Weekly 20140323

30 Python language features and tricks

• Extended unpacking

>>> a, *b, c = [1,2,3,4]
>>> b
[2,3]

• List slices with negative step

>>> a = [1,3,4,5]
>>> a[::-1]
[5,4,3,1]

• Name slices (slice(start, end, slice))

>>> LASTTHREE = slice(-3, None, 2)
>>> a = [1,2,3,4,5,6]
>>> a[LASTTHREE]
[4,6]

• Dictionary comprehension

>>> m = {x: x * 3 for x in range(5)}
>>> m
{0:0, 1:3, 2:6, 3:9, 4:12, 5:15}

• Named tuples

>>> Point = collections.namedtuple('Point', ['x', 'y'])
>>> b = Point(11,13)
>>> b.y
13

• Use capital letter for set variables
• Counter

>>> from collections import Counter
>>> a = Counter([1,1,2,3])
>>> a
Counter({1:2, 2:1, 3:1})
>>> a.most_common(1)
[(1, 2)]

• collections.deque Double-ended queue
• default dict

>>> m = collections.defaultdict(int)
>>> m['a']
0
>>> m = collections.defaultdict(str)
>>> m['a']
''

Simple URL encryption

Append a KEY string (with an easy delimitor) to the ID field, then use base64.urlsafe_b64encode and base64.urlsafe_b64decode

flaskr-tdd

• TDD:

  1. write a test
  2. run the test (it should fail)
  3. write just enough code to pass the test
  4. refactor

• Flask test client

tester = app.test_client(self)
response = tester.get('/', content_type="html/text")
# response.data include all html text

• Flask flash system: get_flashed_messages --> may be useful for client-side validation

• url_for('static', filename = 'style.css')

Install tagbar on vim

• create .vim/ if not exist
• Install exuberant ctags
• Install tagbar

cd ~/.vim
git clone git://github.com/majutsushi/tagbar
mv tagbar/* .  # must be directly under .vim/
sudo rm -r tagbar/
cd doc/
vim tagbar.txt
:helptags .
# quit vim

• edit .vimrc (below is mine):

let g:tagbar_ctags_bin='/usr/local/bin/ctags' # change to ctags bin path, this is on MacOS
let g:tagbar_width=40
noremap <silent> tb :TagbarOpen fj <CR>

• useful key shortcuts:

tb: open tagbar # my key binding
p: go to tag, stay in tagbar
<enter>: go to tag
o: toggle fold
*: open all fold
=: collapse all fold
?: keymap help