http://www.python.org/dev/peps/pep-
0008/.
Use this as a reference whenever in doubt.
22

pep8 - Python style guide checker
pep8 is a tool to check your Python code against
some of the style conventions in PEP 8
$ pip install pep8
$ pip install --upgrade pep8
23

pep8
$ pep8 myprogram.py
24

pep8 (2)
l = [((y,x), x+y) for x in range(4) for y in range(4)]
print l
f = [((x,x),x+x) for x in range(3) ]
print f
pep8 –-first list_comp.py
list_comp.py:1:9: E231 missing whitespace after
','
list_comp.py:5:5: E124 closing bracket does not
match visual indentation
list_comp.py:7:1: W391 blank line at end of file
25

pep8 --show-source list_comp.py
list_comp.py:1:9: E231 missing whitespace after ','
l = [((y,x), x+y) for x in range(4) for y in range(4)]
^
f = [((x,x),x+x) for x in range(3)
^
f = [((x,x),x+x) for x in range(3)
^
list_comp.py:5:5: E124 closing bracket does not match
visual indentation
]
^
list_comp.py:7:1: W391 blank line at end of file
^
pep8 (3)
26

Important pep8 options
Option Description
--first show first occurrence of each error
--show-source show source code for each error
--show-pep8 show text of PEP 8 for each error
(implies --first)
--statistics count errors and warnings
--exclude=patterns exclude files or directories which
match these comma separated
patterns (default:
.svn,CVS,.bzr,.hg,.git)
--filename=patterns when parsing directories, only check
filenames matching these comma
separated patterns (default: *.py)
--benchmark measure processing speed
27

Recommended
To know more about pep8 options type in :
$ pep8 –h
OR
Read the docs
http://pep8.readthedocs.org/en/latest/intro.html
28

References
 http://pep8.readthedocs.org/en/latest/in
tro.html
 https://pypi.python.org/pypi/pep8
 http://www.python.org/dev/peps/pep-
0008/
29

Python Programming Essentials - M31 - PEP 8

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