NullAttributor

Sometimes, you have a deeply nested set of attributes which may not be present somewhere along the chain.  A try/except would be clunky.

>>> class Foo(object): pass
...
>>> f = Foo()
>>> f.a = f
>>> f.a.b = f
>>> f.a.b.c.d
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute 'c'

This is a simple class which allows an expression to be wrapped and unwrapped, with the result at the end being either the value obtained or None.

>>> class na(object):
...     '''
...     A None-Attributator object.  Allows for "safe navigation",
...     similar to the ?. attribute in C#, or undefined in JavaScript.
...     '''
...     __slots__ = ("base",)
...     def __init__(self, base):
...         self.base = base
...     def __getattr__(self, name):
...         return na(getattr(self.base, name, None))
...     def __getitem__(self, key):
...         try:
...             return na(self.base[key])
...         except (KeyError, TypeError, IndexError):
...             return na(None)
...     def __call__(self):
...         return self.base
...
>>>
>>> na(f).a.b.c.d.e()
>>> na(f).a.b['cat']()

loopdefaultdict

Simple idiom for a tree.

>>> import collections
>>>
>>> def loopdefaultdict():
...    return collections.defaultdict(loopdefaultdict)
...
>>> d = loopdefaultdict()
>>> d[1][2][3]
defaultdict(<function loopdefaultdict at 0x02B249B0>, {})
>>> d[1][2][3] = "cat"

Edit: wikipedia has an even better version
http://en.wikipedia.org/wiki/Autovivification#Python

Tree = lambda: defaultdict(Tree)

fun with raise

>>> raise ValueError("wrong value")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: wrong value
>>> try:
...    raise ValueError("wrong value")
... except:
...    raise sys.exc_info()
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError

If you want to really re-raise the exact same message, replace sys.exc_info()[0] with the exception instance.

>>> try:
...    raise ValueError("wrong value")
... except Exception as e:
...    raise (e,) + sys.exc_info()[1:]
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: wrong value

Alas, replacing the traceback object (sys.exc_info()[2]) does not seem to work:

>>> try:
...     json.loads('NOT JSON')
... except:
...     _, _, tb = sys.exc_info()
...
>>> try:
...    1 / 0
... except Exception as e:
...    raise sys.exc_info()[:2], tb
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ZeroDivisionError: <traceback object at 0x02B752B0>

exit() is so pedestrian

C:\Users\kurose\workspace>python
Python 2.7.5 (default, May 15 2013, 22:43:36) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> raise SystemExit("bye")
bye

C:\Users\kurose\workspace>

gEvent friendly REPL

I was surprised how little code this was.  The fileobject module added in gEvent 1.0 and the standard library code module make this trivial.

import sys
import code
from gevent import fileobject

_green_stdin = fileobject.FileObject(sys.stdin)
_green_stdout = fileobject.FileObject(sys.stdout)

def _green_raw_input(prompt):
    _green_stdout.write(prompt)
    return _green_stdin.readline()[:-1]

def run_console(local=None, prompt=">>>"):
    code.interact(prompt, _green_raw_input, local=local or {})

if __name__ == "__main__":
    run_console()

missing the __line__ macro from C?

Frame objects to the rescue!

import sys

def __line__():
    f = sys._getframe().f_back
    return f.f_lineno + f.f_code.co_firstlineno

sigfigs

def _sigfigs(n, sigfigs=3):
    'helper function to round a number to significant figures'
    if n == 0 or math.isnan(n):  # avoid math domain errors
        return n
    return round(float(n), -int(math.floor(math.log10(abs(n))) - sigfigs + 1))