evanescent /ˌɛvəˈnɛsənt/ (adjective):
  1. vanishing; fading away; fleeting.
  2. tending to become imperceptible; scarcely perceptible.
I find it useful to pass code snippets directly to python while on the command line or from within a shell script. Here are some example one liners you can cut & paste to your command shell to try out.

python -c "import this"Display some python wisdom
python -c "from __future__ import braces"Are {} being introduced?
python -c "import heapq; print heapq.__about__"An example of included nuggets of info
math and number formatting
python -c "print '%.f' % 1.2E12"Parse scientific notation and display as integer
python -c "print 2**1234"Arbitrary precision integer calculator
python -c "from decimal import *; d=Decimal('2.0'); print d**1234"Arbitrary precision floating point
python -c "import sys; print sum(int(line) for line in sys.stdin)"Efficiently add integers from stdin
python -c "print reduce(__import__('operator').add, range(1,101))"Alternative method to sum a list
python -c "print sum(1.0/(2<<n) for n in range(10))"Sum a series
information lookup
python -c "import os; print '\n'.join(os.environ['PATH'].split(os.pathsep))"Display the $PATH, one per line
python -c "import os; print os.strerror(42)"Lookup an error code
python -c "import platform; print platform.node()"Print the hostname
python -c "import httplib,pprint; pprint.pprint(httplib.responses.items())"Display a dictionary; e.g. HTTP responses
python -c "import unicodedata as ud; print ud.name(unichr(0x2028))"Unicode character lookup
useful modules
python -m calendarDisplay a calendar
python -m SimpleHTTPServerServe current dir at http://$HOSTNAME:8000/
python -m timeit -htime snippets of code

There are various techniques, and even an online tool for making arbitrarily sophisticated python one liners, but I wouldn't advise obfuscating something sophisticated into a single line, unless it fits naturally. As an interim step to creating an independent python script, you may find something like funcpy or pyline useful. Also the snippets don't have to be resticted to one line and can be expanded as in this example:

python -c "
import os
for e in range(128):
    print '%d\t%s' % (e, os.strerror(e))
" |
grep -Fi supported
Note you may balk at the overhead of starting another interpreter, but I find python caches well, and there are speedup options if required.

Another place I enter small/temporary pieces of python code is at the python interactive prompt, which I have enhanced with a small wrapper script called inpy to add tab completion and an ls function with globbing.
© Apr 15 2009