Python Programming Essentials - M31 - PEP 8

1. http://www.skillbrew.com /Skillbrew Talent brewed by the industry itself Coding Style & Pep 8 Pavan Verma @YinYangPavan Founder, P3 InfoTech Solutions Pvt. Ltd. 1 Python Programming Essentials

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

3. © SkillBrew http://skillbrew.com Readability counts One of Guido's key insights is that code is read much more often than it is written. "Readability counts" 3

4. © SkillBrew http://skillbrew.com Code Layout  Indentation  Tabs or spaces  Maximum length lines  Blank lines  imports 4

5. © SkillBrew http://skillbrew.com 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

6. © SkillBrew http://skillbrew.com Tabs or spaces Spaces are the preferred indentation method Maximum Line Length Limit all lines to maximum 79 characters 6

7. © SkillBrew http://skillbrew.com 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

8. © SkillBrew http://skillbrew.com imports Imports should usually be on separate lines import os import sys import sys, os 8

9. © SkillBrew http://skillbrew.com 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

10. © SkillBrew http://skillbrew.com 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

11. © SkillBrew http://skillbrew.com 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

12. © SkillBrew http://skillbrew.com Whitespace in Expressions and Statements (2) Avoid extra whitespaces in following situations 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

13. © SkillBrew http://skillbrew.com Whitespace in Expressions and Statements (3) Avoid extra whitespaces in following situations spam(1) spam (1) Immediately before the open parenthesis that starts the argument list of a function call 13

14. © SkillBrew http://skillbrew.com Whitespace in Expressions and Statements (4) Avoid extra whitespaces in following situations dict['key'] = list[index] dict ['key'] = list [index] Immediately before the open parenthesis that starts an indexing or slicing 14

15. © SkillBrew http://skillbrew.com Whitespace in Expressions and Statements (5) Avoid extra whitespaces in following situations 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

16. © SkillBrew http://skillbrew.com Whitespace in Expressions and Statements (6) Avoid extra whitespaces in following situations 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

17. © SkillBrew http://skillbrew.com 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. """

18. © SkillBrew http://skillbrew.com 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

19. © SkillBrew http://skillbrew.com 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.

20. © SkillBrew http://skillbrew.com 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.

21. © SkillBrew http://skillbrew.com 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')

22. © SkillBrew http://skillbrew.com 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

23. © SkillBrew http://skillbrew.com 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

25. © SkillBrew http://skillbrew.com 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

26. © SkillBrew http://skillbrew.com 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)] ^ list_comp.py:4:9: E231 missing whitespace after ',' f = [((x,x),x+x) for x in range(3) ^ list_comp.py:4:12: E231 missing whitespace after ',' 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

27. © SkillBrew http://skillbrew.com 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

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

29. © SkillBrew http://skillbrew.com 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