===== System level =====
  * Pass parameters in command line
<code python>
# test.py name_1 name_2 ... name_n
# or python -i test.py name_1 name_2 ... name_n
import sys
parms = sys.argv  # return ['test.py", name_1, ... name_n]
</code>


  * Execute an external command
<code python>
import os
os.system('ls')
</code>


  * time counter (tic/toc)
<code python>
import time
tic = time.time()
... ...
toc = time.time()
elaps = toc-tic
</code>

  * http://stackoverflow.com/questions/860140/whoami-in-python

===== File I/O =====
  * Find the newest/oldest file in a path: http://code.activestate.com/recipes/576804-find-the-oldest-or-yougest-of-a-list-of-files/
===== List/Tuple =====
==== How to reverse zip ====
  * Answer: use zip() in combination with a special unitary operator "*" which release elements from a list/tuple (i.e., effectively it removes "()" or "[]"). ([[http://stackoverflow.com/questions/5917522/unzipping-and-the-operator | Ref1]], [[http://docs.python.org/tutorial/controlflow.html#unpacking-argument-lists | Ref2]]) For example:
<code>
a = [1, 2, 3]
b = ['A', 'B', 'C']
z = zip(a, b)
# to reverse it
a, b = zip(*z)
</code>

===== Dictionary =====
==== Build dictionary  ====
  * [[http://stackoverflow.com/questions/1747817/python-create-a-dictionary-with-list-comprehension | Ref]]
  * In Python 2.6 (or earlier), use the dict constructor:
<code>
d = dict((key, value) for (key, value) in sequence)
</code>
  * In Python 2.7+ or 3, you can just use the dict comprehension syntax directly:
<code>
d = {key: value for (key, value) in sequence}
</code>

===== Plot =====
  * Output plots into a multi-page pdf file
<code python>
from matplotlib.backends.backend_pdf import PdfPages

fnpdf = 'out.pdf'
pdf = PdfPages(fnpdf)

...

for i in range(npage):
    ...
    pdf.savefig()
    show()  # you can remove this line if needed
    
pdf.close()
</code>