Python Safe Calculator

Web-Safe Arithmetic Expressions Evaluator

import math
import re

whitelist = '|'.join(
    # oprators, digits
    ['-', '\+', '/', '\\', '\*', '\^', '\*\*', '\(', '\)', '\d+']
    # functions of math module (ex. __xxx__)
    + [f for f in dir(math) if f[:2] != '__'])

valid = lambda exp: re.match(whitelist, exp)

>>> valid('23**2')
<_sre.SRE_Match object at 0xb78ac218>
>>> valid('sys.exit(100)') == None
True
>>> exp = '23**2'
>>> if valid(exp):
>>>     x = eval(exp)

came across this post via unofficial planet python

Fibonacci, Generators and Python

Python is so beautiful and elegant

See this:

def fib():
    a, b = 0, 1
    while 1:
        yield b
        a, b = b, a+b

if __name__ == '__main__':
    f = fib()
    for i in xrange(1000):
        print f.next()

from: http://www.python.org/dev/peps/pep-0255/

Accumulator Generator

I just came across this PG’s page:

Note: (a) that’s number, not integer, (b) that’s incremented by, not plus.

For python he offers this approach:

class foo:
    def __init__(self, n):
        self.n = n

    def __call__(self, i):
        self.n += i
        return self.n

I thought of it for a few seconds and realized that this class instantiation is not necessary. Same can be achieved this way:

def foo(n):
    def _inci(n, x):
        n+=x
        return n

    return lambda i: _inci (n,i)